Insect Ecology: Concepts to Management 9811917817, 9789811917813

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K. V. Hari Prasad

Insect Ecology: Concepts to Management

Insect Ecology: Concepts to Management

K. V. Hari Prasad

Insect Ecology: Concepts to Management

K. V. Hari Prasad Department of Entomology Acharya N. G. Ranga Agricultural University Tirupati, India

ISBN 978-981-19-1781-3 ISBN 978-981-19-1782-0 https://doi.org/10.1007/978-981-19-1782-0

(eBook)

# The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Dedicated To My Father K.T. Venkatachalapathy (Late) who taught me the first few lessons of Ecology “Live and Let Live and Watch the Surroundings”

Preface

Insects are one among the most diverse groups of organisms on earth colonizing from the freezing cold areas of the Antarctic to the blazing heat of the Namibian desert. The numbers and the diversity of insects have intrigued mankind since long back and the study of insects dates back to time immemorial across various dominant religions of the world. Insects with their sheer number and diversity have become an integral part of several terrestrial and aquatic ecosystem. Of all the living organisms occupying earth’s atmosphere, insects contribute to 20 % of animal biomass. Though Ecology as a subject is an old science, the study of ecology as a separate branch of science has got momentum after important contributions by several ecologists and some noted ones are works of Karl Möbius (Concept of Biocoenosis or Ecological Community); Ernst Haeckel (Ecology and Evolution); Vito Volterra (Mathematical Population Models); Vladimir Vernadsky (Concept of Biosphere); Victor Ernest Shelford (Food Webs, Law of Tolerance); Alfred J Lotka (PredatorPrey Mathematical Models); Charles S Elton (Animal Ecology); G. E. Hutchinson (Niche Concept); Raymond Lindeman’s (Trophic-Dynamic Concept of Ecosystems); and works of Eugene P Odum and Howard T Odum (Co-founders of Ecosystem Ecology). The period after World War II has been a detrimental phase for the field of ecology as cheap and quicker solution by means of synthetic pesticides were available to tackle the problem of insects not only in agriculture but also in veterinary and public health. The publication of Silent Spring in 1962 by Rachel Carson has awakened interest in the field of Ecology among several policy makers across the globe and has paved the way for the development of Integrated Pest Management (IPM). The recent years have witnessed renewed interest in the field of ecology and lead to improvements in IPM with more emphasis on eco-friendly habitat management, into what is being termed as the present day “Ecological Pest Management.” The book Insect Ecology: Concepts to Management starts with a focus on certain basic characters of insects at species level that are responsible for their dominance among the living organisms. In the following chapters, chemical ecology of insects and the evolutionary significance of plant–insect association have been examined along with seasonality of insects, foremost contributing factors responsible for greater migration capacity of locust and monarch butterflies. The subsequent chapters have focused on characters of insects from a population point of view. Several life history parameters of insects are discussed and how majority of the vii

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insects can be conveniently grouped to r and k strategy insects is examined. The study of population dynamics of insects and their role in formulating better pest management strategies have been outlined. The next penultimate chapters deal with community ecology as a matter of community structure and community dynamics or function. “Community Structure” deals with species diversity, richness, and various interactions involved among the organisms of same species (intraspecific) and different species (interspecific) interactions. Within “Community Structure,” the “Concept of Niche” was also introduced. Pattern of ecological succession, interactions at different trophic levels, and how plant secondary metabolites as well as their volatiles affect these interactions have been critically examined in “Community Dynamics.” The ecosystem services provided by the insect have been outlined in the chapter “Ecosystem Ecology” where production system and energy flow among the trophic levels are reviewed. The concept of “Payment for Ecosystem Services (PES)” has been touched upon to give an insight into various policy matters that can affect the ecosystem services provided by Mother Nature. The last chapters of the book deal with “Ecological Pest Management” and effect of “Climate Change on Insects.” The information provided in Chaps. 1–14 gives an idea of how insects have become a part of ecosystem, the pivotal role played by them in various ecosystem services, and how this information can be utilized for formulating better environmentally benign pest management strategies such as Ecological Pest Management. And finally, the role of climate change as one of the main driving forces for the recent outbreaks of insect pest in various ecosystem across globe has been introduced for formulating better pest forecasting models and pest management strategies. Tirupati, India

K. V. Hari Prasad

Acknowledgments

I would like to express my sincere gratitude and appreciation to all the university officials of Acharya N.G. Ranga Agricultural University (ANGRAU), Lam Centre, Guntur, Andhra Pradesh, India who have given me the permission and support for writing this book. My sincere thanks are due to all my PG and PhD students for helping me in reviewing the chapters. Sincere appreciation also goes to all the staff members at the Department of Entomology, S.V. Agricultural College, Tirupati for their constant support and encouragement while writing the book. Finally, without the support of my family members Smt. K. Brunda, my mother; K.C. Gowri Devi, my wife; and my lovely daughters, Sahi and Buddu, it would not have been possible for me to complete this daunting task of writing the book.

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1

History of Ecology and Definitions . . . . . . . . . . . . . . . . . . . . . . . . 1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Terminology Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 1 2 2 4

2

Concepts of Ecology and Ecological Organization . . . . . . . . . . . . . 2.1 Structural Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Theory of Natural Selection and Role of Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Functional Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Concept of Feedback and Homeostasis . . . . . . . . . . . 2.2.2 Concept of Energy Flow Through Ecosystem . . . . . . . 2.3 Ecological Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 5

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5 6 6 8 8 10

3

Introduction to Insect Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 14

4

Factors Responsible for Abundance of Insects . . . . . . . . . . . . . . . . 4.1 Structural Perfections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Developmental Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Protective Adaptations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

17 17 19 20 21

5

Factors Affecting Growth and Metabolism of Insects . . . . . . . . . . . 5.1 Effect of Abiotic Factors on Insect Population . . . . . . . . . . . . . . 5.1.1 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2 Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3 Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4 Rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5 Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6 Atmospheric Pressure . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7 Wind and Air Currents . . . . . . . . . . . . . . . . . . . . . . . .

23 24 24 27 28 28 29 30 30

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5.1.8 Water Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9 Edaphic (Soil) Factors . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Biotic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Food . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Association with Other Organisms . . . . . . . . . . . . . . . . 5.3 Self-regulation of Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Balance of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30 31 31 31 31 33 35 35

Insect–Plant Interactions and Role of Secondary Metabolites . . . . . 6.1 Theories of Insect–Plant Associations . . . . . . . . . . . . . . . . . . . . 6.2 Coevolution of Insect Orders and Plants . . . . . . . . . . . . . . . . . . 6.3 Hexapod Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Herbivory: Also Known as Phytophagy. Greek Word Phyton— Plant; Phagei—To Eat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Feeding Strategies of Herbivorous Insects . . . . . . . . . . 6.4.2 Theories of Herbivore Attacks on Plant . . . . . . . . . . . . 6.5 Insect–Plant Mutualism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Pollination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Plants as Food and Source of Secondary Metabolites (as Defense) for Insect Herbivores . . . . . . . . . . . . . . . . 6.5.3 Sulfur Containing Secondary Metabolites . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37 38 39 41

Insect Chemical Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Insect Chemical Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Semiochemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Pheromones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Classification of Pheromones Based on Their Functions . . . . . . . 7.4.1 Sex Pheromones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Leks and Its Significance . . . . . . . . . . . . . . . . . . . . . . 7.4.3 Epideictic or Dispersion Pheromones . . . . . . . . . . . . . . 7.4.4 Alarm Pheromones . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.5 Host Marking Pheromones . . . . . . . . . . . . . . . . . . . . . 7.4.6 Trail Pheromones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Allelochemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Allomones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.2 Plutella xylostella Diamondback Moth (DBM)–Brassica Plant–Apanteles plutellae . . . . . . . . . . . . . . . . . . . . . . 7.5.3 Allomone in Insects . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Uses of Pheromones in IPM . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.1 Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.2 Mating Disruption . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.3 Mass Trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.4 Male Annihilation Technique or Attract-and-Kill . . . . .

42 43 44 45 45 49 55 57 61 61 62 62 62 63 63 65 66 66 67 67 67 67 68 69 70 70 71 71 72

Contents

7.7 7.8 7.9

Combination of Pheromones and Kairomones . . . . . . . . . . . . . Push and Pull Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Possible Use of Host Marking Pheromone in Pest Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.10 Possible Role of Using Female Annihilation Technique . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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72 73

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74 74 75

8

Seasonality in Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Homodynamic Insects or Life Cycles . . . . . . . . . . . . . . . . . . . . 8.2 Heterodynamic Insects or Life Cycles . . . . . . . . . . . . . . . . . . . . 8.3 Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Patterns of Migration . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Dormancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 Difference Between Torpor and Quiescence . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

77 78 78 78 78 80 81 84

9

Characters of Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Population Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Natality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Mortality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Population Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Survivorship Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Age Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7 Age Pyramids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.8 Rate of Growth of a Population . . . . . . . . . . . . . . . . . . . . . . . . 9.8.1 Logistic Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.9 Population Dispersal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.9.1 Life History Strategies . . . . . . . . . . . . . . . . . . . . . . . . 9.9.2 Crowding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.9.3 Nutritional Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.9.4 Habitat Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.10 Population Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.11 Distribution Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.11.1 Distribution Parameters and Their Calculations . . . . . . 9.12 Population Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.13 Population Isolation and Territoriality . . . . . . . . . . . . . . . . . . . . 9.14 Interactions: Both Intraspecific and Interspecific . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

85 85 86 87 87 87 89 89 90 91 93 93 94 94 94 96 96 98 99 99 99 99

10

Population Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Extrinsic Factors/Density-Independent/Exogenous Process . . . . . 10.1.1 Abiotic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1.2 Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Intrinsic Factors/Density Dependent/Endogenous Process/Biotic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Endogenous Process . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Lateral Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Natural Enemies . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101 103 104 104 105 105 106 112

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10.3

Feedback Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Genetic Breakdown During Flush Period . . . . . . . . . . 10.3.2 Genetic Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Life Tables and Demography . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1 Life Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.2 Construction of Horizontal Life Table . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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117 117 117 117 118 119 121

11

Community Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Community Development . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Community Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Species Diversity, Richness, and Evenness . . . . . . . . . 11.2.2 Species Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 Predator–Prey Interaction . . . . . . . . . . . . . . . . . . . . . 11.2.4 Concept of NICHE . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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125 126 127 127 131 138 153 159

12

Community Functions/Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Community Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.1 Succession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.2 Food Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.3 Food Web . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.4 Tri-Trophic Interaction . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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163 164 164 170 171 175 185

13

Ecosystem Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1 Components of Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.1 Abiotic Components . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.2 Climatic Factors/Weather Parameters . . . . . . . . . . . . . 13.1.3 Biotic Components . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.4 Factors Liming or Affecting the Biotic and a Biotic Component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Ecosystem Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1 Trophic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.2 Ecological Pyramids . . . . . . . . . . . . . . . . . . . . . . . . . 13.3 Ecosystem Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.1 Production System . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.2 Energy Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.3 Biochemical Cycling . . . . . . . . . . . . . . . . . . . . . . . . 13.3.4 Ecosystem Services . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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189 190 190 190 190

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192 192 193 193 194 196 197 198 200 205

Ecological Pest Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.1 Causes for Outbreak of Pests in Agro-Ecosystems . . . . . . . . . . 14.2 Development of Pest Control Strategies . . . . . . . . . . . . . . . . . 14.2.1 Era of Traditional Approaches (Ancient–1938) or Subsistence Phase . . . . . . . . . . . . . . . . . . . . . . . . . .

. 209 . 210 . 212

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14.2.2 Era of Pesticides (1939–1975) or Exploitation Phase . . 14.2.3 Crisis Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.4 Collapse of Control Systems or Disaster Phase . . . . . . . 14.2.5 Environmental Contamination . . . . . . . . . . . . . . . . . . . 14.2.6 Era of IPM (1976 Onwards) or IPM Phase . . . . . . . . . . 14.3 Integrated Pest Management (IPM) . . . . . . . . . . . . . . . . . . . . . 14.3.1 Concepts of IPM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4 Present IPM scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.5 Ecological Pest Management (EPM) . . . . . . . . . . . . . . . . . . . . . 14.5.1 Factors Influencing EPM . . . . . . . . . . . . . . . . . . . . . . 14.5.2 Soil Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.5.3 Habitat Management . . . . . . . . . . . . . . . . . . . . . . . . . 14.6 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6.1 Rice Pest Management by Ecological Engineering in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.7 Payment for Ecosystem Services (PES) . . . . . . . . . . . . . . . . . . 14.7.1 PES or Payment for Environmental Services Policy in Rice—Vietnam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Climate Change and Insect Ecology . . . . . . . . . . . . . . . . . . . . . . . 15.1 Direct Effect of Climatic Change on Insect and Tritrophic Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.1 Effects on Physiology, Biology, and Life History . . . . 15.2 Indirect Effect on Insects Through Effects on Plants . . . . . . . . 15.2.1 Plant Phenology . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

213 213 213 214 214 214 215 216 217 218 218 218 219 219 220 220 221

. 223 . . . . .

224 224 225 225 226

About the Author

K. V. Hari Prasad has obtained his graduation in Agriculture and post-graduation in Entomology from Marathwada Agricultural University, Parbhani, Maharashtra, India. He has obtained his PhD from the University of Reading, Reading, United Kingdom. He is the recipient of ASPEE fellowship for his post-graduation and Felix fellowship for his PhD. He has worked in ICRISAT in the capacity of Scientific Officer, and presently he is working as Associate Professor, Department of Entomology, S.V. Agricultural College, Tirupati, Acharya N.G. Ranga Agricultural University, Lam, Guntur, Andhra Pradesh, India. His area of research is Host Plant Resistance to Insects and Insect Ecology. He has guided over 10 post-graduate and PhD students as major and minor chairman.

xvii

1

History of Ecology and Definitions

Abstract

History of ecology and its importance as separate branch of biology is discussed. The earliest work of ecology and how the subject of ecology has shaped into multidisciplinary subject and pioneer works of several ecologists were discussed and terminologies frequently used have been outlined. Keywords

History · Ecology

1.1

History

From time immemorial ecology was not given a status of major subject but always dealt as a branch of biology and has been oscillating between biological (Botany and Zoology) and physical sciences. Even when it was taught as a branch of biology, ecologists themselves were divided depending on their own lines of specializations such as plant ecologist and animal ecologist. The works of several ecologist such as Karl Möbius (concept of Biocoenosis or ecological community); Ernst Haeckel (Ecology and evolution); Vito Volterra (Mathematical population models); Vladimir Vernadsky (concept of Biosphere); Victor Ernest Shelford (Food webs, law of Tolerance); Alfred J Lotka (Predator– prey mathematical models); Charles S Elton (Animal Ecology); G. E. Hutchinson, known as “father of modern ecology” (Niche concept); Raymond Lindemans work on the trophic-dynamic concept of ecosystems; works of Eugene P Odum and Howard T Odum (Co-founders of ecosystem Ecology) and publication of Eugene Odum’s textbook, Fundamentals of Ecology, have bridged the gaps and divisions ecology and projected ecology as a unified major subject with a proper scientific experimentation and background. # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_1

1

2

1

History of Ecology and Definitions

The publication of Silent Spring in 1962 by Rachel Carson has once again kindled the interest of environment as a unified subject among the minds of various scientific communities and public as a large and lead to what many believe as Environmental Movement culminating into present Modern Ecology. Like many fields of contemporary biology, ecology is a multidisciplinary subject combining several biological, physical, and social sciences. The rise of Modern Ecology as contemporary science is witnessed by the publications of journals exclusively dedicated to ecological research such as Ecology, Ecology Monographs, and establishment of several ecological societies across the world. In addition to the basic research, the present-day ecology is also gaining importance in applications such as ecologically sustainable management practices, conservation biology, population dynamics, and services such as ecosystem services. For details into the history of ecology, the readers are requested to go through some of the reviews such as McIntosh (1985) and Santangelo and Bramanti (2006).

1.2

Definitions

The word Ecology was first coined by a German Biologist Ernst Haeckel in 1869. Ecology in Greek means “oikos” meaning home or environment and “logos” means understanding or to study. Haeckel described ecology as “the study of the natural environment including the relations of organisms to one another and to their surroundings” (Haeckel, 1869). Charles Elton in 1927 defined ecology as “the study of animals and plants in their relation to their habits and habitats.” Simpson (1964) defined ecology as “Given an existing population structure and an existing ecological situation, and given the genetic variation of the population as it moves through time, the action of selection seems to be fully deterministic.” Peter W. Price (1975) has defined ecology as “study of environment for evolutionary process” that composes of dynamics within a given environment, the functional relationship between organisms and the analysis of control mechanisms in the ecosystem. Krebs (1985) defined ecology as “the scientific study of the interactions that determine the distribution and abundance of organisms.” Eugene P Odum (1953) described ecology as “the study of the structure and function of ecosystems.” This definition of Odum have been widely used even today. In simple words, Ecology is the study of organisms in relation to the surroundings in which they live. These surroundings are called environment that includes both physical and biological entities (Chapman & Reiss, 1999).

1.3

Terminology Used

Species: Group of populations that actually or potentially interbreed with each other constitutes a species.

1.3 Terminology Used

3

Population: A group of potentially interbreeding individuals at a given locality (Mayr, 1963). Speciation: A method by which evolution advances. Niche: A set of biotic and abiotic conditions in which a species is able to persist and maintain stable population. Habitat: The habitat is the physical area where a species lives. Community: Coexisting interdependent population. Ecosystem: Community interrelation to its physical environment. Life system: Part of an ecosystem that determines the existence, abundance, and evolution of a particular population. Biomes: Regional ecosystem types such as grassland, desert, and deciduous forest. Biosphere: Biological system that includes all of earth’s living organisms interacting with the physical environment. Adaptation: Any quality of an organism, population, or species that increases its chances of living viable progeny. Fitness: The ability of an individual or a population to leave a viable progeny in relation to the ability of others. Autecology: Study of an individual organism or species and its relation to biotic and abiotic factors. Autecology is also known as population ecology. Synecology: Study of group of organisms or species in a community and its relation to biotic and abiotic factors. Synecology is also known as community ecology. Biota: Organisms living in a particular habitat or geographical area. Biotic potential: Every living organism exhibits an innate capacity to survive, reproduce, and multiply, called as the “biotic potential” or “maximum reproductive power” the term coined by Chapman in 1928 (“An inherent capacity of an organism to survive and multiply under optimum environmental condition”). Carrying capacity: Carrying capacity refers to the maximum population size that a given environment can support for an indefinite period or on a sustainable basis. Environmental resistance: The limitations forced by the environment in terms of shortage of food, space, intra- and interspecific competition, abiotic factors is collectively known as “environmental resistance” that keeps the ever-growing population of a living organism under check. Homeostasis: The constant interaction between carrying capacity and environmental resistance that keeps the balance of population of a living organism. Diversity: Number of species available in a community and the relative abundance of individuals within each species. Distribution: Spatial arrangement of a biological taxon is known as distribution. Dispersal: The movement of individual away from their area of origin or from centers of high population density also called as species range. Migration: It involves mass movement of entire population from one area to another to overcome the unfavorable condition in the native place and returning of entire population to the native place when once normal conditions return, for example, locust, monarch butterfly.

4

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History of Ecology and Definitions

Emigration: Outward movement of living organism from a population. Immigration: Inward movement of living organism into a population. Species evenness: Distribution of individuals among species. Species richness: Distribution of different species within a community irrespective of the number of individuals with each species. Trophic level: The relative position of a living organism within a food chain. Demography: The process of birth, death, immigration, and emigration that determine the size, fluctuations, and age structure of population. Metapopulation: A regional group population of a species connected occasionally. A metapopulation is a population of populations distributed in discrete habitat patches that are linked by occasional dispersal. Deme: A population of organisms within which the exchange of genes is completely random. Deme is not a closed population, but genetic drift occurs by immigration and emigration.

References Chapman, J. L., & Reiss, M. J. (1999). Ecology: Principles and applications. Cambridge University Press. Haeckel, E. (1869). Ueber Entwicklungsgang u. Aufgabe der Zoologie. Jenaische Zeitschrift, 5, 353–370. Mayr. (1963). Animal species and evolution. Harvard University Press. McIntosh, R. (1985). The background of ecology: Concept and theory. Cambridge University Press. Santangelo, G., & Bramanti, L. (2006). Ecology through time, an overview. Rivista di Biologia, 99, 395–424.

2

Concepts of Ecology and Ecological Organization

Abstract

How natural selection shapes ecosystem and how materials and energy pass from one trophic level to another within the gambit of structural and functional concepts of ecology are discussed. The importance of biotic potential in increasing the population of organisms and role environmental resistance and feedback mechanisms to contain this ever-increasing populations is being reviewed. A brief representation of hierarchical ecological organization of living organisms is also outlined. Keywords

Natural selection · Homeostasis · Biotic potential · Environmental resistance · Energy flow · Feedback mechanism

Ecology is based on two concepts: one termed as structural concepts and the other termed as functional concepts.

2.1

Structural Concepts

2.1.1

Theory of Natural Selection and Role of Environment

In any given population of living organisms, the phenotypes of individual organisms are molded by the interaction between their genetic makeup and the environment. The genetic characters are imprinted in “gametes” and are transferred from parents to offspring. Though these genetic characters are unique to all the living organisms, the expression of these characters (genotype) is continuously molded by the surrounding environmental conditions. The interactions of these genetic characters with that of # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_2

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Concepts of Ecology and Ecological Organization

Fig. 2.1 Schematic representation of theory of natural selection

environment and the resultant outcome is expressed as phenotypic character (phenotype) and organisms/populations that withstood the environmental conditions of “that” period survive and the population is forwarded further as envisaged by “law of survival of fittest” (Fig. 2.1). However, the “fittest” may not be “fittest” all the time unless they maintain the genetic diversity that suits the changing environment. The genetic diversity among the population is generally maintained by two factors: first genetic mutation and the second recombination of the existing genes in the population mostly through sexual reproduction. Within this genetic diversity, genotypes that are expressed as vigorous phenotypes or fittest are generally selected and the populations are forwarded unequivocally. However, these selections are also not permanent due to changing environmental conditions and the cycle continues. This molding of population by means of natural selection is the first basic concept of ecology.

2.2

Functional Concept

2.2.1

Concept of Feedback and Homeostasis

The first concept says that on the basis of survival of fittest, a particular genotype or a population is/are selected and forwarded. Under a given favorable environment with little competitions for food and space, all the living organisms exhibit an innate capacity to survive, reproduce, and multiply called as the “biotic potential” or “maximum reproductive power” the terms coined by Chapman in 1928 (“An inherent capacity of an organism to survive and multiply under optimum environmental condition”). The “biotic potential” or innate capacity to increase depends on 1. Initial population: More the initial population of an organism, more will be its progeny. 2. Fecundity: It is the average number of eggs laid by a female in its lifetime. More the fecundity, more will be the resultant population.

2.2 Functional Concept

7 Biotic potential

Population of host insect/Natural enemies

900 750

Population of Host insect

600 450

Population of Natural enemy

300 150 0

Biotic environmental resistance

Homeostasis 1

4

7

10

13

16

19

22

Generations

Fig. 2.2 Hypothetical representation of “Balance of Life” or “Homeostasis”

3. Sex ratio: It is the ratio of the total female population and is represented by number of females/total number of males and females. Up to certain threshold, more the proportion of females, more is the multiplication capacity. 4. Number of generations per unit time/year: Obviously, more the number of generations per unit time, the larger will be the resultant population. Based on the above factors, the biotic potential can then be represented by the formula: Biotic potential ¼ PðfsÞn : P ¼ Initial population, f ¼ Fecundity, s ¼ Sex ratio, n ¼ Number of generations in a unit time. It has been estimated that with no competition for food and space and with favorable environmental conditions, the progeny from a pair of Drosophila could cover the whole Indian subcontinent and Myanmar with a solid piece of cake of flies. However, this seldom happens in nature and the populations are maintained at equilibrium by a factor known as “environmental resistance” or “feedback mechanism,” which includes both biotic factors viz., food, natural enemies, diseases, etc., and abiotic factors viz., temperature, humidity, rainfall, etc. along with spatial and temporal occupancy. These feedback mechanisms can be positive or negative. Resource constraint is an example of negative feedback mechanism whereas the increase of natural enemy population in relation to the size of host population is an example of positive feedback mechanisms. The constant interaction of these feedback mechanisms keeps a check on the population buildup resulting in Balance of Nature or Homeostasis (Fig. 2.2).

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Concepts of Ecology and Ecological Organization

The proportion of a population which is normally eliminated as a result of environmental resistance is known as “normal coefficient of destruction,” which can be expressed by the formula: Coefficient of destruction Qn ¼ 1 


1 n s fn

:

where Q ¼ coefficient of destruction, n ¼ number of generations per year or unit time, s ¼ sex ratio, when the population is taken as 1, f ¼ fecundity. These two mechanisms, i.e., biotic potential and environmental resistance generally operate in cycles thus keeping the population of all living organisms in stable condition known as “Balance of life” or “General equilibrium” or “Homeostasis.”

2.2.2

Concept of Energy Flow Through Ecosystem

The first law of thermodynamics states that energy can be neither created nor destroyed. The energy can only change form or move from one object to another. The second law of thermodynamics or law of entropy says that in every energy transformation, potential energy is reduced because heat energy is lost to the system in the process. Thus, as the food passes from one organism to another, a portion of potential energy contained in the food supply is reduced step by step until all the energy in the system is dissipated as heat (Fig. 2.3). Though the energy lost and cannot be recycled, chemical atoms and molecules are not dissipated and remain in the ecosystem through food chains and food webs indefinitely unless natural calamities such as floods, erosion, and fire remove the chemicals from the ecosystem (Fig. 2.3). Concept of energy flow through ecosystems forms the third concept of ecology.

2.3

Ecological Organization

“Ecological organization or hierarchical ecology,” describes biological organization from atoms to biosphere. Each level in the hierarchy represents an increase in organizational complexity within each object composed primarily of previous level basic unit. Atom Smallest chemical unit composed of protons, electrons, neutrons, e.g., oxygen (O) Molecule A group of one or more atoms held together by chemical bonds, e.g., oxygen (O2) Biomolecular complex Group of molecules that are biologically complex, e.g., DNA, RNA, protein, lipid, carbohydrate

Acellular or precellular

(continued)

2.3 Ecological Organization

9

Fig. 2.3 Flow of energy in ecosystem

Organelle A functional group of biomolecules e.g., biochemical reaction A specialized subunit within the cell that has a specific function Organelles are endosymbiotic in origin, e.g., mitochondria, plastid Cell Basic unit of life and are a group of functional organelles Unicellular: Bacteria Archae bacteria Multicellular: Plants/Animals Tissue Functional grouping of cell Organ Functional grouping of tissue Organism (Species) Basic living system with functional grouping of various organs Population A group of organisms of same species capable of interbreeding

Subcellular

Cellular

Multicellular

(continued)

10

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Concepts of Ecology and Ecological Organization

Community or biocoenosis Grouping and interaction of various populations Ecosystem Community interrelation to its physical environment Biome Continental scale grouping of ecosystems Biosphere/Ecosphere (All life on earth + physical environment) Biocoenosis/Biogeocoenosis Life and earth functioning together Odum and Barrett (2005)

Reference Odum, E. P., & Barrett, G. W. (2005). Fundamentals of ecology (5th ed.). Thomson Brooks/Cole.

3

Introduction to Insect Ecology

Abstract

Certain physiological adaptation of insect to survive extreme climatic conditions were discussed considering dominance and abundance of insect in various ecosystems. The incredible role played by insects as part of ecosystem is outlined with information on role of insect as a key insect plant interaction, role of insects as herbivores, as decomposers are discussed. The importance of insect as major source of pollination for many tropical plants and their importance as biological control agent in limiting populations of various native and alien species of insects and weeds were also given. Keywords

Adaptations · Herbivory · Recycling · Biological control · Pollination · Recycling · Biological diversity

Ecology as a subject is as rich and diversified as that of nature itself and encompasses several disciplines both quantitative and qualitative emanating from an individual cell and its multitrophic interaction with other cells spatially and temporally to biosphere that include all the living organisms on earth and its relation to its physical environment. Insect, as an organism though small is an integral and intricate part of ecology and ecosystem. Insects have been on earth for more than 350 million years before man (roughly about 150,000 years) started colonizing the earth. This group of organisms belonging to Class Insecta and Phylum Arthropoda of Animal Kingdom has surpassed every other living organism with its sheer number, size, diversity, adaptations, etc. Around 70 to 90% of all known animal species are insects. Around 10% of the animal biomass of the world are ants and another 10% composed of termites. This means that “social insects” would make up an incredible 20% of the total animal # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_3

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3 Introduction to Insect Ecology

biomass of this planet. Recent estimates suggest that there are about 5.5 million species of insects and 6.8 million species of other terrestrial arthropods. Of these 5.5 million species of insects, beetles itself contribute roughly 25% (1.5 million species) (Stork et al., 2015). Insects occupy every corner of the world from the places with extreme heat to extreme cold. Insects such as Belgica Antarctica a dipteran midge can survive freezing temperature (as low as 50  C) prevailing in Antarctica by producing (25% of their body weight) cryptoprotectant substances such as glycerol, alanine, and trehalose in their hemolymph. Whereas adults and larva of Scatella thermarum of order Diptera lives in the hot springs at 48  C. From the time immemorial, insects have been viewed and studied from economic perspective to humankind. Most of the studies are usually made with reference to their habits and habitats and on the basis of which insects can be classified into three convenient groups harmless, harmful, and beneficial insects. This classification, however, is not a rigid one and is often subject to alterations depending on conditions. Certain insects which are considered not harmful may under some other favorable conditions become serious pests and vice versa. Harmless insects such as moths, butterflies, and other insects enrich the biodiversity and are an important part of ecosystem services provided by insects. Beneficial insects can be again classified as useful insects such as pollinators, parasitoids, predators, and scavengers and productive insects such as honey bee, silkworm, and lac insect that are directly or indirectly helping mankind in their quest for survival on this planet. And finally harmful insects are the ones that cause economic loss to human beings and their belongings and can be referred as insect pests and can be grouped on the basis of the associations with other living organisms such as insect pests of crops, animals including human beings, and also as vectors transmitting various dreadful plant and animal (including man) diseases. With its diversity and richness, insects have been playing several important functions in ecology and biodiversity to name a few as follow: Herbivory: By phytophagy or herbivorous insects play a major role in shaping and regulating plant community composition which in turn affects the abundance and composition of carnivores, thus regulating the ecosystem at every trophic level. Insect herbivores form approximately 25% of all the herbivores on earth (Brues, 1946; Strong et al., 1984). Recycling of soil nutrition: Termites, dung rollers, and other soil inhabiting insects which are detritivores, recycle most of the plant and animal waste and thus help in maintaining the soil structure and fertility. These soil insects are the chief regulators of litter and soil organic matter and thus facilitate a regular supply of nutrients to primary producers. Dung rollers have been introduced artificially for litter management in most of the developed countries such as Australia. Several dung beetles such as Onthophagus gazella; Euoniticellus intermedius from African continent have been introduced to Australia during 1971 and 1975 to manage the wet cattle dung and since then have been established (Waterhouse, 1974). Dung beetles in most of the tropical forest ecosystem also aid in seed dispersal by burying the seeds along with the dung and thus protecting them from rodents. Aiding in pollination: Majority of angiosperms or flowering plants

3

Introduction to Insect Ecology

13

(around 85%) directly or indirectly depend on insects for pollination. Evolution of winged insects has been the driving force in evolution of angiosperms and vice versa (Shusheng et al., 2008). The coevolution has been so intricate in case of fig and chalcid wasps that neither of them survives without the other (Machado et al., 2005). Biological control: Insects as natural enemies have contributed significantly to managing some of the most important pest of crops. Some of the successful examples are use of Rodolia cardinalis for managing Icerya purchasi cottony cushion scale on citrus in California and since then around 200 invasive insect pests and 50 weeds have been successfully managed across the world through biological control, with benefit: cost ratios (ranging from 5:1 to >1000:1) (Cock et al., 2016; Naranjo et al., 2015). An interesting example of how a biological control program had led forest conservation has been given by Wyckhuys et al. (2019). An accidental introduction of mealybug Phenacoccus manihoti (pseudococcidae; Hemiptera) in Thailand has resulted in 18% reduction of cassava yield during 2009–2010 and led to increase of domestic price in Thailand. This has resulted in the surrounding countries such as Cambodia, Lao PDR, Myanmar, and Vietnam an increase of cassava cultivated area by cutting down forests resulting in deforestation. Consequently during 2010, a host-specific parasitoid Anagyrus lopezi (Encyrtidae; Hymenoptera) was released for managing the mealybug menace and the outbreak of mealybugs were managed resulting stabilizing the domestic market resulting 31–95% slowdown of deforestation in neighboring countries. Biodiversity and Ecological balance: Because of ubiquity of insects, they invariably form a key component of most of terrestrial ecosystems (Shurin et al., 2006). In case of cyclic ecological succession where herbivores and their natural enemies are involved insect have an indispensable role to play. Insect as Food: Insects as food is as old as human history. Insect has been a source of food for many of the rural and tribal people of the most of the tropical and subtropical countries of the world (Ramos-Elorduly, 2009; Srivastava et al., 2009; Yen, 2009a, 2009b). A pictorial depiction of edible insects across different countries in the world has been recently published by Shockley and Dossey (2013). The importance of insect as human food, i.e., entomophagy has been globally accepted as one of the alternative sources to meet the nutritional requirement of most of the under-nourished children in developing and under-developed countries (Gahukar, 2011). With so many fascinating facts in its basket, insects are bound to become an integral and intricate part of ecology and are often very difficult to separate insects from ecology and ecology from insects. As rightly pointed out by Edward O. Wilson. “If all mankind were to disappear, the world would regenerate back to rich state of equilibrium that existed ten thousand years ago; if insects were to vanish, the environment would collapse into chaos.” Most of the early years of insect ecology has focused on life history strategies and their evolution, interactions with other living organisms such as herbivory, pollination, predatory–prey interaction, and life tables. The population dynamic studies such as life tables have provided valuable information on key mortality factors which forms the basis for most of pest management strategies in tropical agro

14

3 Introduction to Insect Ecology

ecosystems (Price, 1997). Not until recently the agro ecosystem services provided by insects were included within the scope of insect ecology. The first few chapters of the book deal with basic concepts of ecology and the hierarchy of biological/ecological organization and the rest of chapters are exclusively devoted to significance and importance of insect and its role in ecology starting with its significance as an individual organism, insect as species and its role in population dynamics, importance of insects in shaping up of ecosystem and their multitrophic interactions with living and non-living world as that of herbivory, food chains and food webs, predatory–prey interactions, tritrophic interactions, ecosystem services provided by insects and finally how these information can be effectively utilized for formulating environmentally sound ecological pest management strategies. Although each chapter is necessarily separate, it is important to understand that insect ecology as a series of interlocking systems. For instance, to fully understand how an insect population can be transformed from insect to pest, information on insects in relation to its host plants, natural enemies, competitors, and role of microand macroclimatic conditions is essential though they are outlined and discussed as separate chapters. Interactions of insects with other biotic and abiotic components is an integral part of population ecology as well as community ecology and by being some themes or tropics might be repeated as per the case may be.

References Brues, C. T. (1946). Insect dietary. In: An account of the food habits of insects. Cambridge: Harvard University Press, 466 pp. Cock, M. J. W., Murphy, S. T., Kairo, M. T. K., Thompson, E., Murphy, R. J., & Francis, A. W. (2016). Trends in the classical biological control of insect pests by insects: an update of the BIOCAT database. Biocontrol, 61(4), 349–363. https://doi.org/10.1007/s10526-016-9726-3 Gahukar, R. T. (2011). Entomophagy and human food security. International Journal of Tropical Insect Science, 31, 129–144. Machado, C. A., Robbins, N., Gilbert, M. T. P., & Herre, E. A. (2005). Critical review of host specificity and its coevolutionary implications in the fig/fig-wasp mutualism. Proceedings of the National Academy of Sciences, 102(Supplement 1), 6558–6565. https://doi.org/10.1073/pnas. 0501840102 Naranjo, S. E., Ellsworth, P. C., & Frisvold, G. B. (2015). Economic value of biological control in integrated pest management of managed plant systems. Annual Review of Entomology, 60, 621–645. Price, P. W. (1997). Insect ecology (3rd ed.). Wiley. Ramos-Elorduly, J. (2009). Anthropo-entomophagy: cultures, evolution and sustainability. Entomological Research, 39, 271–288. Shockley, M., & Dossey, A. (2013). Insects for human consumption. In: Mass production of beneficial organisms, pp 617–652. https://doi.org/10.1016/B978-0-12-391453-8.00018-2. Shurin, J. B., Gruner, D. S., & Hillebrand, H. (2006). All wet or dried up? Real differences between aquatic and terrestrial food webs. Proceedings of the Royal Society B: Biological Sciences, 273(1582), 1–9. https://doi.org/10.1098/rspb.2005.3377 Shusheng, H., Dilcher, D. L., Jarze, D. M., & Taylor, D. W. (2008). Early steps of angiosperm– pollinator coevolution. Proceedings of the National Academy of Sciences, 105(1), 240–245. https://doi.org/10.1073/pnas.0707989105

References

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Srivastava, S. K., Babu, N., & Pandey, H. (2009). Traditional insect bioprospecting—as human food and medicine. Indian Journal of Traditional Knowledge, 8, 485–494. Stork, N. E., McBroom, J., Gely, C., & Hamilton, A. J. (2015). New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods. PNAS, 112, 7519–7523. Strong, D. R., Lawton, Jr., J. H., & Southwood, T. R. E. (1984). Insects on plants: Community patterns and mechanisms. Oxford: Blackwell Scientific. 313 pp Waterhouse, D. F. (1974). The biological control of dung. Scientific American, 230(4), 100–109. Wyckhuys, K. A. G., Hughes, A. C., Buamas, C., Johnson, A. C., Vasseur, L., Reymondin, L., Deguine, J. P., & Sheil, D. (2019). Biological control of an agricultural pest protects tropical forests. Communications in Biology, 2, 10. Yen, A. L. (2009a). Edible insects: traditional knowledge or western phobia? Entomological Research, 39, 289–298. Yen, A. L. (2009b). Entomophagy and insect conservation: some thoughts for digestion. Insect Conservation, 13, 667–670.

4

Factors Responsible for Abundance of Insects

Abstract

Factors responsible for dominance of insects on earth have been examined by outlining insects and their structural perfections, developmental characters, protective adaptation, etc. with few important examples. Keywords

Cuticle · Hexapod locomotion · Sense organs · Trachea · Fecundity · Parthenogenesis · Morphological · Physiological · Behavioral adaptation

The following are some of the contributing factors for the abundance of insects on earth (Chapman, 1988; Pant & Ghai, 1981; Richard & Davies, 1977; Triplehom & Johnson, 2005).

4.1

Structural Perfections

1. Exoskeleton: The insect body has an outer exoskeleton or body wall made up of cuticular protein called as chitin. The following are different functions of cuticle insects: (a) Protection from desiccation or water loss from the body (b) Protection from physical or mechanical injuries (c) Maintaining the shape and size of the body (d) Providing area for muscle attachment (e) Giving strength to the body appendages such as legs for performing specific functions as digging, swimming, and preying. (f) A variety of cuticular structures are parts of mechano- and chemoreceptors and help in insect communication. # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_4

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Factors Responsible for Abundance of Insects

2. Small size: It helps the insects to exploit different ecological niches inaccessible for other animals. Insects, due to their small size, require less space (for shelter), food, and energy for their survival, and can easily escape from their natural enemies. The smallest insect species within the family, Dicopomorpha echmepterygis, is only 0.139 mm long belonging to family chalcididae of Hymenoptera. 3. Quicker speciation: Because of hard exoskeleton, smaller size, and short life cycle there is a chance of quicker species formation (a large number of species at a faster rate). Changes that occur during the process of evolution through variation in their habitat or habits will be maintained or continued to several generations resulting in the development of more species from a genus. 4. Functional wings: Two pairs of wings that are present on meso- and metathoracic segments are mainly helpful for taking flight from one place to another in search of food, shelter, or to find a mate, to oviposit or to get protection from their natural enemies. Functional wings is one of the characters that made the insects to dominate animal world on earth’s land mass. 5. Hexapod locomotion: The presence of three pairs of legs gives stability while moving by keeping the equilibrium of the body. Because of this hexapod locomotion, insects can walk any type of surface. This mechanistic hexapod locomotion is the basis for designing many robots that can be used in areas of natural calamities such as earthquakes for the purpose of tracking. 6. Compound eyes: Most of the adult insects and nymphs consist of compound eyes as visual organs which possess number of hexagonal units known as ommatidia, corresponding to the cornea of an individual eye or lens. The compound eyes are present on either side of the head capsule of an adult insect and also in the nymphs of Exopterygota. The number of ommatidia varies from 1 in the worker of ant, Ponera punctatissima to over 10,000 in the eyes of dragonflies. Because of the presence of number of ommatidia in the compound eyes, even if some or few ommatidia get damaged, the insect does not lose the power of vision. 7. Scattered sense organs: The sense organs viz., visual organs, gustatory organs, organs of touch, etc. are distributed on different parts of the body such as antennae, eyes, mouth parts in the head, legs with claws on thorax, tympanum, and cerci in the abdomen. This scatteredness on all parts of the body prevents the chance of all being damaged. 8. Decentralized nervous system: The nervous system is so decentralized that insects can be artificially stimulated to walk, fly, feed, mate, or oviposit even if some parts of the body are removed or damaged. 9. Direct respiration: Insects respire by means of thin elastic air tubes known as trachea which open outside, on the body surface through spiracles. The presence of these trachea allows free supply of oxygen to the insect and makes it to be an efficient terrestrial or aerial arthropod. In addition to trachea, insects are also employed with other respiratory organs such as tracheal gills, spiracular gills, rectal gills, and blood gills with which insects can breathe in aquatic ecosystem

4.2 Developmental Characters

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and survive effectively. Along with direct respiration, insects also respire through mechanisms such as cutaneous respiration and plastron respiration. 10. Enteronephric excretion: In insects, excretion is mainly by means of malpighian tubules which open in between midgut and hindgut. This arrangement is well suited for water conservation as well as for the absorption of unwanted waste metabolites at a quicker rate. The by-product of excretion for insects in terrestrial ecosystem is uric acid and for aquatic insects the excretory product is ammonia.

4.2

Developmental Characters

1. High fecundity: Fecundity is defined as the egg laying capacity of a female insects. It helps to increase the population at faster rate. Social insects are animals with the highest rate of fecundity in terrestrial ecosystems. It is estimated that a queen termite (Termes bellicosus) can lay about 30,000 eggs per day and can live for around 10 years through a mechanism called physogastry where the abdomen reaches maximum size to accommodate a large number of eggs. 2. Method of reproduction: Insects can reproduce both sexually and parthenogenetically. This parthenogenetic reproduction coupled with high fecundity helps insects to increase their populations to large numbers, when all the biotic and abiotic factors are favorable. One character which is very unique in some insects is called Alternation of generation: Some insects such as aphids reproduce by parthenogenesis during summer when there is availability of ample food material and during winter under conditions of food shortage and harsh climatic conditions they reproduce sexually. 3. Controlled reproduction: Though insects possess high fecundity, there is also high degree of control over reproduction by reducing the number of females that can lay eggs. This character is mostly seen in social insects such as honey bees and termites. In social insects, such as honey bees, the queen decides whether it has produced the fertilized or unfertilized eggs thus can decide the sex of the offspring. 4. Short life cycle: Most insects have very short life cycle, i.e., 2–4 weeks which help insects to complete a large number of generations in a definite period of time. Drosophila melanogaster, a common fruit fly has been used in most of the genetic studies because of its shortest life cycle, i.e., they can complete all the life stages, i.e., from egg to adult within 14–16 days. This is also one of the reasons for quicker speciation in insects. 5. Specificity of food: There is diversity in food habits among different species of insects. As they differ in their preference for a particular type of food, there will not be any competition among themselves. Less competition for food increases their chances of survival and further multiplication. It is generally said that insects feed on anything and everything that is edible and has nutrients ranging from plants, animals, excreta, dead bodies, etc.

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Factors Responsible for Abundance of Insects

6. Zenith of evolution: During the process of evolution, insects have shown a high degree of specialization to the extent that there is division of labor, polymorphism, etc. that make them to be efficient in their struggle for existence.

4.3

Protective Adaptations

For protecting themselves from adverse environmental conditions or natural enemies, insects have developed or attained some adaptations including 1. Morphological adaptations: The body color and shape of some insects make them look like part of the plant, thereby protecting themselves from natural enemies, for example, stick insects and leaf insects. The morphological adaptation can also observe in the eggs that are produced by insects. The adult insects make sure that the eggs deposited are protected from abiotic and biotic factors such natural enemies. Ootheca is a structural covering comprising proteins and tannins that harden around the eggs for their protection produced by insects such as praying mantids and cockroaches. Some insects such as whiteflies and green lacewing produce stalked eggs to escape from natural enemies. 2. Physiological adaptations: Some insects produce or release poisonous or unpleasant odors from their body or possess warning coloration by imitating certain distasteful insects, for example, stink bugs have specialized exocrine glands located in the thorax or abdomen that produce foul-smelling hydrocarbons. Larvae of swallowtail butterflies have eversible glands called osmeteria, located just behind the head when disturbed they release repellent volatile and waves their body back and forth to ward of intruders. Some blister beetles (Meloidae) produce cantharidin, a strong irritant and blistering agent that has defense functions (Carrel et al., 1993). Cantharidin is present in all the life stages of the insect. In case of larvae when disturbed they ooze as oral fluid, in adults cantharidin present in blood reflexively discharged from leg joints. Insects also have physiological adaptations to feed on plants producing toxic secondary metabolites. These insects can detoxify the toxic plant compounds in their gut and acquires the energy needed for its growth and metabolisms and such insects feeding on toxic plants are called specialized insects or adapted insects. This physiological adaptation is one among the reasons for the development of resistance in insects to synthetic pesticides causing huge economic loss to the farmers and other stake holder in forest, urban and medical entomology. 3. Behavioral adaptations: It is a defense strategy adopted by some insects through feigning death or imitating the voice of dangerous insects. For example, Colorado potato beetles when disturbed, draw their legs beneath and drop to the ground and pretend as if dead. 4. Construction of protective structures: Some insects construct shelter with the available plant material for protecting themselves from adverse conditions, natural enemies and to store food material for use during the period of scarcity. For

References

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example, cases/bags in case of case worms/bag worms, web spinners, Termatoria in case of termites, honey comb in case of honey bees.

References Carrel, J. E., McCairel, M. H., Slagle, A. J., Doom, J. P., Brill, J., & McCormick, J. P. (1993). Cantharidin production in a blister beetle. Experientia, 49(2), 171–174. Chapman, R. F. (1988). Insects: Structure and function. Cambridge University Press. Pant, N. C., & Ghai, S. (1981). Insect physiology and anatomy. ICAR. Richards, O. W., & Davies, R. G. (1977). Imm’s general text book of entomology (Vol. I and II). Chapman and Hall. Triplehom, C. A., & Johnson, N. F. (2005). Borror and De Long’s introduction to the study of insects. Thomson Brooks/Cole Publishing. Snodgrass, R. E. (2001). Principles of insect morphology. CBS Publishers & Distributors.

5

Factors Affecting Growth and Metabolism of Insects

Abstract

Factors that affect growth and metabolism is discussed in detail. The role of abiotic factors such as temperature, humidity, rainfall, light, and edaphic characters on growth, development, and the survival of insect at extreme climatic conditions is being explained. Distribution, dispersal, and migration of insects in relation to abiotic factors are dealt in brief. The effect of biotic factors such as food and association with other organisms has been outlined, and these topics were discussed in detail in the succeeding chapters. Finally, the role of selfregulation in maintaining the insect population under a normal balance or balance of nature has been examined. Keywords

Temperature · Optimum activity zone · Survival · Extreme low and high temperature · Distribution · Movement · Moisture · Adaptations · Humidity · Rainfall · Photoperiodism · Food · Associations · self-regulation · Balance of life

These factors can be grouped under three major headings as follows. However, most of the time these factors do not act alone on growth, metabolism, and multiplication of insects and there are always intricate interactions among these factors. For example, temperature can act as a limiting factor for the growth, metabolism, and multiplication of insects affecting insects directly or indirectly affecting the plants that are sources of food material to insects. 1. Climatic factors or abiotic factors 2. Biotic factors 3. Self-regulations

# The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_5

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Factors Affecting Growth and Metabolism of Insects

5.1

Effect of Abiotic Factors on Insect Population

5.1.1

Temperature

5.1.1.1 Development Temperature is the most important physical factor which determines the duration of the various stages in the insect life cycle and consequently the number of generations. Temperature acts on insects in two-fold manner as follows: 1. By acting directly on the survival and development which determine the abundance of an insect pest 2. Indirectly by affecting the growth and nutritious status of host plants which in turn affects the insect growth and development Depending on the maintenance of body temperature, animal kingdom is divided into the following: 1. Warm-Blooded Animals (Homeothermic): These animals maintain a constant body temperature within certain narrow limits irrespective of the temperature variations in the external environment. These are also called as “Endothermic animals” because they rely on internal source of heat to compensate the lost heat to cooler surroundings, for example, mammals. 2. Cold-Blooded Animals (Poikilothermic): These animals are not capable of maintaining constant body temperature. They do not have internal mechanism of temperature regulation, and therefore their body temperature varies with that of the surroundings. These are also called “Ectothermic animals” as they depend on the environment than the metabolic heat to raise their body temperature, for example, insects. 3. Socio-homeothermic Animals: These organisms maintain their body temperature slightly above the atmospheric temperature and are able to air condition their nests. They maintain their own temperature inside their colony irrespective of the temperature outside, for example, honey bees. The range of temperature within which an insect survives better and at which normal physiological activities takes place is known as “Temperature Preferendum of Preferred Temperature.. Aquatic insects generally have a relatively consistent temperature range along which the insect survives. However, most of the insects living in terrestrial climatic zone are faced with extreme temperate fluctuations. For most of terrestrial insects, rates of metabolic activity generally increase with an increase in temperature. For most of terrestrial insects of which some are notable crop pests, the influence of temperature on growth and metabolism can be placed into various zones. Vannier (1994) has given a scale called thermobiological scale that divides temperature into different zones that affect growth, metabolism, and multiplication of insects (Fig. 5.1).

5.1 Effect of Abiotic Factors on Insect Population

Permanent torpor zone maximum

25

45°C

Temporary torpor zone maximum 35°C Supra Optimal activity zone 20°C

Optimal activity zone

Infra Optimal activity zone 10°C Temporary torpor zone minimum 0°C

Developmental zero

Permanent torpor zone maximum

Fig. 5.1 Thermobiological scale

1. Optimal activity zone: The temperature between 10 and 35
C where normal growth and metabolism of most of the terrestrial insects takes place is known as optimal activity zone. This zone is again divided into “infra optimal activity zone,” i.e., between 10 and 20
C mostly for insects occupying temperate climate zones and “supra optimal activity zone,” i.e., between 20 and 35
C for most of insects occupying tropical climatic zones. 2. Temporary torpor zone minimum and maximum: Temperature between 10 and 0
C is known as temporary torpor zone minimum where the growth and metabolism of insects slowdown in descending scale and 0
C where the growth and metabolism of most of the terrestrial insects’ stop is known as “Developmental zero.” Temperature between 35 and 45
C is known as temporary torpor zone maximum where again the growth and metabolism of insect slows down in ascending scale. At temporary torpor zones, the growth and metabolism of insects slow down; however, when the insects are brought back to optimum activity zone, the insects regain their lost growth and development and hence the denotation “temporary torpor zone.” 3. Permanent torpor zone: Temperature above 45
C (in ascending scale) and 0
C (in descending scale) is known as permanent torpor zone where the growth and metabolism of the insects stop permanently and will not be regained even if they were brought back to optimal activity zone.

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Factors Affecting Growth and Metabolism of Insects

5.1.1.1.1 Survival of Insects at Low Temperature Insects living in temperate zones can tolerate temperature as low as 50
C. These insects known as freeze-tolerant insects survive by preventing the formation of ice crystals in the intracellular fluids. The presence of ice-nucleating lipids and lipoproteins in major proportion in the intracellular fluids inhibits the supercooling of the intracellular fluids. As these compounds are present in smaller quantities in the extracellular fluids, ice formation takes place in the extracellular fluids which draws water molecules from intracellular fluids thereby dehydrating the cells and lowering the freezing points of the intracellular fluids. Some insects empty their guts before the onset of freezing temperatures, thus preventing the ice formation of food particles and preparing the insect to enter in resting stage, i.e., hibernation. Certain insects such as Belgica Antarctica, a dipteran midge, can survive freezing temperature (as low as 50
C) prevailing in Antarctica by producing (25% of their body weight) crypto protectant substances such as glycerol, alanine, and trehalose in their hemolymph. 5.1.1.1.2 Survival of Insects at High Temperature Adults and larva of Scatella thermarum of order Diptera lives in the hot springs at 48
C. Australian Montane grasshopper Kosciuscola can change the body color from black during nighttimes to pale blue during daytime and thereby regulates the absorption of heat. Evaporative cooling is the most common phenomenon observed in sociohomeothermic insects such as honey bees where by the temperature of the colony can be lowered to 5–8
C of outside temperature. 5.1.1.1.3 Fog-Basking Beetles Namibian desert beetle Onymacris unguicularis (Tenebrionidae; Coleoptera) that lives in extreme hot temperature survives by literarily drinking water. The insect elytra and abdomen have small depressions by which the insect is able to collect water from early morning humid fog. During dawn, the insect stands on its head and exposes elytra and abdomen to the fog and the droplets that condense on the small depressions will be collected through furrows on the elytra that opens up near the mouth and thereby the insect literarily drinks water. 5.1.1.1.4 Behavioral Thermoregulation Terrestrial insects such as dragonflies bask on exposed areas during early morning hours which are cooler and during the warmer period of the day these hide to escape the heat. Most of the leaf-webbing insects such as Western tent caterpillar Malacosoma californicum constructs webbings to regulate temperature and heat.

5.1.1.2 Fecundity Fecundity of majority of insects will be maximum at moderately high temperatures and declines at upper and lower limits of favorable temperature. Aphids remain

5.1 Effect of Abiotic Factors on Insect Population

27

parthenogenetic under high temperature and many hours of sunshine while the opposite condition gives rise to oviparous forms.

5.1.1.3 Distribution, Dispersal, and Movement 5.1.1.3.1 Distribution Mediterranean fruit fly Ceratitis capitata cannot tolerate temperature below 10
C. Females will not oviposit when temperatures drop below 16
C and development of egg, larval, and pupal stages stop at 10
C which might be the reason why this insect could not establish in England and Northern Europe where the average temperature goes below 10
C. The distribution of this insect is restricted by dryness in south of Spain, Portugal, and Northern Africa and by cold in Northern Europe. In the 1990s, few leaf-mining insects of Gracillariidae have occurred in Central Europe including Poland. The horse-chestnut leafminer Cameraria ohridella have become a major concern on horse chestnut owing mainly to increase of temperatures besides accidental introduction (Jaworski & Hilszczanski, 2013). 5.1.1.3.2 Dispersal and Movement Most insects try to move away from unfavorable temperatures. The rice weevil Sitophilus oryzae (Linnaeus) is found in the upper layers of bins irrespective of whether the initial infestation started at the depth of the bin or at surface due to rise in temperatures, i.e., when the temperature reaches 32
C, the adults migrate to cooler upper layer. Mass flight of desert locust Schistocerca gregaria (Forskal) or migration starts at 17–22
C; however, when the temperature is between 14 and 16
C, the migration becomes occasional. At high temperature, locusts expose minimum body surfaces to sun’s rays by lying parallel to them while they expose maximum body surface to sun’s rays at low temperature lying at right angle to them.

5.1.2

Moisture

Water content in insect body varies from less than 50% to more than 90% of the total body weight. This moisture content varies from different insects and within insects’ different stages of insects. Aquatic larvae contain about 98% while insects such as Tribolium spp., Sitophilus spp. which feed on dry food material constitute about 50%. Water is generally lost through spiracles and integument. Similar to that of temperature preferendum insects also have a tendency to congregate within a narrow range of humidity known as preferred humidity. However, this range varies from insect to insect and within different stages of insects. Insects cannot afford to lose more water than they take, and hence they conserve water depending on the situation.

5.1.2.1 Adaptations to Conserve Moisture 1. Body pigments: Insects develop dark pigment in cooler areas which help to absorb more heat from sun for raising body temperature. This aids in getting rid

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2.

3. 4. 5.

6. 7.

5

Factors Affecting Growth and Metabolism of Insects

of excessive moisture from the body. Light color in desert insects helps to reflect sun’s rays and save them from excessive evaporation. Integument: Well-developed integument and fused sclerites in beetles and weevils aid in conserving body moisture. Waxy coating of integument also saves from excessive evaporation. Winglessness: Grasshoppers and crickets in arid regions have poorly developed wings and some are wingless by which the area of evaporation is reduced. Pilocity: Dense hairs on the body prevent evaporation. Form of body: Oval and compressed body of some desert beetles protects them from hot winds. Some desert insects have burrowing habit by which they go into deeper layers of soil when sufficient moisture is not available. By reabsorption of water from products of excretion. Some insects links Amsacta spp. enter into aestivation when dry conditions prevail. The fall of water content of body below a certain minimum amount proves disastrous to insects and if it is considerably above the normal (in very wet places) harmful effects like disease outbreaks are noticed in insects.

5.1.3

Humidity

Unlike in temperature, there are no definite ranges of favorable humidity to all insects. Different species and their different immature stages have their own range. Humidity affects the speed of development, fecundity, and color of insects. If water content of the insect body is high, dry air accelerates the development. Locusts sexually mature quicker and the number of eggs laid are more at 70% R.H. Rhinoceros beetle develops dark chitin in moist air and light chitin in dry air. Survival is indirectly affected by extremely high humidity conditions that favor the spread of diseases in insects.

5.1.4

Rainfall

Relative humidity is dependent on rainfall. The total amount of rainfall distribution in time influences the abundance of insects in an area. More than 12.5 cm rain during May–June results in an increase in soil moisture which is not favorable to the cutworms and hence forced to come out of the soil and fall a ready prey to their parasites and predators. On the other hand, if the rainfall is less than 10–12.5 cm during summer, cutworms remain protected in soil and there is outbreak of the pest in the next season. Hence, the outbreak of pest can be forecasted if the number of wet days (0.8 cm) during May–July is noted. If there are less than 10 wet days, there will be an increase of cutworms in the following year. If there are more than 10 wet days, there will be a decrease. Desert locust does not lay eggs and even if laid does not hatch unless soil has sufficient moisture. Rainfall also plays an important role in the movement of swarms of desert locust. Saturated condition of moisture is injurious for the development of

5.1 Effect of Abiotic Factors on Insect Population

29

spotted bollworm Earias fabia Stoll. Red pumpkin beetle Aulacophora foveicollis (Lucas) withholds eggs until it come across moist soil. Rain induces emergence of most of the insects from soil. For example, ants, termites, red hairy caterpillar, and root grub beetles emerge out from the soil after the receipt of rains.

5.1.5

Light

Sunlight is the greatest single source of energy for almost all biological systems. Light as an ecological factor has been defined as all shorter wavelengths of radiant energy up to and including the visible spectrum which is measurable. Wavelengths of visible parts of spectrum range from 4000 (Violet) to 7000 (Red) Angstroms. Light is a nonlethal factor. It helps in orientation or rhythmic behavior of insects, bioluminescence, period of occurrence, and inactivity. The different properties of light that influence organisms are illumination, photoperiod, wavelength of light rays, their direction, and degree of polarization. In insects, visible and ultraviolet light influences the following: 1. Growth, moulting, and fecundity: silkworms develop faster in light than in darkness. Late-age worms survive better in 16-h light and 8-h dark periods. However, young-age worm prefers 16-h darkness and 8-h light period. Grubs of Trogoderma also develop more rapidly in light. Moths of spotted bollworm of cotton and red hairy caterpillar lay most of their eggs during periods of darkness. 2. Other activities: Orientation of animals through directed movements by light is called phototaxis which also depends on temperature, moisture, food, and age. Green leafhopper, Nephotettix spp. is attracted to light on hot and humid evenings but is indifferent to it during dry weather. Chafer beetle and, many moths pass the day in concealment. Cockroaches hide during daytime. Dusk is the most usual time for flight and copulation of moths, for emergence of winged white ants, etc. Some insects can detect and respond to polarized light. Honeybees can perceive the degree of polarization of light in different regions of sky and use this information in combination with the other factors in determining the source of pollen and nectar. Based on daily activity cycle, insects or animals are categorized as follows: Diurnal: Insects which are active during daylight hours Nocturnal: Insects which are active at night Crepuscular: Insects which are active at dusk Matinal: Insects which are active during dawn Photoperiodism: The number of hours of light in a day length (24 h) is termed as photoperiod and the response of organisms to the photoperiod (length of the day) is known as photoperiodism. Or the response of any living organism to environmental rhythm of light and darkness is known as photoperiodism.

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Factors Affecting Growth and Metabolism of Insects

Insects living at higher latitudes frequently show a “long-day” photoperiodic response in which activity, development, and reproduction occur during the short nights of summer (long day) but an overwintering diapause occurs when autumnal night lengths exceed a critical value (Saunders, 2002). In case of the blow fly Calliphora vicina Robineau-Desvoidy a maternally operating photoperiod regulates larval diapause provided that the temperature of the larval environment falls below about 15
C. This type of summer-active or “long-day” response is dominated by the ecologically important critical night length (or day length) separating the short summer nights, inducing continuous non-diapause development from the longer autumnal nights leading to diapause. Diapause induction is dependent on the actual number of hours of photoperiod phases and duration of scotophase (a dark phase in a cycle of light and darkness). European corn borer (Ostrinia nubilalis) is a long-day insect and larval diapause is induced by naturally occurring photoperiods with scotophases of 10–14 h. 3. Oviposition: Light stimulates oviposition in mantids and inhibits in Periplaneta sp. 4. Pigmentation: In dark areas, pigmentation develops in insects, i.e., dark color develops in dark areas. Bioluminescence: Famous luminous insects are the glow-worms and fireflies. The enzyme luciferase in the presence of oxygen and adenosine triphosphate (ATP) promotes the oxidation of luciferin. This causes the production of light in insects. In most cases, females produce flash of light to attract males for mating.

5.1.6

Atmospheric Pressure

It is generally of little importance. Locust shows great excitement and abnormal activity about half an hour before the occurrence of storm when the atmospheric pressure is low. Drosophila flies stop moving when put under vacuum.

5.1.7

Wind and Air Currents

Most of the insects will not take flight when speed of wind exceeds the normal flight speed. Air currents, especially in the upper air being strong, carries many insects like aphids, white flies, scales, etc. to far-off places and is an important factor in dispersal. Air movement may also be directly responsible for death of insects. Severe wind coupled with heavy rains cause mortality and moisture evaporation from body surface of insects.

5.1.8

Water Currents

Many aquatic insects adapt themselves to either still or moving water currents. Various genera of mayflies are classified as still and rapid forms. The type of

5.2 Biotic Factors

31

gases dissolved in water current also determines the type of insect species. Caddisfly and mayfly larvae are found in water bodies with high oxygen content while mosquito larvae are found in water bodies having very less oxygen content.

5.1.9

Edaphic (Soil) Factors

Loamy soils allow digging and burrowing operation and are usually favorable for insects. Agrotis sp. lives in soil of fairly light texture in which they move around freely in response to daily or seasonal temperature and moisture changes. Wellaerated soil is prerequisite for most of the soil-inhabiting insects such as termites and white grubs.

5.2

Biotic Factors

5.2.1

Food

Each insect species has certain nutritional requirements for completion of its life cycle. Under normal conditions, there is a happy adjustment between the host and particular species of insect. But in the event of sudden increase in population, the densities of population become too high to be supported by the food available in the area. Hence, competition for food as well as space will be there. According to nutritional requirements, insects are categorized into: 1. Omnivorous: Which feed on both plants and animal, for example, wasps, cockroaches. 2. Carnivorous: Which feed on other animals as parasites and predators, for example, predators (ladybird beetles and mantids), parasites–ticks on animals. 3. Herbivorous: Which feed on living plants (crop pests) and these can again be categorized into (a) Polyphagous: Insects which feed on a wide range of cultivated and wild plants, for example, locusts, grasshoppers. (b) Monophagous: Insects which feed on single species of plants, for example, rice stem borer, mango nut weevil. (c) Oligophagous: Insects which feed on plants of one botanical family, for example, diamondback moth, cabbage butterfly. 4. Saprophagous (Scavengers): Insects which feed on decaying plants and dead organic matter, for example, Drosophila flies, house flies, scarabaeid beetles.

5.2.2

Association with Other Organisms

Include beneficial and harmful insects. Associations of individuals of the same species are known as intraspecific relations and it may be beneficial. These

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Factors Affecting Growth and Metabolism of Insects

association are viz., associations of two sexes, parental care, associations of social insects, etc. Phenomenon like overcrowding is harmful since shortage of food and space results. Disease outbreak may occur. Cannibalism may occur, for example, praying mantids, larvae of Helicoverpa, Tribolium feeds on its own eggs. Associations of individuals of different species are known as interspecific relations and these may be beneficial or harmful.

5.2.2.1 Beneficial Associations 1. Symbiosis: Interrelationship between organisms of different species which live in close union without harmful effects are known as symbiosis, each member being known as symbiont. 2. Commensalism: One insect is benefited by living on or inside another insect without injuring the other and is known as commensal and it lives on the surplus food or the waste food of its host, for example, gall-forming insects. When the commensal uses its host as a means of transport, the phenomenon is termed as phoresy. Most insect orders have members that participate in phoresy; however, the Diptera and Coleoptera form some of the most extensive phoretic associations with vertebrates, other insects, and mites. They can participate in phoresy as phoronts, as well as phoretic hosts. An interesting example of an insect as a phoretic host is the case of a common phoretic nematode, Pelodera coarctata, and its dung beetle host, Aphodius. As a dung pat deteriorates and dries, a special resistant phoretic nematode larva is produced that attaches to visiting dung beetles. The phoretic nematodes remain in a dormant state on the beetles until the beetles arrive at a fresh dung pat. Then the nematodes emerge, become active, and begin a new population of free-living nematodes, For example, a mite–beetle phoretic relationship. The mites on the head and body of the Nicrophorus beetle are likely Poecilochirus (Mesostigmata: Parasitidae), which feed on nematodes in the beetles’ nest chamber. 3. Mutualism: When both the symbionts are benefited by the association, it is known as mutualism, for example, ants and aphids, termites and flagellates. 5.2.2.2 Leaf-Cutter Ant-Fungus Garden Mutualism Leaf-cutter ants (Acromyrmex or Atta) are chief harvesters of leaf material and acts as the dominant herbivores of the neotropics. The success of leaf-cutter ants is derived largely from their mutualism with the fungus, Leucoagaricus gongylophorous, which is cultivated within the fungus gardens as the ants’ sole food source. This mutualism is frequently parasitized, however, by another fungus (Escovopsis) that can invade and overwhelm the colony. Leaf-cutter ants (specifically, Acromyrmex) have evolved a defense against Escovopsis via a second mutualism with a bacterium (Pseudonocardia) that selectively suppresses the growth of the invading fungus. The fungus garden, therefore, represents a complex, quadripartite symbiosis (ant-fungusfungus-bacterium), with major impacts on tropical food web ecology.

5.3 Self-regulation of Density

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5.2.2.2.1 Harmful Associations Those that live with the expense of other living organisms are parasites and predators. Parasite: Parasite is one which attaches itself to the body of the other organisms either externally or internally for nourishment and shelter at least for a shorter period if not for the entire life cycle. The organism which is attacked by the parasite is called host. Parasitoid: An insect parasite of arthropod is parasitic only in immature stages, destroys its host in the process of development and free living as an adult or parasitoid is an insect that feeds on the body of another insect or arthropod during the larval stage of their life cycle and adult are a free-living insect, no longer dependent on the host. Parasitization: It is the phenomenon of obtaining nourishment at the expense of the host to which the parasite is attached. Parasites can be grouped into depending on the nature of host as follows: 1. Zoophagous: That attack animals (Cattle pests) 2. Phytophagous: That attack plants (Crop pests) 3. Entomophagous: That attack insects (Parasitoids and Predators) Predators and Predatism: A predator is one, which catches and devours smaller or more helpless creatures by killing them in getting a single meal. Insect killed by predator is known as prey. Insect Predator: It is defined as the one, which is large in size, active in habits, has the capacity for swift movements, and have many structural adaptations with well-developed sense organs to catch the prey. Most of the predators remain stationary and sedentary and suddenly seize the pray when it comes within its reach, for example, antlions. Most of the general predators feed upon a large number of small insects every day and have cryptic colorations and deceptive markings, for example, praying mantids and robber flies.

5.3

Self-regulation of Density

The following are different mechanisms through which an organism self-regulates its density (a) (b) (c) (d)

Natality Dispersal Mortality Cannibalism

Living organism will multiply enormously if the environment is optimum. Different organisms multiply at different rates. Every organism has an inherent capacity

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5

Factors Affecting Growth and Metabolism of Insects

to survive, reproduce, and multiply in numbers. The extent to which a species can multiply in a given period of time if no adverse factors interfere is called its “Biotic potential” which is also known as “Maximum reproductive power.” This concept was first introduced by R.N. Chapman in 1928. The population growth of any living organism is denoted by N t þ 1 ¼ N t þ Bt  d t þ I t  E t where Nt + 1 ¼ population size at time t + 1; Nt ¼ population size at time t; B ¼ births (natality); D ¼ deaths (mortality); I ¼ immigrants; E ¼ emigrants. The population growth can also be denoted as N t ¼ N 0 ert : Nt ¼ number at time t, r ¼ biotic potential, N0 ¼ number at time 0, e ¼ base of natural log, 2.71828, and t ¼ period of time being studied. Some insects like termite queens and house flies lay a large number of eggs while others lay very few eggs. Some insects reproduce very fast. Mustard aphid has over 40 generations a year. If all survive, a single pair of house flies may produce 191, 010, 000, 000, 000, 000, 000, flies from April to August which if spread over the entire earth form a layer about 14 m deep. Similarly, a progeny of a pair of Drosophila flies produced in a year would cover the whole of Indian subcontinent and Myanmar with a solid cake of flies. Such is the biotic potential of insects when there is no interference of biotic and abiotic factors of the environment. The above-mentioned population growth is generally known as “Exponential population growth” which occurs very rarely. Most of the time the population reaches a maximum limit known as “Carrying capacity” where “environmental resistance” acts on the growing population and brings down the ever-increasing population. Carrying capacity refers to the maximum population size that a given environment can support for an indefinite period or on a sustainable basis. The limitations forced by the environment in terms of shortage of food, space, intra- and interspecific competition, role of abiotic factors collectively is known as “environmental resistance.” This environmental resistance changes the structure of population growth from exponential to logistic growth. ðK  N Þ dN ¼ rN K dt N ¼ number of individuals; dN ¼ instantaneous rate of change; r ¼ biotic potential, t ¼ time; K ¼ carrying capacity.

References

5.3.1

35

Balance of Life

In nature, there are two sets of tendencies, namely the biotic potential tending to increase the population and the environmental resistance tending to reduce the population. As such there is a constant interaction between these two opposing forces and then maintains a dynamic equilibrium known as “Balance of life.” It is evident from the above that in any case, the insects or other animals never attain the high density which they are potentially capable of doing which is because of environmental limiting factors like abiotic factors comprising mainly temperature and humidity which at too high or too low levels adversely affect insects. Natural disturbance like heavy rain, hailstorms, snow, sandstorms, dust storms, and very high wind velocity is adverse to insect life. Biotic factors, i.e., limitation of food, competition for food and space, and natural enemies act adversely depending on the density of population.

References Jaworski, T., & Hilszczanski, J. (2013). The effect of temperature and humidity changes on insects’ development their impact on forest ecosystems in the expected climate change. Forest Research Papers, 74(4), 12–21. Saunders, D. S. (2002). Insect clocks. Elsevier. Vannier, G. (1994). The thermo biological limits of some freezing tolerant insects: the supercooling and thermo-stupor points. Acta Oecologica, 15, 31–42.

6

Insect–Plant Interactions and Role of Secondary Metabolites

Abstract

The chapters open with theories of insect–plant association where several theories were discussed. A flow chart of coevolution of plants and insects from the evolution point of view is given with a special note on “hexapod gap.” The structural and functional coevolution of insects in relation to plant evolution is briefly discussed. Evolution of herbivory in insects considering plant as the source of food material to insects and several theories of insect herbivory has been outlined and discussed. Role of insects belonging to different orders, as principal pollinator agent of several terrestrial cross-pollinated plants is reviewed under “Entomophily.” A brief note on deceptive pollination in insects is also touched upon. Plants through the process of evolution has learnt to defend themselves from herbivore attack particularly insects and accordingly role of plant secondary metabolites as a defense against insect herbivore has been discussed in length. Several plant secondary metabolites, their by-products and their role as defense against insect herbivore has been discussed in length. Keywords

Coevolution theories · Coevolution of plants and insects · Monophagy · Polyphagy · Feeding strategies · Nitrogen limitation theory · Stress hypothesis · Climate release hypothesis · Plant vigor hypothesis · Entomophily · Fig-wasp association · Deceptive pollination · Terpenes · Phenols · Alkaloids · Cyanogenic glucosides · Glucosinolates · Lectins · Non-protein amino acids

# The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_6

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6.1

6

Insect–Plant Interactions and Role of Secondary Metabolites

Theories of Insect–Plant Associations

Several theories of insect–plant association have been envisaged by several workers. In this chapter we are putting forth the theories that were proposed so far and most widely accepted and debated and probably led to development of another new theory. An attempt is made to given insight of these theories to the readers nevertheless the author does not support or denounce any one particular theory. The insect–plant association can be broadly grouped into three (Menken et al., 1996) 1. Coevolution 2. Diffused coevolution 3. Sequential evolution Coevolution (Ehrlich & Raven, 1964): According to this theory, the reciprocal selection between insects and plants has induced both chemical diversification and resistance in plants and food specialization in insects. A plant species that has evolved a new chemical defense due to selection by a range of herbivores reduces herbivory leading to diversification and the selection pressure of the “new” plant species evolves adaptation in insect herbivore leading to diversification and the reciprocal evolution continues. The theory is based on two typical cases of co-speciation in order coleoptera viz., insects belonging to the genus Phyllobrotica, tribe Galerucinae of Chrysomelidae family feeding which are monophagous and exclusively on Scutellaria spp. of plants belonging to Lamiaceae and insects of genus Tetraopes of cerambycidae feeding exclusively on milkweeds. The phylogenetic studies of correlation between the host plant and insect species in both the cases had a very strong association suggesting parallel diversification or co-speciation (Miller and Wenzel, 1995). Diffuse coevolution/Guild evolution/multispecies coevolution (Thompson, 1994). This theory describes about coevolution from the context of community. As community is an interaction various species living together, evolution of one species leads to evolution in other species interacting with the former species that ultimately lead to community evolution. This theory seems to hold good if the number of species interacting in a community is limited both spatially and temporally and becomes more difficult to predict coevolution lineage if communities and the species interacting within a community is vast (Thompson, 1994). Sequential evolution (Jermy, 1976): This theory suggests that evolution in insect herbivores follows that of their host plants without affecting the evolution of plants as such. Since most of adults of lepidopteran insects such as moths and butterflies use plant surface volatiles as cue for locating and ovipositing on a suitable host plant, it is logical to think that any changes in the host plant phytochemistry does bring behavioral changes in the ovipositing adults culminating into speciation.

6.2 Coevolution of Insect Orders and Plants

6.2

39

Coevolution of Insect Orders and Plants

Table 6.1 Table 6.1 chronological Evolution of plants and insects Geographical period 2000 MYA

2000–1000 mya

Paleozoic era 510–439 mya Ordovician period

439–425 mya

425 mya Silurian period

Plants/Characters Prokaryotes (bacteria and archae) Small cells and lack mitochondria and chloroplast, DNA not enclosed in nucleus Cyanobacteria (blue green algae) that forms stromatolites were the main contributing factors for increase in oxygen concentration Endosymbiont theory formation of eukaryotes Eukaryotes are either unicellular or multicellular. The cells of eukaryotes contain nucleus enclosing chromosomes and other cell organelles such as mitochondria, ER, Golgi complex and chloroplasts

Insects/Characters

Earliest evidence of appearance of land plants mostly near the seashores where the plants were submerged but their sporangia are held above the water for dispersal and further colonization Bryophytes (mosses and liverworts) Bryophytes are seedless, non-vascular plants without any distinct stem, root, and leaves and are anchored to the land by rhizoids. Mosses and liverworts are poikilohydric, i.e., they dry when environment is dry and in wet environment become active, grow and multiply Pteridophytes (vascular plants) (ferns, horsetails, and club mosses) These are vascular plants and also known as lycophytes and are homoihydric (self water/moisture regulation). These are first true plants to develop on land. They have true roots, stem and leaves however they do not reproduce by seeds but instead by spores through sporangia (continued)

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Table 6.1 (continued) Geographical period 408–362 mya Devonian period Late Devonian period 362–290 mya Carboniferous period Late carboniferous period

280 mya Permian period

Plants/Characters Pteridophytes and progymnosperms intermediate between seedless vascular plants and seeded vascular plants) First seed bearing gymnosperms evolved Wet warm and swampy environment most congenial for spore bearing pteridophytes First conifers (gymnosperms) evolved (conifers, cycads, pine) Gymnosperms have naked seed and seeds do not develop within ovary as does by Angiosperms Most of the gymnosperms are dioecious except for conifers which are mostly monoecious Gymnosperms

The Mesozoic era 245–208 mya Gymnosperms started replacing Triassic period earlier fern forests during this period 208–145 mya Climax of gymnosperms (Most of the Jurassic period gymnosperms are majorly wind Early Jurassic pollinated requiring copious amount period pollen which comes as high Middle metabolic cost for the plants. Because Jurassic period of these reasons most of the gymnosperm plants are close to each other and occurs as a close proximity

Insects/Characters Collembolan insects that were feeding on plant juices through puncturing

Insects have acquired flight ability Ephemeroptera, Orthoptera, ancestral Hemiptera and Mecoptera Odonata

Homoptera and Heteroptera Characteristic beak like mouthparts were well suited to suck the sap from phloem which was close to the outer surface of the stem During this period there was shift from pteridophytes dominance to Gymnosperm’s dominance resulting in reduction in number of insects having chewing mouth parts such as orthoptera Evolution of Neuroptera, Mecoptera, Trichoptera, and primitive Coleoptera Primitive hymenoptera with normal mandibulate mouth parts (parasitic hymenoptera) Odonata Orthoptera Diptera Hymenoptera (primitive hymenoptera) Dermaptera, Phasmatidae, Thysanoptera Well-established plant feeding behavior of insects Hymenoptera (gall insects, fig wasps, etc.) Cecidomyiidae Primitive Lepidoptera (micropterigids caterpillar feeds on mosses and liverworts and adults feed on pollen) (continued)

6.3 Hexapod Gap

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Table 6.1 (continued) Geographical period 145–65 mya Cretaceous period

Plants/Characters Insects/Characters Rise of angiosperms and associate Monotrysia and Ditrysia of pollinator insects Lepidoptera Angiosperms have closed seed with And social insects such as Apis, ants, reserved food material for Vespidae, and Bombus overcoming adverse periods and production of nectar and colored flowers for attracting insects has resulted in less wastage of pollen and increase in success rate of pollination. Cenozoic era—Modern (from 65 mya) Angiosperms has dominated the early cretaceous period the dominance is being continued till date. Angiosperm trees have given rise to grasses and hedges (Niranjan & KhushGurdev, 1995)

6.3

Hexapod Gap

Insects appeared on earth around 385 mya (million years ago) as per evolutionary dating. The earliest insect that was discovered in the fossil records was a wingless collembolan insect. After this discovery in the fossil records, for the next 60 million years (i.e., when winged insects have started colonizing the earth) not even a single fossil record of insect was found and this huge evolutionary gap between 385 million and 325 million years ago is known was baffling many evolutionary biologists and is known as “Hexapod gap.” After this Hexapod gap, i.e., after 325 mya winged insects started dominating the tropical word and presence of wings is considered as one of the key factors responsible for abundance of insects. The earliest hypothesis that tried to decipher this mystery is proposed by Yale geochemist Robert Berner (2009). The author compared the ratio of carbon and oxygen in the ancient rocks and fossils and has simulated a model according to which the atmospheric oxygen during the initiation of hexapod gaps was below 15% (the modern atmospheric oxygen concentration is about 21%) and according to him this was responsible for either extreme decline (so little that none were found in fossil records across globe) of the insects. However, a recent study by contradicted the claim by Robert Berner (2009). By taking into account of more extensive carbon records (Schachat et al., 2018), the new report says that the dip in atmospheric oxygen during the late Devonian period) (during hexapod gap) does not fit the into the new simulated model one of the study’s coauthors, Stanford paleontologist Jonathan Payne, stated, “What this study shows is that environmental inhibition by low oxygen can be ruled out because it is not compatible with the most current data” (Than, 2018). So now the scientist is back

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to square one where the answer for hexapod gap is again intriguing and remaining as mystery as ever. Insect–plant interactions can be broadly divided into two categories. 1. Herbivory or phytophagy 2. Insect–plant mutualism (a) Pollination (b) Plant–insect food for defense relationship

6.4

Herbivory: Also Known as Phytophagy. Greek Word Phyton—Plant; Phagei—To Eat

Herbivorous insects generally recognize the food sources by tactile, olfactory cues which triggers the impulse to take first bite. This process is further proceeded by gustatory stimuli present on mouth parts that decides whether the insect has to continue to feed or not. Most of the phytophagous insects generally use visual or olfactory (odor) cues to locate a host plant either for feeding or depositing eggs. Some insects are attracted by the shape of or color of various parts of a plant species, for example, red spheres, attract adult apple maggots, white pans of water attract aphids, and bright yellow sticky traps attract whiteflies and leafhoppers. Insects are attracted by plant odors such as Green Volatiles (GV) which are byproduct of plants primary metabolism. Some phytophagous insects are also attracted by the volatiles of secondary plant compounds such as mustard oils in crucifers and cardenolides in milk weeds. These secondary plant metabolites were basically developed to deter the herbivores, however unknowingly became as a token stimulus for a certain group of insects known as “specialist” insects. These specialist insects developed ability to detoxify the plant secondary metabolites with the help of MFOs, GST etc. Phytophagy can be grouped into following types-based host plants they consume 1. Monophagous: Restricted to a single host genus. For example, YSB Scirpophaga incertulas on paddy; Mango hoppers Amritodus atkinsoni; Mango nut weevil Sternochetus mangiferae. 2. Oligophagous: Feeding on plants belonging to one particular or related family. For example, insects feeding on cruciferous plants such as Plutella xylostella; cucurbit plants Raphidopalpa foveicollis. These insects are also known as specialist insects. 3. Polyphagous: Insects feeding on more than one family or group of plants. For example, Helicoverpa armigera, Spodoptera litura, Red hairy caterpillar, army worms, Conogethes punctiferalis, Lymantria dispar. At least half of the estimated 2–ten million described species of extant insects are herbivores or phytophagous feeding on plants. Despite the richness of phytophagous

6.4 Herbivory: Also Known as Phytophagy. Greek Word Phyton—Plant; Phagei—To Eat

43

species, herbivory or phytophagy occurs only in nine orders of the 29 orders in insects they are. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Orthoptera (grasshopper) and relative forms >80% Phasmatodea—Stick insects >95% Thysanoptera—Thrips >80% Hemiptera—True bugs and soft bodies insects >80% Psocoptera—Bark lice Coleoptera—Beetles ¼ 35% Hymenoptera—Saw flies ¼ 15% Lepidoptera—Butterflies and moths >95% Diptera—flies ¼ 30%

Monophagy vs. Polyphagy Historically, it has been viewed that the majority of insect herbivores are monophagous (70% of species), but this view is largely based on insects on temperate regions. Insects such as aphids, plant hoppers, butterflies and agromyzid flies fall into this category. However recent review of insects from tropical regions revealed that the levels of monophagy may be less than it has been in temperate regions. There is an argument to the support of this view that herbivores can be monophagous locally, but on geographical scale they may be polyphagous where they specialize on different host plants in different geographical regions. For example, Oregon swallowtail butterfly Papilio oregonius—Monophagous Feeding on compositae Artemisia dracunculoides Papilio brevicunda—Oligophagous Feeding on several genera of Apiaceae Papilio zelicaon—Polyphagous Feeding on 69 plants in 32 genera in two plant families

6.4.1

Feeding Strategies of Herbivorous Insects

Insects feeding on different plants have different types of feeding strategies. They are (a) Chewers: Having biting and chewing type of mouth parts. The main orders are Lepidoptera, Orthoptera, Phasmatodea, Coleoptera, and Hymenoptera. Leaves, shoots, stem, bark, flowers, fruits, roots, etc. are the plant parts eaten. Mandibles of the insects that belong to chewers are highly sclerotized. (b) Mining and boring Mining: Insects that live in between the two epidermal layers of leaf. Damage appears as mines, tunnels, blotches, or blisters. Diptera: Liriomyza; Lepidoptera: GLM, Tuta absoluta, Coleoptera: Paddy hispa

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Boring: Very strong mandibles. For example, stem borers, stalk borers, wood borers, fruit borers, etc. (c) Sap sucking: Most of these insects have mouth parts modified into tubular structures that can penetrate up to the vascular bundles. Phloem sucking: Most of these are hemipteran insects and majority of them also act as vectors for major plant diseases. Xylem sucking: Feeds on xylem which has very low levels of nitrogen. For example, cicada, spittle bug (d) Gall forming insects: Dipteran galls–mango, paddy, sorghum, sesamum. Insects belonging to Hemiptera, Hymenoptera, Lepidoptera, and Coleoptera also produce galls. (e) Seed predators: Insects that feed on seeds and their parts.

6.4.2

Theories of Herbivore Attacks on Plant

6.4.2.1 Nitrogen Limitation Theory Nitrogen in a plant is one of many plant nutrients that are vital to herbivores. The large difference in N content of plant and insect tissues may be the major reason why less than one third of insect orders and higher taxa of terrestrial arthropods achieved the ability to feed on seed plants (Southwood, 1973). The N present in protein of insect and mite ranges from 7 to 14%. Whereas in case of plants and plant parts, it never reaches 7%. Phytophagous insects are faced with low levels of plant nutrients, and hence their success in growing and reproducing depends on their ability to efficiently ingest, digest, and metabolize N with optimum levels of leaf water (75–95%). The water content of the food has a strong effect on efficiency of conversion of ingested food (ECI) for insects. Sap feeding insects generally have higher ECI than chewing insects. 6.4.2.2 Stress Hypothesis White (1969) formulated plant stress hypothesis (PSH). According to him, plants under stress become more susceptible to herbivore attack because of reduced protein synthesis and increased availability of amino acids in the tissues. Because N2 is limited in most of the plant species under normal condition, stress-induced plants increase the availability of plant nitrogen, resulting in improved growth and reproduction of phytophagous insects, resulting in population outbreaks. It has also been postulated that insects feeding on vascular bundles with low concentration of allelochemicals were suggested to have more positive effects of water stress than free-living chewing insects. This could be due to the fact that water stress condition also increases the levels of plant allelochemicals. 6.4.2.3 The Climatic Release Hypothesis (Wellington, 1954a, 1954b) According to this theory, a series of abiotic conditions that may be favorable for herbivore survival and multiplication which at the same time unfavorable to plant species (thus inducing stress) may lead to increased insect herbivory and insect pest outbreaks.

6.5 Insect–Plant Mutualism

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6.4.2.4 Plant Vigor Hypothesis Suggested by Price (1991), plant vigor hypothesis is originated from few specific cases as (a) Gall inducing insects that require rapidly proliferating plant tissues for better performance and accordingly as tissue growth is dramatically reduced when plants experience stress, gall formers often perform very poorly on stressed plants. (b) Several reports from field scientists of more attack of insect herbivores on young, tender, and open grown forest trees rather on mature trees. (c) Variability in levels of damage caused by insect herbivores on plant species under different growth regimes. Plant species that are generally grown under high nutrient management leads to more insect herbivore attack however can compensate the damage because of high vigor (Coley, 1983). The authors also argued that plants under poor nutrient management regimes grow slowly and have to invest more on production of physical defense such as thick cuticle and chemical defense such as secondary metabolites to protect themselves from herbivores and hence become a poor host for an insect herbivore.

6.5

Insect–Plant Mutualism

6.5.1

Pollination

There is ample compelling evidence to demonstrate that there was a convergent evolution between rise of angiosperms or flowering plants and insect pollinators. Pollination syndromes involving insects have played a significant role in the evolution and radiation of the angiosperms during cretaceous period (Crepet, 1979a, 1979b; Crepet & Friis, 1987; Grimaldi, 1999) though contradicted by some (Gorelick, 2001; Percy et al., 2004). During the mid-Cretaceous period, angiosperms have evolved special floral adaptations to permit the clumping of pollen which serves as reward to visiting insects thus conversely has resulted in an increase in specialized pollinators. Hu et al. (2008) have estimated that around 86% of 29 basal extant angiosperm families are zoophilous and in particular entomophilous. The rise of angiosperms during early cretaceous and thereafter has conversely led to the development of insect pollinators as these plants were providing sufficient energyrich food materials such as nectar, pollen, and other protein supplements that fulfilled the dietary requirement of ovipositing insect adults. The general benefits of insect pollination (entomophily) over anemophily (air/wind). 1. Increase in pollination efficiency. 2. Reduction of pollen wastage. 3. Successful pollination under those conditions which are unsuitable for wind pollination. 4. Maximization number of plant species in a given area.

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6.5.1.1 Entomophily or Insect Pollinators 6.5.1.1.1 Cantharophily (Pollination by Beetles) Beetles constitute one of the oldest group of insects and were abundant during upper Jurassic or lower cretaceous period, when angiosperms started colonizing the earth. Pollen eating habit of beetles became evident during Jurassic period when gymnosperms dominated the earth. Most of the flowers that attract beetles are dull colored, few visual attractions however with strong odor and ample pollen. Most of these beetles are smooth bodied and are not well suited to carry pollen on their bodies except carrion beetles. Carrion beetles are attracted by strong rotten smell of Amorphophallus flowers of Araceae, and their mouth parts are modified to collect pollen and the presence of numerous hairs help in pollen adhering and cross pollination. 6.5.1.1.2 Mycophily (Pollination by Flies) There is a great variation of the morphological changes that occur in insects belonging to order Diptera. Wasp like syrphids have a typical short fly like proboscis with which it collects and chews pollen. Flies belonging to family Tabanidae have proboscis reaching to a length of 50 mm. Several unspecified flies are attracted by plants having primitive blossoms with open nectar and short tubes such as umbellifers. The male sexes of blood-sucking dipterans are frequent visitors of plant blossoms. The females of these insects feed on blood (protein rich) needed for the development of gonads and deposition of eggs. However, males of these insects frequently visit small flowers and feed on carbohydrate-rich nectar as the source of energy for survival. Sapromyophilus Flowers (Aristolochia, Arum, and Stapelia spp.) that are pollinated by flies mostly have brown- or dark-colored flowers and emit a strong pungent odor of rotten flesh that attracts the flies for oviposition though the resulting progeny may not survive because of the absence of necessary food material. These flies are generally termed as dung flies or carrion flies. 6.5.1.1.3 Pollination by Hymenopteran Insects The mouth parts of most of hymenopterans excluding ants have a lapping tongue of about 1–3 mm that is used for lapping the nectar. The flowers visited by the wasps and bees have blossoms with open nectar and extra floral nectarines. Bees can be trained to distinguish two to three colors whereas wasps can be trained to distinguish between odors. 6.5.1.1.4 Myrmecophily (Pollination by Ants) Ants are fond of carbohydrate-rich food material and also protein-rich food material as they also rear broom or colony and hence also feed on pollen as a source of protein. Because of gregarious and aggressive nature of ants, they are often called ant

6.5 Insect–Plant Mutualism

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guards as they drive away unwanted visitors of flowers. The presence of ants crawling around blossoms may lead to autogamy or geitonogamy. True myrmecophily occurs in ants living in desert ecosystems. The plants belonging to family Euphorbiaceae that dominate deserts have extra floral nectaries and flowers bloom only for a shorter period of time. Desert ants frequently visit these plants for this extra floral nectar and help in cross pollination (Honeypot ant Myrmecocystus mexicanus). 6.5.1.1.5 Melittophily (Pollination by Bees) Honey bees by way of their structural and behavioral adaptations by far the most successful pollinators compared to all other insects. They collect the pollen by legs and are called podilegid. Bee-pollinated flowers tend to have very attractive, colorful, and fragrant flowers. Most of these flowers have nectar guides which function to attract and orient bees towards the flowers; in some cases, these nectar guides also become landing sites for the bees and improves efficiency of pollination. Solitary bees of Prosopididae have short mouth parts and eat pollen directly and regurgitate it for food. The leaf cutter bees of Megachilidae are gastrologic, i.e., they collect pollen on the ventral side of the body. Bumble bees with their hairy body and long proboscis makes a very good candidature as pollinator. 6.5.1.1.6 Psychophily (Pollination by Butterflies) Butterfly-pollinated flowers have attractive colors and fragrance with no nectar guides. However, they are equipped with long nectar-filled tubes or spurs that only insects with long tubular proboscis can access thus aiding in pollination. 6.5.1.1.7 Phalaenophily (Pollination by Moths) Flowers that are pollinated by moths mostly open during nighttime as most of the moths are nocturnal and have large varyingly colored fragrant flowers. Similar to butterfly-pollinated flowers, these flowers also have nectar tubes or spurs. Some of the plants are exclusively moth pollinated as observed in case of plant species of Yucca and relatives (Agavaceae), which are exclusively pollinated by yucca moths (Parategeticula and Tegeticula spp.). Yucca moths, besides pollinating Yucca flowers, also deposit their eggs within the ovary of Yucca plant species and have an obligatory interdependent interaction.

6.5.1.2 Fig–Wasp Association Fig plants have closed inflorescence, the syconium, having both male and female floral parts which is site of fig–wasp interaction. Fig wasps belonging to family Agaonidae of Hymenoptera have obligate correlation with fig plants for their survival. The pollen-filled female wasps enter receptive syconia through a narrow tunnel called the ostiole and then pollinate the female flowers, during which they also oviposit into some fig ovaries which are later modified as gall for the development of wasp larvae. The ovaries that have been pollinated by the pollen develops into seeds during which time the wasp larvae develops within the galls and complete

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their life cycle. Male wasps hatch first and use their extensible genitalia to mate with females, who are still within their own galls. During which time some of male flowers also mature and produce pollens so that the mated female wasps acquire pollen before dispersal. The wingless male wasps bite holes through the syconium wall, enabling the females to disperse and search for new, receptive syconia, and the male wasp dies within syconia thereafter (Cook & Rasplus, 2003).

6.5.1.3 Deceptive Pollination Deceptive pollination systems or syndromes attract pollinators without providing rewards such as nectar (Jersáková et al., 2006). Pollinators may be attracted by signals that mimic resources such as food or mates (Schaefer & Ruxton, 2009; Gaskett, 2013). The following are different types of deceptive pollination techniques adapted by plants: 6.5.1.3.1 Brood-Site Deception Plants that employ this type of deception have characters that attract the pollinators for oviposition or deposition of eggs. Brood-site deceptive pollination relies on insects such as Diptera and Coleoptera (carrion flies and beetles) that usually oviposit in decaying organic matter (Urru et al., 2010, 2011). Pollinators are lured with signals such as odors that mimic oviposition substrates. Some orchids such as Satyrium pumilum attracts carrion flies by mimicking the smell of rotten flesh as food source and frequent visits by these flies help in pollination. Carrion beetles are attracted by strong rotten smell of Amorphophallus flowers of Araceae, and their mouth parts are modified to collect pollen and the presence of numerous hairs help in pollen adhering and cross pollination. Flowers (Aristolochia, Arum, and Stapelia spp.) that are pollinated by flies mostly have brown or dark colored flowers and emit a strong pungent odor of rotten flesh that attracts the flies for oviposition though the resulting progeny may not survive because of absence of necessary food material. These flies are generally termed as dung flies or carrion flies. Dracula orchids (Epidendreae: Pleurothallidinae) have flowers that are blood red color and emit fungal aromas and resemble mushrooms (Kaiser, 2006; Dentinger & Roy, 2010; Endara et al. (2010). Two orchid species, Dracula felix and Dracula lafleurrii, are predominantly visited by drosophilid flies that usually reproduce in fungi (Endara et al., 2010) and thus help in cross pollination. 6.5.1.3.2 Sexual Deception Hammer orchids, i.e., genus Drakaea of family Orchidaceae release pheromones in the air that mimics the female pheromone of thynnid wasp of family Thynnidae of Hymenoptera. The male wasps are attracted by these scents and try to copulate with the flowers during which the pollen gets attached to their body and when the same wasps visit another flower and during the process of pseudo copulation distribute the pollen and thus help in pollination. Plants belonging to family Orchidaceae majorly employ deceptive pollination syndromes. This group of plant represents a divergent view of coevolution of plants

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and insect pollinators. The evolution between orchid plants and pollinators represents a unilateral evolution in plants with no reciprocal or coevolution in pollinators. These plants have a unique adaptation of not wasting biologically expensive pollen or nectar and at the same time fulfilling the function of cross pollination with various deceptive techniques already evolved in plants that have been proved worthy of cross pollination. For more details on deceptive pollination in orchids, the reader can refer to Jersáková et al. (2006); Gaskett (2011, 2013).

6.5.2

Plants as Food and Source of Secondary Metabolites (as Defense) for Insect Herbivores

At least half of the estimated 2–10 million described species of extant insects are herbivores or phytophagous feeding on plants. Despite the richness of phytophagous species, herbivory or phytophagy occurs only in nine orders of the 29 orders in insects which indicate that being phytophagous may not be the most preferred life history or feeding strategy of insects. This “not most preferred” strategy probably stems from. 1. The fact that most terrestrial plants are nutritionally poor. Plant tissue contains lower concentrations of nitrogen than insect tissue and obtaining sufficient nitrogen appears to be a fundamental problem for insect herbivores (McNeill & Southwood, 1978; Mattson, 1980) as envisaged by nitrogen limitation theory of Southwood, 1973. These primary differences are reflected in the low assimilation and growth efficiencies of phytophagous insects compared with those of predatory insects (2–38% for phytophages vs. 38–51% for predators; Southwood, 1973). 2. Insect herbivores have to overcome the physical and chemical defenses employed by different group of plants to protect them from general herbivores and insects in particular. This has been outlined in sequential theory of coevolution between plants and insect herbivores. Green plants convert carbon dioxide and water soil nutrients in the presence of sunlight to various metabolic products such as sugars, amino acids, proteins, and lipids, which are known as primary metabolites. These primary metabolites are utilized by the plants for growth and various physiological functions. Green plants also synthesize compounds known as secondary metabolites which are mostly the by-products of primary metabolites. However, the production or synthesis of secondary metabolites comes at a high cost for the plants. These secondary metabolites play a major role as plant defense against herbivores in general and insects in particulars (Bigger herbivores always have chance to change the food preference between preferred and not preferred host plants. However, insect herbivore owing to their small size and less dispersal have a very narrow range of food preference called “niche”).

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Owing to their high metabolic cost, most of these secondary metabolites are produced in a dynamic way after the insect herbivore damage via various signaling pathways and degrade at a rather very quick rate. In addition to defense role, these secondary metabolites also involved in various ecological interactions between plants, insects, and environment. Functions of Secondary Metabolites 1. They protect plants against herbivores and plant pathogens. 2. They serve as attractants or deterrents to herbivores. 3. They also act as agents of plant–plant competition and plant–microbe symbiosis. Secondary metabolites can be divided into three main chemically distinct groups.

6.5.2.1 Terpenes The generic name “terpene” was originally applied to the hydrocarbons found in turpentine, the suffix “ene” indicating the presence of olefinic bounds. Terpenes or Terpenoids (which are also known as isoprenoids) are terpenes with an oxygen moiety and additional structural rearrangements (these two terms used interchangeably) are generally derived from a union of 5-carbon elements called C5 units or isoprene unit (CH2¼C-CH3-CH¼CH2). The basic structural element of terpenes is also called Isoprene units because terpenes at high temperature give rise to isoprene. The insecticidal activity of terpenes is due to their action as antifeedants or deterrents or toxins or as modifiers of insect development, for example, sterols and phytoecdysone. 6.5.2.1.1 Monoterpenes(C10) In gymnosperms such as pine and fir, monoterpenes accumulate in resin ducts found in needles, twigs, and trunks, mainly α-pinene, β-pinene, limonene (Plants of Rutaceae (also called citrus fruits) and myrecene (Verbenaceae), which are toxic to insects such as bark beetles of Scolytidae and other insect herbivores. Natural pyrethroids in angiosperms from chrysanthemum flowers. Both natural and synthetic pyrethroids are important insecticides used for management of domestic as well as crop pests. Another monoterpene citronella acts as feeding deterrant for several leafhoppers (Saxena and Basit, 1982). Monoterpenes such as menthol from peppermint has several medicinal properties. 6.5.2.1.2 Sesquiterpenes (C15) These are aliphatic or cyclic isoprenoid C15 compounds. Sesquiterpene Lactones

Most widely distributed in the glandular trichomes and latex ducts of Asteraceae of Compositae family. These chemicals are highly toxic to several lepidopteran caterpillars, flour beetles, and grasshoppers (Prasifka et al., 2015). ABA (Abscisic acid) which is a sesquiterpene that plays a primary role in the initiation and maintenance of seed and bud dormancy. Defense system in plants

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against UV–B is activated in which ABA acts as downstream in the signaling pathways. In plants, ABA has a role in mediating induced plant resistance which is elicited either by insect herbivore of pathogen damage. Gossypol is another phenolic cadinene-type sesquiterpene present in the pigment glands of cotton plants. When these gland cells are removed from cotton plants, the plants become more susceptible to insect pest attack. Gossypol has proved to be toxic and causes antibiosis effect to a number of lepidopteran caterpillars such as Heliothis virescens and Spodoptera littoralis. 6.5.2.1.3 Phytojuvenile Hormone Plants are also the source of several juvenile hormone analogs that are originally synthesized in insects for regulating growth and development. Juvabione, historically known as the “paper factor,” has been found in the wood of balsam fir tree Abies balsamea (Slama & Williams, 1965). Juvabione is a phyto-sesquiterpene analogous insect juvenile hormone because it has the ability to mimic juvenile activity and affects insect reproduction and growth. This phytohormone plays an important role in conifers as the second line of defense against insect herbivores. Several phytojuvenoids such as sesamin, juvocimene, juvadecene have been isolated from several plant species and paved the way for the development of synthetic insecticides affecting growth and development of insects. Phytohormones such as precocene I and II isolated from Compositae plant Ageratum houstonianum have proved to have antijuvenile properties in insects (Bowers, 1991). 6.5.2.1.4 Diterpenes (C20) Diterpenes comprise four C5 isoprenoid units leading to C20 compound. These diterpenes are non-volatile and are found as resin ducts of higher plants. Abietic acid is a diterpene found in coniferous pines and leguminous trees. It is present in or along with resins in resin canals of the tree trunk. When these canals are pierced by sucking insects, the outflow of resin may physically block feeding and serve as chemical and physical deterrent. 6.5.2.1.5 Triterpenes (C30) Triterpenoids are composed of six C5 isoprene units. Some of the major triterpenoids in plants associated with defense are cucurbitacins, limonoids, and saponins. Cucurbitacins are a group of 20 tetracyclic triterpenoids isolated from plants of family cucurbitaceae. Cucurbitacin acts as toxic compound for most of the insect and mite pests such as leaf beetles, stem borers, and red spider mites but acts as phagostimulant to cucumber beetle (Balkema-Boomstra et al., 2003). These chemicals are either constitutive or induced by insect herbivore damage have been shown to have exert acute and sublethal toxicity, as well as deterrents effects for feeding and oviposition in insects (Agrawal et al., 1999; Tallamy et al., 1997). Citrus plant (Rutaceae)–bitter tasting liminoid having antiherbivory properties and deters feeding of leaf-cutting ants. Azadirachta indica is known to have

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antiherbivory (i.e., insecticidal antifeedant) activities and also acts as phytoecdysone having effect on insect moulting. 6.5.2.1.6 Saponins The presence of both terpene and sugar element in one molecule gives saponins the foamy properties of detergent. Saponins have been shown to have toxic effect to several species of mites, lepidopterans, and beetles. Tea saponin extracts from Camellia oleifera seeds have shown contact and stomach toxicity for the lepidopteran pest of tea plantation, Ectropis oblique (Cui et al., 2019). 6.5.2.1.7 Tetraterpenoids Tetraterpenoids consist of eight isoprene units and have the molecular formula C40H64. The most common tetraterpenoids are carotenoids, which are natural fat-soluble pigments. 6.5.2.1.8 Polyterpenes Polyterpenoids are polymeric isoprenoid hydrocarbons, which consist of more than eight isoprene units. These chemicals are naturally found in latex ducts of rubber plants and known to possess several insecticidal properties. For example, carotenoids and rubber known to possess antiherbivory properties.

6.5.2.2 Phenols The term, phenolics, is used to describe a group of structurally diverse plant secondary metabolites. Phenolics includes metabolites derived from the condensation of acetate units (e.g., terpenoids), those produced by the modification of aromatic amino acids (e.g., phenylpropanoids; cinnamic acids, lignin precursors, hydroxybenzoic acids, catechols, and coumarins), flavonoids, isoflavonoids, and tannins. Several thousand polyphenolic compounds are found in plants, synthesized via the shikimic acid-derived phenylpropanoid and/or polyketide pathways. They have a basic structure consisting of benzene ring with a hydroxyl group attached, without any nitrogen-based functional group. Polyphenols not only contribute to the flavor, color, odor, astringency, oxidative stability, and bitterness of different plant parts, but also play a critical role as plant chemical defenses (Singh et al., 2021). Grain aphid (Sitobion avenae F.) infestation in winter triticale (Triticosecale Wittm) seedlings induces bioactive compounds such as phenolic acids providing resistance. Kariyat et al. (2019) showed that 3-deoxyanthocynadin (flavonoids) present in wild type sorghum (Sorghum bicolor (L.) Moench Family: Gramineae) caused significantly higher mortality and reduced population growth in corn leaf aphid (Rhopalosiphum maidis Fitch), when compared to null mutants devoid of them. Chlorogenic acids in chrysanthemum (Dendranthema grandiflorum (Ramat.) effectively defend against thrips, pisatin (flavonoid) deters pea-aphid (Acyrthosiphon pisum Harris) in pea, and ferulic acid in rice imparts resistance against brown planthopper (Nilaparvata lugens Stål.). There is a strong correlation between the constitutive concentrations of catechol-based phenolics in strawberry leaves and

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resistance to the two-spotted spider mite (Tetranychus urticae) (Luczynski et al., 1990). 6.5.2.2.1 Coumarin Coumarin derivatives can be classified as simple coumarins, furocoumarins, dihydrofurocoumarins, pyranocoumarins (linear and angular), phenylcoumarins, and biscoumarins (Venugopala et al., 2013; Borges et al., 2010). Most of the coumarins exhibit antifeedant properties against Spodoptera littoralis, Myzus persicae, and Rhopalosiphum padi (Barrero et al., 2013). Furocoumarins are abundant in members of family Umbelliferae. Normally, these compounds are not toxic but when activated by UV-A light become toxic to most of the fungal diseases. 6.5.2.2.2 Lignin Lignin is an integral part of cell wall of vascular plants. Lignification gives mechanical strength to plants and at the same time also helps to resist insect herbivore and pathogen attack. Lignins act as physical defense by toughness and deters herbivore feeding and is also relatively indigestible to insects and pathogens. Most of the lignins reduce the digestibility of insect herbivores. 6.5.2.2.3 Flavonoids and Anthocyanins Flavonoids are aromatic compounds that absorb UV light and some visible light and hence are brightly colored, for example, Anthocyanins. Some flavonoids are colorless such as isoflavonoids. Flower colors rich in flavonoids promote flower pollination and fruit and seed dispersal, mainly perform the function of pigmentation and defense. The pigmentation in flowers, young leaves, etc. protect them from UV radiation. 6.5.2.2.4 Tannins Tannins are compounds with an astringent taste. Tannins are protein-binding agents. Tannins play a significant role as defense against herbivores. The most probable mode of action of tannins is coagulation of mucoproteins in the alimentary canal of insect herbivores. These are general toxins that significantly reduce the growth and development of many insect herbivores and also act as feeding deterrent to most herbivores. The defensive properties of tannins are generally attributed to their ability to bind to proteins. The role of tannins imparting resistance to the larvae of the Winter moth Operophtera brumata L., Geometridae was reported by Feeny (1970). 6.5.2.2.5 Nitrogen Containing Secondary Metabolites They include alkaloids, cyanogenic glucosides, and non-protein amino acids most of which are biosynthesized from common amino acids.

6.5.2.3 Alkaloids A large family of N containing secondary metabolites such as alkaloids are found in 20% of vascular plants such as herbaceous dicots. The alkaloids are present in low

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quantities in monocots and gymnosperms. Common alkaloid containing plants are from leguminaceae, liliaceae, solanaceae, and amaryllidaceae. Alkaloids have been divided into three major classes as follows: 1. True alkaloids which are derived from amino acids, basic in nature and contains nitrogen in the heterocyclic ring. For example, nicotine present in tobacco and atropine, cocaine, morphine, quinine, etc. 2. Pseudo alkaloids are also basic in nature but are not derived from amino acids. For example, caffeine—seeds and leaves of cocoa coffee, tea. 3. The protoalkaloids are derived from amino acids, are basic but the nitrogen is not in a heterocycle, for example, the phenylethylamine-derived alkaloids such as mescaline. Common potato species contains alkaloids such as solanine, solanidine, and demissine that show resistance to Colorado potato beetle and leafhoppers (Harborne, 1993a, 1993b). Alkaloids such as quinolizidine act as feeding deterrents against a number of herbivores including insects, molluscs, and mammals (Petterson et al., 1991). The common potato (Solanum tuberosum) contains the alkaloid solanine, which has no effect on Colorado beetle Leptinotarsa decemlineata. However, alkaloid demissine found in another species of potato Solanum demissum showed a resistant reaction to the same insect pest. It has been suggested that Colorado beetle was able to detoxify alkaloid solanine but not demissine. Increasing concentrations of the alkaloids solanidine in various cultivars were positively correlated with a reduction in leafhopper infestation (Sanford et al., 1990). Barley contains alkaloids gramine in the leaves that are associated with resistance to aphids Schizaphis graminum (kanehisa et al., 1990). Some insects detoxify these toxic compounds or store them on their bodies and use them as defense against their natural enemies as observed in Utetheisa ornatrix (ornate moth) where the insects detoxify the Pyrrolizidine alkaloids and store them in their bodies as defense against a lacewing predator Ceraeochrysa cubana (Eisner et al., 2000).

6.5.2.4 Cyanogenic Glucosides These compounds are ubiquitously present in gymnosperms and angiosperms including monocots and dicots. The basic structure of glucoside is R-CH-CN which is derived from an aromatic or branched chain amino acid such as tyrosine (dhurrin in Sorghum bicolor), valine and isoleucine (linamarin and lotaustralin) in Lotus japonicus (lotus) and Manihot esculenta (cassava), and phenylalanine (amygdalin and prunasin in Rosaceae, including apples, plums, cherries, peaches, and strawberries). These compounds are widely distributed in nature both in monocots such as sorghum and barley and dicots such as cassava and clover. Glucosides are generally present in vacuoles and when tissues are broken down these glucosides are exposed to enzymes such as b-glucosidases and hydroxyl nitrile lyases which hydrolyze the glucosides leading to release of cyanide which is a respiratory poison. Presence of these glucosides gives protection for these plants

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from herbivores (Zagrobelny et al., 2004). Studies on insect behavior have shown that these cyanogenic glucosides can act as either feeding stimulant or feeding deterrent for specialist and generalist insects as the case may be. Presence of these compounds in crop plants have detrimental effect on both animals and human beings. Cassava which is a staple food for many of the developing countries can be stored for a longer period without much damage from insect herbivores and pathogens owing to the presence of the cyanogenic glucosides. People have learned methods to overcome these toxic compounds by various means of food processing that makes cassava edible and used as a staple food. Both plants and animals including some specialized insects have developed mechanisms to breakdown these cyanides into non-toxic compounds. The enzyme “Rhodanese” converts cyanide and thiosulfate to sulfate and thiocyanate which is less toxic. Several contrasting reviews and opinions exist on the role of cyanogenic glucosides as defense against insect herbivores, to name a few. Larvae of the southern army worm, Spodoptera eridania, preferred to graze on cyanogenic glucoside-containing plants and grew better when cyanide is present in their diet (Brattsen et al., 1983). Contrast with earlier report, Bellotti and Arias (1993) reported that the presence of cyanogenic glucosides improved the resistance of cassava tubers to the cassava root bug Cyrtomenus bergi Froeschner.

6.5.3

Sulfur Containing Secondary Metabolites

6.5.3.1 Glucosinolates Glucosinolates are sulfur- and nitrogen-containing compounds found in plants of Brassicaceae and Capparales. In intact plants, GLC are located in a separate compartment away from a specific thioglucosidase enzyme, i.e., myrosinase which is present in a separate myrosin cell. When plant cells are disrupted either by herbivore damage or mechanical damage, the enzyme myrosinase comes in contact with GLC and hydrolyze them to produce a variety of volatile products such as isothiocyanates. These breakdown products are also known as “mustard oils” and are responsible for the pungent taste and odor of the cruciferous vegetables. There are reports of significant negative correlation between GLC content of 27 plant species and fecundity of aphids such as Lipaphis erysimi (Malik et al., 1983; Singh et al., 2003). However, some of the specialized insects such as flea beetle and cabbage seed weevil Ceutorhynchus assimilis have clearly turned GLC defense mechanism against the plants, using the presence of defense compounds to identify and locate the host plant (Bartlet et al., 1992). Parasitoids of the specialist feeders are also attracted by volatiles such as isothiocyanates which are released by these plants when attacked by insect herbivore. GLC in oilseed rape is dynamic, both the amount and type of GLC vary with plant age, plant tissue, and development status. In leaves of cruciferous plants, GLC content is high in young, developed tissues reaching its maximum in fully opened

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leaves and then declines later. Many studies also reported that GLC can be induced by factors such as herbivore attack and mechanical damage.

6.5.3.2 Lectins and Phytoalexins 6.5.3.2.1 Lectins Lectins are proteins produced by living organisms that react with sugar residues. Most lectins have two or four subunits and each unit has a specific sugar-binding site. This property makes lectins agglutinate or precipitate glycoprotein. More than 600 species of Leguminaceae contains lectins mostly in the seed cotyledons. Most of the lectins are involved in resistance to insect herbivore (Macedo et al., 2015). Lectins from seeds of kidney bean Phaseolus vulgaris caused lethal mortality to the larvae of bruchid beetle Callosobruchus maculatus (Gatehouse et al., 1984). Some plants such as snowdrop produce lectins that act as defense proteins and bind to carbohydrate or carbohydrate-containing proteins. After ingestion by insects, lectins bind to epithelial cells in the midgut and interfere with nutrient absorption. 6.5.3.2.2 Phytoalexins First identified in potato after the infection of Phytophthora infestans. These are secondary metabolites produced or induced as response to fungal or bacterial infection in plants (Muller and Borger (1940)). Production of these compounds is localized at the site of infection and is induced more rapidly in resistant genotypes compared to susceptible genotypes. Some of the examples of phytoalexins that are induced in plants against the infection of plant pathogens are Viniferin, Dianthalexin, Pisatin, Phaseollin, Maakiain, Scoparone, Avenalumins, etc.

6.5.3.3 Non-protein Amino Acids Many plants in particular legumes also contain unusual amino acids called non-protein amino acids that are not incorporated into proteins but are present in free form and act as defensive substances (Rosenthal, 1991). Canavanine is a widely distributed analog to arginine found in both trees and forage legumes. (Arginine is an alpha amino acid that is used in the biosynthesis of proteins. Arginine plays an important role in cell division, healing of wounds, release of hormones, and removal of ammonia from the body.) Both canavanine and its break down product canaline are effective substrates for enzymes which utilize arginine and therein lies their toxic effect (Rosenthal, 2001). The arginyl-tRNA synthetase of most organisms cannot distinguish between arginine and canavanine, resulting in incorporation of canavanine into proteins, which leads to deleterious effects. However, some insects, such as Caryedes brasiliensis (bruchid beetle) and Sternechus tuberculatus (curculionid weevil) have an arginyltRNA able to distinguish between protein and non-protein amino acids. Additional examples are covered in the recent review by Huang et al. (2011). For more details into plant secondary metabolites and their role in herbivory, the reader is requested to go through some excellent reviews provided by Bennett and

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Wallsgrove (1994); Bernay (1998); Yactayo-Chang et al. (2020); Rosenthal (1979); Rosenthal and Berenbaum (2012).

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Dentinger, B. T. M., & Roy, B. A. (2010). A mushroom by any other name would smell as sweet: Dracula orchids. McIlvainea 19:1–13. Ehrlich, P. R., & Raven, P. H. (1964). Butterflies and plants: A study in coevolution. Evolution, 18, 586–608. https://doi.org/10.1111/j.1558-5646.1964.tb01674.x Eisner, T., Eisner, M., Rossini, C., Iyengar, V. K., Roach, B. L., Benedikt, E., & Meinwald, J. (2000). Chemical defense against predation in an insect egg. Proceedings of National Academy of Sciences United States of America, 97, 1634–1639. Endara, L., Grimaldi, D. A., & Roy, B. A. (2010). Lord of the flies: pollination of Dracula orchids. Lankesteriana, 10, 1–11. Feeny, P. P. (1970). Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars. Ecology, 51, 565–581. Gaskett, A. C. (2011). Orchid pollination by sexual deception: pollinator perspectives. Biological Reviews, 86, 3375. Gaskett, A. C. (2013). Colour and sexual deception in orchids: progress towards understanding the functions and pollinator perception of floral colour. In P. Bernhardt & R. Meyer (Eds.), Darwin’s orchids: Then and now. University of Chicago Press. Gatehouse, A. M., Dewey, F. M., Dove, J., Fenton, K. A., & Pusztai, A. (1984). Effect of seed lectins from Phaseolus vulgaris on the development of larvae of Callosobruchus maculatus; mechanism of toxicity. Journal of the Science of Food and Agriculture, 35(4), 373–380. Gorelick, R. (2001). Did insect pollination cause increased seed plant diversity? Biological Journal of the Linnean Society, 74, 407–427. Grimaldi, D. (1999). The co-radiations of pollinating insects and angiosperms in the Cretaceous. Annals of the Missouri Botanical Garden, 86, 373–406. Harborne, J. B. (1993a). Phytoalexins and phytotoxins. In Introduction to ecological biochemistry|| Higher plant–lower plant interactions (pp. 264–297). Academic Press. https://doi.org/10.1016/ B978-0-08-091858-7.50014-0 Harborne, J. B. (1993b). Introduction to ecological biochemistry (4th ed.). Academic Press. Hu, S., Dilcher, D. L., Jarzen, D. M., & Taylor, D. W. (2008). Early steps of angiosperm-pollinator coevolution. Proceedings of the National Academy of Sciences United States of America, 105, 240–245. Huang, T., Jander, G., & de Vos, M. (2011). Non-protein amino acids in plant defense against insect herbivores: Representative cases and opportunities for further functional analysis. Phytochemistry, 72, 1531–1537. Jermy, T. (1976). Insect–host-plant relationship—Co-evolution or sequential evolution. In The host-plant in relation to insect behaviour and reproduction (pp. 109–113). Springer. Jersáková, J., Johnson, S. D., & Kindlmann, P. (2006). Mechanisms and evolution of deceptive pollination in orchids. Biological Reviews, 81, 219–235. Kaiser, R. (2006). Flowers and fungi use scents to mimic each other. Science, 311, 806–807. Kanehisa, K., Tsumuki, H., Kawada, K., & Rustumani, M. (1990). Relations of gramine contents and aphid populations on barley lines. Applied Entomology and Zoology, 25(2), 251–259. Kariyat, R. R., Gaffoor, I., Sattar, S., Dixon, C. W., Frock, N., Moen, J., Moraes, C. M. D., Mescher, M. C., Thompson, G. A., & Chopra, S. (2019). Sorghum 3-deoxyanthocyanidin flavonoids confer resistance against Corn Leaf Aphid. Journal of Chemical Ecology, 45, 502–514. Luczynski, A., Isman, M. B., & Raworth, D. A. (1990). Strawberry foliar phenolics and their relationship to development of the two spotted spider mite. Journal of Economic Entomology, 83, 557–563. Macedo, M. L. R., Oliveira, C. F., & Oliveira, C. T. (2015). Insecticidal activity of plant lectins and potential application in crop protection. Molecules, 20(2), 2014–2033. Malik, R. S., Anand, I. J., & Srinivasachar, D. (1983). Effects of glucosinolates in relation to aphid (Lipaphis erysimi) fecundity in crucifers. International Journal of Tropical Agriculture, 1, 273–278.

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Abstract

This chapter gives an insight to chemical ecology of insects and its role in insect– plant interactions. Chemicals produced by insects can be broadly classified as semiochemicals and pheromones. How these pheromones viz., sex, aggregate, trail, host marking, and others affect the behavior, the emitter, and receiver and their benefit of advantage to the organisms involved have been outlined. Leks and its significance as intraspecific communication between individuals of same species has been dealt in brief. The role of these pheromones in Integrated Pest Management is discussed. The role of semiochemicals in communication with herbivores in particular insect herbivore has been dealt in detail. How these semiochemicals individually and in their combination with pheromones can be utilized in a better, environment-friendly Insect Pest Management strategies are discussed. Keywords

Sex pheromones · Aggregate pheromones · Leks · Epideictic pheromone · Alarm pheromone · Host marking pheromone · Trials pheromones · Allomone · Kairomone · Synomone · IPM · Monitoring · Mating disruption · Mass trapping · Push and pull strategy

7.1

Introduction

Chemical ecology has been defined as “the promotion of an ecological understanding of the origin, function and significance of natural chemicals that mediate interactions within and between organisms” (Harborne, 2001). Discovery of pheromones and kairomones has paved the way for the development of this field of chemical ecology. # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_7

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7.2

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Insect Chemical Ecology

Insect chemical ecology can be studied from two different perspective. 1. How do insects defend themselves against array of plant secondary metabolites that are toxic to insect herbivores (mostly ditrophic interaction)? 2. How do insects and plants communicate among themselves and between them (mostly tri trophic or multitrophic interactions)—semiochemical-based interaction? Ditrophic interaction, i.e., insect defense against plant secondary metabolite has already been discussed under Chap. 6 and here we focus more on semiochemicalmediated interactions between insects and plants and among insects.

7.3

Semiochemicals

Insects have colonized this earth for many millions of years and one of the important characters that facilitate this dominance of insect over other organisms is that of chemical communication. In order to survive in a complex heterogenic ecosystem intricated with several flora and fauna, insects have to depend on chemical communication starting from host plant selection, oviposition stimulants and deterrents, and interactions among themselves and between insects and plants. This chemical communication has been termed as semiochemicals. Semiochemicals are chemicals that mediate interactions between organisms. termed ‘semiochemicals’ (from the Greek semeion, a mark or signal). The term semiochemicals was first proposed by Law and Regnier (1971). Semiochemicals are species specific and are divided into pheromones (communication is intraspecific) and allelochemicals (communication is interspecific) (Fig. 7.1).

7.3.1

Pheromones

Pheromones (Gk. phereum, to carry; horman, to excite or stimulate) are released by one member of a species to cause a specific interaction with another member of the same species. The first semiochemical to be chemically characterized was the sex pheromone of the silkworm moth (Bombyx mori) in 1959 by Adolf Butenandt after sacrificing around 50, 000 female moths and years of hard work. Bombykol is also the first pheromone chemically characterized (EZ-10,12-)-hexadecadienol (Butenandt et al., 1961). The term pheromone was coined by Karlson and Luscher in 1959. The authors defined pheromones as “substances secreted to the outside of an individual and received by a second individual of the same species in which they release a specific reaction.” Pheromone can also be called as ectohormone as they are released outside the body of an organism.

7.4 Classification of Pheromones Based on Their Functions

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Semiochemicals Intra specific

Interspecific

Pheromones

Allelochemicals

1. Sex pheromones

2. Aggregate pheromones 3. Alarm pheromone 4. trail pheromone 5. Host marking pheromone

1. Allomone: -ve to Emier 2. Kairomone: -ve to receiver 3. Synomone :-ve to both emier and receiver 4. Apnemone: from non living material: +ve to receiver

Fig. 7.1 Classification of semiochemicals

There are two distinct types of pheromones: releasers and primers. Releaser pheromones initiate immediate behavioral responses in the receiving organism upon reception, such as alarm, defense, aggregation, attraction, marking of territories, and trail following. Primer pheromones cause physiological changes in the receiving organism ultimately resulting in a behavior response after a longer period of time such as the development of a particular caste or sexual maturation, which eventually modifies the organism’s behavior, as seen in case of queen substance released by queen honey bee to arrest the ovarian development in case of worker honey bees. On the basis of interactions mediated, pheromones are again classified as sex pheromone, aggregate pheromone, alarm pheromone, trial pheromone, and host marking pheromone.

7.4

Classification of Pheromones Based on Their Functions

7.4.1

Sex Pheromones

These are chemicals that are released by an organism outside the body that elicits response in the opposite sex. Depending on the sex of the releaser, they are called female sex pheromone or male sex pheromone. Most of the sex pheromone discovered so far are female sex pheromone of the insects belonging to the order Lepidoptera. Male sex pheromones though uncommon do occur in some insects belonging to Coleoptera, Lepidoptera, and Diptera.

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Of 1314 species of insects with confirmed attraction responses to identified pheromones (Mayer & McLaughlin, 1990), female sex pheromone was found in 1260 species of insects and only 54 of those 1314 species produce or use maleproduced sex pheromone of which 40 of these species are coleopterans, 9 are dipterans, 2 are hemipterans, and 3 are lepidopterans (Mayer & McLaughlin, 1990). Female sex pheromone, their biochemistry and role in IPM have been extensively reviewed. However, the information available on male sex pheromones is rather scanty.

7.4.1.1 Male Sex Pheromone/Attractant/Aggregate Pheromone The word attractant has been here in addition to sex pheromone to denote those characters of male sex pheromones. Most of the male sex pheromones do the function of attracting males or attracting both the sexes or acting as aggregate pheromones for feeding and oviposition. Additionally, male sex pheromones use mixture of host plant odor and the male sex pheromone for attracting potential opposite sex as well as both the sexes. There is a clear contrasting difference between male and female insects in terms of mating behavior. Female insects that have to oviposit or produce young ones spend most of their energy in searching for a better place for oviposition by avoiding marking pheromones of other species and selection of suitable host for better survival of their progeny and hence spend much less time in searching for potential male, and this searching is being compensated by release of sex pheromones. However, as majority of male insects lack parental instincts, male insects divert their energy in searching for a potential female. 7.4.1.2 Male Sex Pheromones and Host Plant Odors Much of the initial knowledge and information on male sex pheromones comes from bark beetles of scolytidae and weevil insects of curculionidae. In many scolytids, few beetles that encounter the host plant make galleries and release attractant pheromones that contain both insect and host plant compounds inviting either female or both the sexes. As the case may be males are attracted by a potential food site for feeding and females are attracted by several reasons such as finding host plant for feeding, finding potential male mate, and finally finding a potential host plant for deposition of their fertilized eggs. This mechanism is clearly established in case of bark beetles. When both the sexes are attracted, mass attack on the host plant is initiated and is one of the main mechanisms of breaking the host plant defense against herbivores. Ips paraconfuses Lanier initially feeds on the host tree and the production of male sex pheromone gets stimulated after sufficient feeding on host plant and pheromone released attracts the female insects that will have access to a potential mate and site for egg laying. Most of the insects that produce male sex pheromone invariably either attract male or both the sexes and the rate of attraction increases with the addition of host plant substances or cues. Out of 21 curculionid species producing male sex pheromone, 19 of them act as both sex attractant and aggregate pheromone as in case of cotton boll weevil Anthonomus, where male boll weevils produce a pheromone blended in the frass after feeding on the buds and squares of the cotton plant. The

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pheromone attracts potential mates and other sex to both feeding and oviposition sites, as well as to the male itself. Few lepidopteran insects also produce male sex pheromones; however, the mechanisms are not similar to that of weevil insects. The cabbage looper Trichoplusia ni performs two mate finding strategies involving attractive pheromones. Males of these insects produce male sex pheromone (stimulated after arriving on to the host plant and host plant odor also synergizes attraction of females) that is attractive to both and females early in the night and several hours after this phenomenon, female insects release a pheromone attractive only to males (Lenczewski & Landolt, 1991). It is believed that most male insects producing sex pheromones do with additional incremental advantage to the attracting females. These incremental advantages include location of oviposition site; and incase of insects feeding on plants with toxic compounds such as salt-marsh caterpillar and Creatonotos spp., (that feeds on plants containing alkaloids and either sequester or store them in their bodies as protection against predators), male insects supports the female insect with incremental advantage of transferring the sequestered plant toxins from male spermatodes to females so as to protect themselves from natural enemies. The papaya fruit fly, Toxotrypana curvicauda Gerstaecker, also uses the same strategy of weevil insects as described above where mate location involving a maleproduced pheromone that provides females simultaneous access to males and hosts (Landolt, 1997).

7.4.2

Leks and Its Significance

Leks can be defined as clusters of small territories where males congregate and display in order to attract mates. This unique phenomenon occurs in several animals like mammals, birds, fish, and insects. Contrary to what we have seen in earlier section where male sex pheromones act as resources of mating and egg laying for female insects, leks are non-resourceful and are mainly performed to attract potential females. In real terms, leks can be termed as real male sex pheromones where the function is only for mating. Leks in other words can also be understood as territory marking by animals to mark their supremacy and added advantage of access to the females is being noticed in many animals. If a new male has entered a new territory, it has to overcome the aggressive behavior of the native male and gain supremacy over the territory. In insects, lekking is observed in two insect groups, tephritid fruit flies (Diptera: Tephritidae) and Hawaiian Drosophila (Diptera: Drosophilidae). Tephritid fruit flies such as Caribbean fruit fly Anastrepha suspensa (Loew) and the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), males form leks on tree foliage that may be formed in part by male attraction to male pheromones. Although they may occur near host trees with oviposition sites, leks of fruit flies are considered to be independent of female resources. Females are attracted and visit leks solely for courtship and mate selection.

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Two types of lekking are generally observed in insects. One is aggregations which is substrate based where males spend most of their time clustering periodically at particular stations most probably patches with food source. Mating may be initiated on the substrate or in the air. Substrate-based aggregations that occur on the top of hills and mountains are referred to as hilltopping aggregations, whereas all other substrate-based aggregations are termed non-hilltopping. The second type of lekking is aerial aggregations where male insects cluster or swam in groups regardless of location or substrate and mating is generally imitated in the air within the swarms. Shelly and Whittier (1997) have given the list of insects exhibiting both substrate and aerial aggregation. For details on leks, their mechanisms. and their role in insect mating systems, the author is directed to look into reviews by Shelly and Whittier (1997); Shelly (2018); Wickman and Rutowski (1999).

7.4.3

Epideictic or Dispersion Pheromones

These are chemicals that are produced in order to avoid aggregation in most cases to prevent overfeeding or grazing that may ultimately limit the food source. In case of western pine beetles Dendroctonus brevicomis that attack ponderosa pine (Pinus ponderosa) tree, the pioneer insect to start forming galleries produces α-pinene and myrcene that act as aggregate pheromone and attract both male and female in large numbers that attack the host tree and breakdown the tree resistance. After mating and when the number of beetles reach maximum, both the sexes produce antiaggregation pheromone verbenone, trans-verbenone, and ipsdienol which deter further beetles from landing close by, encouraging spacing out for the trees and when the resources are dwindling these chemicals also repel the insects. Host marking pheromones that space out food resource can also be termed as epideictic pheromone.

7.4.4

Alarm Pheromones

Alarm pheromones are volatile substances produced by insect in response to the presence of predators to warn conspecific individuals. These chemicals are ubiquitously present in several eusocial insects such as ants and honey bees. Alarm pheromones were first reported in ants by Goetsch in 1934. Since then, several alarm pheromones have been identified in insects other than social insects such as aphids. (E)-β-Farnesene, alarm pheromone is released by several species’ aphids such as Acyrthosiphon pisum, Aphis gossypii, Aphis craccivora, and Rhopalosiphum maidis. When conspecific individuals receive the alarm pheromone, either they move away from the plant surface or drop off from the plant surface thus protecting themselves from predators (Basu et al., 2021). It has been observed that some predators and parasitoids of aphid use these alarm pheromones as kairomones to locate the host insects what is called as evolutionary arm race between insects and natural enemies.

7.5 Allelochemicals

7.4.5

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Host Marking Pheromones

Many phytophagous insects and insect parasitoids mark their feeding and oviposition sites with chemicals called marking pheromones so that the conspecific individuals can avoid these sites as means of reducing competition. These chemicals have been identified in insects belonging to orders Diptera, Lepidoptera, Coleoptera, and Hymenoptera. The adult Lepidopteran insects such as Pieris brassicae and Pieris rapae avoid the sites or leave the place where conspecific females have already deposited the eggs. This they do by using visual sense (as the eggs are big and visual) and also by receiving cues from the site of oviposition (Schoonhoven, 1990). Not only cues released by adult females but also cues from frass of feeding caterpillars can also deter conspecific females in depositing their eggs. In case of fruit flies of family Tephritidae of order Diptera, the adult fruit flies after oviposition on to the fruits smear the surface of the fruit with marking pheromone by its ovipositor, and in some cases these marking pheromones are also present in the feces that deter conspecific females from depositing eggs on the same fruit (Edmunds et al., 2010).

7.4.6

Trail Pheromones

These pheromones are produced by foraging ants, termites, and larvae of some lepidopteran insects. Dufour’s gland, the venom gland, are generally involved in the production of trail pheromones in eusocial insects such as honey bees and ants. They are essentially used to indicate sources of requisites to other members of the colony. Most of the trail pheromones are secreted along with alarm pheromones and mainly used for foraging by conspecific in insects in eusocial insects. An excellent review by Czaczkes et al. (2015) have given insights to role of trail pheromones not only involved in foraging but also in maintaining and regulating colony behavior. Interested readers are requested to go through the work of Czaczkes et al. (2015).

7.5

Allelochemicals

Allelochemicals can be divided into three categories:

7.5.1

Allomones

A substance produced by the organism that contacts with an individual of another species and evokes in receiver the behavior or physiological reaction that is favorable to the emitter. An allomone is defined by Brown (1968) as a chemical substance, produced or acquired by an organism, which, when it contacts an individual of another species in the natural context, evokes in the receiver a behavioral or

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physiological reaction adaptively favorable to the emitter. Plant allelopathy that prevents the growth of development of other plants in the surroundings are also examples of allomone in plants. Allomones can be beneficial and antagonistic. Most of the plants that produce chemical cues or scents, nectar etc. have mutualistic association with pollinators and thus are beneficial for plants in cross pollination and even in dispersal of seeds. Plants producing secondary metabolites can also be called allomones as they deter the herbivores from feeding on plants. The role of plant secondary metabolites acting as allomone is obscure. Plant secondary metabolites in their original stage (intact plant secondary metabolites) are not emitted or released to elicit a behavioral change in the receiver. However, when a herbivore attacks the plants because of their toxicity they repel the herbivore and in this way the emitter is getting the benefit. On the basis of their feeding strategy insects can be classified as general insects feeding on many numbers of plant species and specialized insects or adapted insects that feed exclusively on plants containing toxic secondary metabolites. These specialized insects such as insects feeding on brassica plants having glucosinolates or insects feeding on milk weed plants containing cardenoloids and so on, in the process of co evolution started using these toxic compounds and deriving nutrionally valuable compounds and have adapted to feed on these plants containing toxic compounds. For example sinigrin a plant secondary metabolite of brassica plants though act as deterrant to most general insects, also acts as phago stimulants for insects such as Plutella xylostella, which are speciliased insects on these crops. By looking at these different ways of interactions between plant secondary metabolites and insect herbivore, one can deduce a fairly rough conclusion that for general insects these plant toxic secondary metabolites act as allomones as here the emitter, i.e., plant is benefitted. Whereas for a adapted or specialized insect these toxic chemicals acts as kairomone as the receiver insects are benefitted in this scenario. Things become more complicated if you bring the plant volatiles that are released after the attack of insect herbivores.

7.5.2

Plutella xylostella Diamondback Moth (DBM)–Brassica Plant– Apanteles plutellae

For the adapted insect Plutella xylostella, the intact glucosinolate in brassica plant acts as kairomone as some of these compounds act as phagostimulants for P. xylostella. However, when these plants are fed by the insect, plant tissues break and the intact glucosinolates come in contact with an enzyme myrosinase which is stored separately in the vacuoles of the plant cell. When these myrosinase enzymes come in contact with GLC, GLC breaks and produces volatile compounds such as isothiocyanates and nitriles. These volatiles attract the adults of a wasp parasitoid Apanteles plutellae that locate the host insect and deposit eggs within their bodies thus killing insect herbivore and saving the brassica plant. In conclusion while in ditrophic level interaction between brassica plant and DBM, intact GLC acted as kairomone as the receiver in our case DBM is benefitted; however at third trophic

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level the breakdown product of GLC in brassica acted as allomone for the plant as it is benefitting the emitter and at the same time it is also acting as kairomone for the parasitoid as the adults are benefitted in locating the host insect and hence they can also be called as synomones as both the emitter and receiver are benefitted.

7.5.3

Allomone in Insects

The insects that release toxic chemicals to ward off the predators or exhibiting mimicry to avoid predation are some of the examples of insect allomones. Insects that release repugnatory odors as in case of Heteropteran bugs and toxic chemicals as in case of bombardier beetle, reflex bleeding in case of blister beetle and several lepidopteran insects that sequester or store plant toxins are some of the examples of allomones in insects. Many eusocial insects such as ants, bees, and wasps have poisonous stings for defense against natural enemies or intruders and these chemicals can also be termed as allomones. Lomamyia latipennis is an insect belonging to family Berothidae of order Neuroptera. The adult lacewings which are also known as beaded lacewings because their wing pattern is not predaceous and they generally feed on plant nectar. However, the female insect after mating deposits eggs near the termite colonies, the larvae of these insects feed on termites which they immobilize with an aggressive allomone. The first instar larvae approach termites and waves the tip of its abdomen near the termites’ head. The termite becomes immobile in 1–3 min and completely paralyzed for a short period of time. The berothid then feeds on the paralyzed prey. The later instars feed in a similar manner and are able to paralyze multiple termites at the same time. Adult berothid lacewings are not predatory and feed on nectar like most lacewings. However, female berothids lay egg clusters on logs, stumps, or trees infested with termites. The tiny, newly hatched larvae crawl across the wood seeking cracks and crevices that will lead to the termites within.

7.5.3.1 Kairomones A substance produced by the organism when it comes in contact with an individual of another species, evokes a reaction which is favorable to the receiver. Kairomones include the majority of attractants, phagostimulants, and other substances that mediate the positive responses of predators to their prey, herbivores to their food plants, and parasites to their hosts. Most of the male sex pheromones that use host plant or tree compounds discussed erstwhile in this chapter falls under the category of kairomones (Brown et al., 1970). 7.5.3.2 Synomones A substance produced by an organism, when contacts with individual of another species, evokes in the receiver behavioural reaction that is favourable to both emitter and receiver. α-pinene and myrcene which is produced by damaged pine trees are kairomones for species of bark beetles of Dendroctonus as they are attracted for location of feeding sites but the same chemicals are also attractive to pteromalid

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hymenopteran wasp that parasitise these beetles. So the same chemical α-pinene and myrcene produced by pine plants act as synomone.

7.5.3.3 Apneumones Apneumones (from Greek word “a-pneum” ¼ breathless or lifeless) emitted by a non-living source, causing a favorable behavioral or physiological reaction to a receiving organism, are harmful to other species that may be found either in or on the non-living material. Apneumones were suggested by Nordlund and Lewis (1976). Rare cases of these allelochemicals have been found later in the literature. 7.5.3.4 Parapheromones These are chemicals that have synthesized from plants but exhibiting attraction for many insects’ pest. Parapheromones are chemical compounds, originally derived from plants, that attract sexually mature males and females to some extent (Sivinski & Calkins, 1986). These parapheromones and their analogs have been used in various pest management strategy of fruit flies in mating disruption, mass trapping, etc. Extracts of Ocimum sanctum and Zieria smithii, which were observed to be natural attractants for males of fruit flies (Bactrocera dorsalis) contain methyl eugenol as the major component. Similarly, other compounds such as cuelure and trimedlure were identified from plants which act as attractants for other species of Bactrocera (Bakthavatsalam, 2016).

7.6

Uses of Pheromones in IPM

7.6.1

Monitoring

Most of the identified sex pheromones have been commercially synthesized and have been used widely for insect pest monitoring and forecasting of insecticidal schedule. The active ingredient of the pheromone is generally impregnated onto dispensers which are then placed into a trap of suitable size and shape and used in insect pest detection as well as monitoring. A number of male insect or female insects that were caught in these traps were used to establish threshold for suitable pest management option including chemical control. Some common types of pheromone dispensers currently in use include hollow fibers, plastic laminates, impregnated ropes, twist ties, wax formulations, polyethylene vials, sol-gel polymers, and rubber septa. Recently, nano gel formulations were also being used which have added advantage of slow release of chemical and do not require frequent changing of septa. Depending on the type of insect species which is of interest various traps have been designed such as funnel trap, delta trap, probe trap, and sticky trap. In most of the surveys for pest free areas (PFA), pheromones traps are used to monitor invasive insect species and to schedule management options. Sex pheromones have been successful in monitoring several insect pests across the world of which majority of them belong to order Lepidoptera (Bakthavatsalam, 2016).

7.6 Uses of Pheromones in IPM

7.6.2

71

Mating Disruption

Use of synthetic pheromones to confuse the insects and break their communication needed for location of potential mate is the main motive of mating disruption. Being low cost input, this technology has been used in many are wide management programs. El-Ghany (2019) has postulated four mechanisms of mating disruption. They are 1. Competitive attraction (false trail following): semiochemical substances draw the attention of the males away from wild females thereby following a false trail. 2. Confusion of males (camouflage): confusion occurs due to saturation of the environment with semiochemical substances causing random flight patterns and thereby missing the female position and effectively blocking mating. 3. Sensory desensitization: adaptation of the male antennal receptor system or habituation of the central nervous system as a neurophysiological effect processing due to overexposure to semiochemical substances (continuous and high background concentration). 4. Disguise (emigration of males prior to mating): males emigrate from the area due to excess pheromone, causing them to be unavailable for mating with virgin females. The most successful cases of pest management by mate disruption have been reported in gypsy moth Lymantria dispar, the codling moth Cydia pomonella, the Mediterranean flour moth Ephestia kuehniella Zeller, the Indian meal moth Plodia interpunctella (Hübner), Tuta absoluta in contained glasshouses, and others (Rizvi et al., 2021). Specialized Pheromone and Lure Application Technology (SPLAT) a wax-based formulation having sustained-release of pheromone leading to the mating disruption has been used successfully in case of cotton bollworm Pectinophora gossypiella (Saunders) in India (Sreenivas et al., 2021).

7.6.3

Mass Trapping

By increasing the number of traps within a unit area (an extension of insect pest monitoring), one can trap insects in large numbers and if coupled with any toxicant such as in attract-and-kill method, male insects can be lured en masse and can be eliminated thus depriving the female insect of their potential mate and developing subsequent generation. Mass trapping with aggregate pheromones is more efficient as compared with that of sex pheromone as in former both the sexes are attracted as observed in case of mass trapping of R. ferrugineus (Jayanth et al., 2007) and coffee white stem borer Xylotrechus quadripes (Chevrolet). This technique has been used against pests such as Cydia pomonella (L.), Zeuzera pyrina (L.), and Cossus cossus (L.) in orchards; Spodoptera littoralis (Boisduval) and Pectinophora gossypiella (Saunders) in cotton and oilseed; bark beetles; palm weevils; corn rootworms; Ephestia spp. and Plodia interpunctella (Hubner.) in stored products and food

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industries; or the gypsy moth Lymantria dispar (L.), and boll weevil Anthonomus grandis (Boheman) as invasive species (Rizvi et al., 2021). Bakthavatsalam (2016) has given a list of insects where mass trapping technique is successfully implemented. In India, mass trapping with sex pheromones have been effectively utilized for insect pests such as paddy yellow stem borer Scirpophaga incertulas (Walker) (Cork, Iles, et al., 2005), brinjal shoot and fruit borer Leucinodes orbonalis where installation of 25–30 traps/ac, containing a blend of E-11-hexadecenyl acetate and E-11-hexadecenol (100:1) along with insecticidal sprays such as neem, effectively managed the pest (Cork, Alam, et al., 2005).

7.6.4

Male Annihilation Technique or Attract-and-Kill

This strategy can be used both for pheromone based and/or kairomone based traps where the in addition to detection and monitoring, simultaneous kill of the insect can also be achieved. This technique has been used for long-term management of several insect pest such as codling moth, bark beetle, and others. This technology is majorly used for eradication of invasive species, for example, boll weevils and tephritid fruit flies. For fruit fly species such as Bactrocera dorsalis (Hendel), male attractant/lure methyl eugenol (ME) combined with spinosad an insecticide (Manoukis et al., 2019) is used as male annihilation technique and several satisfying results of reduced pest infestation results were reported. One drawback in using this technology is it can be used efficiently if the pest population is considerably low or moderate; however, if the pest population is very high and if the pest has high migration capacity erroneous results are expected.

7.7

Combination of Pheromones and Kairomones

Presence or addition of plant kairomones can enhance the attractiveness of some insectpheromones in what is termed as synergism. This mechanism is mostly observed in case of coleopteran beetles because most of the aggregate pheromones of beetles contain plant volatile as one of the components (Tewari et al., 2014). Number of cotton boll weevils Anthonomus grandis attracted increased in traps containing aggregate pheromone and blend of green leaf volatiles of cotton (Dickens, 1989). Use of pheromones in combination with plant-derived kairomones was tested for their efficacy of attraction. Traps baited with a 3.0/3.0 mg pheromone/ kairomone blend caught significantly more codling moth males and total moths (including females) than traps baited with either compound alone in apple orchards (Knight et al., 2005). As there were combination of both sex pheromone and kairomones both male and female moths were attracted.

7.8 Push and Pull Strategy

7.8

73

Push and Pull Strategy

The “push-pull” strategy is a concept of combining both attractant and repellent properties of host plant and insect pest. In this strategy, the pests are repelled or deterred away from the main crop (push) by using repellent or deterrent stimuli and at the same time insect pests are simultaneously attracted (pull), using trap crops where they are concentrated and can be managed. The term “push-pull” was first conceived as a strategy for insect pest management by Pyke, Rice, Sabine, and Zaluki in Australia in 1987. Originally used in management of cereal borers in maize and sorghum such as Chilo partellus, Eldana saccharina, Busseola fusca, and Sesamia calamistis. A push-pull strategy for managing cereal stem borers in Africa was developed by scientists of the International Centre of Insect Physiology and Ecology (ICIPE) in Kenya in collaboration with Rothamsted Research in the United Kingdom. The strategy involves combined use of repellent and attractant plants for the stem borers of interest. The push-pull strategy for cereal stem borers involved trapping stem borers on highly attractant trap plants (pull) Napier grass (Pennisetum purpureum), Sudan grass (Sorghum vulgare sudanense) and repelling ovipositing moths away from the host plants using repellent intercrops (push) such as desmodium (Desmodium uncinatum and Desmodiu mintortum). Desmodium crop was also found to suppress the population of weed striga through allelopathy. Molasses grass is used as intercrop with maize that not only reduced infestation of maize by stem borers, but also increased stem borer parasitism by a natural enemy, Cotesia sesamiae. The results have shown that in areas where only stem borers are a problem, there was an increase of 20% yield for the farmers while in areas where both stem borer and striga prevails, there was an increase of 50% yield in maize (Khan & Pickett, 2004). Since then, several programs have designed and tested in the field with quite promising results. Cook et al. (2007) have given detail account of push and pull strategy adopted in various crops in several countries and some of them are discussed here. For managing Helicoverpa armigera and Helicoverpa punctigera in cotton, neem seed kernel extracts were used as repellent on the crop to repel the adults from oviposition and intercultivated with an attractive trap crop, either pigeon pea (Cajanus cajan) or maize (Z. mays) (pull) to pull the insects from cotton. Trap crop efficiency was increased by application of a sugar-insecticide mix. Trap crops, particularly pigeon pea, reduced the number of eggs on cotton plants in target areas. The results have revealed that the push-pull strategy was significantly more effective than the individual components alone and reduced the number of eggs 3 days after application of the bait by 92%, 40%, and 78%, respectively, against the untreated control when pigeon pea was at its most attractive stage (Cook et al., 2007). The interest in this technology has gained momentum after technology developments in identification and synthesis of many plant volatiles as well as pheromones and the technique has been renamed as “stimulo-deterrent diversion tactic.”

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Except the work report of Cook et al. (2007) on management of H. armigera and H. punctigera on cotton in Australia most of the other push and pull strategies employed the new concept as “stimulo-deterrent diversion tactic,” where several aggregate pheromones or synthetic host plant chemicals were used as both attractants and repellents to achieve the same results as envisaged in the traditional push-pull strategy for pest management. By taking similar approach of push and pull principle, in apple orchards, perimeter-row trap trees were baited with grandisoic acid (aggregation pheromone) and benzaldehyde (fruit volatile) to attract plum curculio Conotrachelus nenuphar adults and were given spot application of insecticide management. Compared with standard full block insecticide applications for the pest, limiting treatment to pheromone-baited “trap” trees provided satisfactory suppression of fruit injury plum curculio (Leskey et al., 2008).

7.9

Possible Use of Host Marking Pheromone in Pest Management

In a field experiment, the synthetic host marking pheromone of European cherry fruit fly Rhagoletis cerasi have been sprayed on uninfested fruit orchards and up to 98% of reduced infestation was observed (Aluja & Boller, 1992). Similarly, Edmunds et al. (2010) were able to isolate, characterize, and synthesize host marking pheromone of Mexican Fruit Fly Anastrepha ludens and in a field experiment on plum trees, and spraying of this host marking pheromone has drastically reduced infestation by another fruit fly Anastrepha obliqua. However, one of the bottlenecks in using this technology for pest management is the identification of compounds acting as marking pheromones, their biochemistry, and synthesis. It took 15 years of research by Hurter et al. (1987) to finally be able to chemically characterize and synthesize European cherry fruit fly host marking pheromone. For more details into semiochemicals and their use in pest and disease management, one can go through excellent reviews provided by Bakthavatsalam (2016), Ezzat et al. (2019), El-Ghany (2019), and Rizvi et al. (2021).

7.10

Possible Role of Using Female Annihilation Technique

So far female sex pheromones have been used for male annihilation or attract-andkill technique where the pheromone is baited with a suitable inspection for killing the attracted male. Recent developments in the field of insect chemical ecology have resulted in identification and synthesis several male sex pheromone/aggregate pheromones. The role of use of these male sex pheromones for female annihilation can be a possible option where we are aiming at killing the females that are responsible for egg laying and development of net generation. The possibilities of exploring the female annihilation technique may pave a way for a new paradigm in the field of IPM.

References

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References Aluja, M., & Boller, E. F. (1992). Host marking pheromone of Rhagoletis ceras: Field deployment of synthetic pheromone as a novel cherry fruit fly management strategy. Entomologia Experimentalis. Applicata, 65, 141–147. Bakthavatsalam, N. (2016). Semiochemicals. In: Omkar (Ed.) Ecofriendly pest management for food security. Academic Press, pp. 563–611. Basu, S., Clark, R. E., Fu, Z., Lee, B. W., & Crowder, D. W. (2021). Insect alarm pheromones in response to predators: Ecological trade-offs and molecular mechanisms. Insect Biochemistry and Molecular Biology, 128, 103514. https://doi.org/10.1016/j.ibmb.2020.103514. Epub 2020 Dec 23. PMID: 3335957. Brown, W. L. J. (1968). A hypothesis concerning the function of the meta pleural glands in ants. American Naturalist, 102, 188–191. Brown, W. L., Eisner, T., & Whittaker, R. H. (1970). Allomones and kairomones: Transspecific chemical messengers. BioScience, 20(1), 21–22. https://doi.org/10.2307/1294753 Butenandt, A., Beckamnn, R., & Hecker, E. (1961). Uber den Sexuallockstoff des Seidenspinners 0.1. Der biologische Test und die Isolierung des reinenSexuallockstoffes Bombykol. HoppeSeyler’sZeitschrift für PhysiologischeChemie, 324, 71–83. Cook, S. M., Khan, Z. R., & Pickett, J. A. (2007). The use of push-pull strategies in integrated pest management. Annual Review of Entomology, 52(1), 375–400. https://doi.org/10.1146/annurev. ento.52.110405.091407 Cork, A., Alam, S. N., Rouf, F. M. A., & Talekar, N. S. (2005). Development of mass trapping technique for control of brinjal shoot and fruit borer, Leucinodes orbonalis (Lepidoptera: Pyralidae). Bulletin of Entomological Research, 95(6), 589–596. Cork, A., Iles, M. J., Kamal, N. Q., Choudhury, J. C. S., Rahman, M. M., & Islam, M. (2005). An old pest, a new solution: commercializing rice stem-borer pheromones in Bangladesh. Outlook on Agriculture, 34(3), 181–187. Czaczkes, T. J., Gruter, C., & Ratnieks & Francis L.W. (2015). Trail pheromones: An integrative view of their role in social insect colony organization. Annual Review of Entomology, 60(1), 581–599. https://doi.org/10.1146/annurev-ento-010814-020627 Dickens, J. (1989). Green leaf volatiles enhance aggregation pheromone of boll weevil, Anthonomous grandis. Entomologia Experimenta Applicata, 52, 191–203. Edmunds, A. J. F., Martin, A., Diaz-Fleischer, F., Patrian, B., & Hagmann, L. (2010). Host Marking Pheromone (HMP) in the Mexican Fruit Fly Anastrepha ludens. CHIMIA International Journal for Chemistry, 64(1), 37–42. https://doi.org/10.2533/chimia.2010.37 El-Ghany, N. (2019). Semiochemicals for controlling insect pests. Journal of Plant Protection Research, 59, 1–11. https://doi.org/10.24425/jppr.2019.126036_rfseq1 Ezzat, S. M., Jeevanandam, J., Egbuna, C., Merghany, R. M., Akram, M., Daniyal, M., Nisar, J., & Sharif, A. (2019). Semiochemicals: A green approach to pest and disease control. In C. Egbuna & B. Sawicka (Eds.), Natural remedies for pest, disease and weed control (pp. 81–89). Academic Press. https://doi.org/10.1016/B978-0-12-819.304-4.00007-5 Goetsch, W. (1934). Untersuchungenüber die zusammenarbeitimameisenstaat. Zurgeographie, okologie und systematicTiere., 28, 319–401. Harborne, J. B. (2001). Twenty-five years of chemical ecology. Natural Product Reports, 18(4), 361–379. Hurter, J., Boiler, E. F., Stadler, E., Blattman, H., Buser, R., Bosshard, N. U., Damm, L., Kozlowski, M. W., Schoni, R., Raschdorf, F., Dahinden, R., Schlumpf, E., Fritz, H., Richter, W. J., & Schrexber, J. (1987). Oviposition-deterring pheromone in Rhagoletis cerasi L.: purification and determination of the chemical constitution. Experientia, 43, 157–164. Jayanth, K. P., Mathew, M. T., Narabenchi, G. B., & Bhanu, K. R. M. (2007). Impact of large-scale mass trapping of red palm weevil Rhynchophorus ferrugineus Olivier in coconut plantations in Kerala using indigenously synthesized aggregation pheromone lures. Indian Coconut Journal, 38(2), 2–9.

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Karlson, P., & Lüscher, M. (1959). Pheromones: A new term for a class of biologically active substances. Nature, 183, 55–56. Khan, Z. R., & Pickett, J. A. (2004). The ‘push-pull’ strategy for stemborer management: a case study in exploiting biodiversity and chemical ecology. In G. M. Gurr, S. D. Wratten, & M. A. Altieri (Eds.), Ecological engineering for pest management: Advances in habitat manipulation for arthropods (pp. 155–164). CABI. Knight, A. L., Hilton, R., & Light, D. M. (2005). Monitoring codling moth (Lepidoptera: Tortricidae) in apple with blends of ethyl (E, Z)-2,4-decadienoate and codlemone. Environmental Entomology, 34, 598–603. Landolt, P. J. (1997). Sex attractant and aggregation pheromones of male phytophagous insects. American Entomologist, 43(1), 12–22. https://doi.org/10.1093/ae/43.1.12 Law, J. H., & Regnier, F. E. (1971). Pheromones. Annual Review of Biochemistry, 40, 533–548. Lenczewski, B., & Landolt, P. J. (1991). Temporal partitioning of dual sexual attraction strategies in Trichoplusiani (Lepidoptera: Noctuidae). Annals of Entomological Society of America, 84, 124–130. Leskey, T. C., Pinero, J. C., & Prokopy, R. J. (2008). Odor-baited trap trees: A novel management tool for plum curculio (Coleoptera: Curculionidae). Journal of Economic Entomology, 101, 1302–1309. Manoukis, N. C., Vargas, R. I., Carvalho, L., Fezza, T., Wilson, S., Collier, T., & Shelly, T. E. (2019). A field test on the effectiveness of male annihilation technique against Bactrocera dorsalis (Diptera: Tephritidae) at varying application densities. PLoS One, 14(3), e0213337. https://doi.org/10.1371/journal.pone.0213337 Mayer, M. S., & McLaughlin, J. R. (1990). Insect pheromones and sex attractants. CRC Press. Nordlund, D. A., & Lewis, W. J. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions. Journal of Chemical Ecology, 2, 211–220. Pyke, B., Rice, M., Sabine, B., & Zalucki, M. P. (1987). The push-pull strategy behavioural control of Heliothis. Australian Cotton Grower, 9, 7–9. Rizvi, S. A. H., George, J., Reddy, G. V. P., Zeng, X., & Guerrero, A. (2021). Latest developments in insect sex pheromone research and its application in agricultural pest management. Insects, 12(6), 484. https://doi.org/10.3390/insects12060484 Schoonhoven, L. M. (1990). Host-marking pheromones in lepidoptera, with special reference to two Pieris spp. Journal of Chemical Ecology, 16, 3043–3052. https://doi.org/10.1007/ bf00979611 Shelly, T. D. (2018). Sexual selection on leks: A fruit fly primer. Journal of Insect Science, 18(3), 9. Shelly, T., & Whittier, T. (1997). Lek behavior of insects. In J. Choe & B. Crespi (Eds.), The evolution of mating systems in insects and Arachnids (pp. 273–293). Cambridge University Press. https://doi.org/10.1017/CBO9780511721946.017 Sivinski, J. M., & Calkins, C. (1986). Pheromones and parapheromones in the control of tephritids. The Florida Entomologist, 69(1), 157–168. https://doi.org/10.2307/3494757 Sreenivas, A. G., Markandeya, G., Harischandra Naik, R., Usha, R., Hanchinal, S. G., & Badariprasad, P. R. (2021). SPLAT-PBW: An eco-friendly, cost-effective mating disruption tool for the management of pink bollworm on cotton. Crop Protection, 149, 105784. Tewari, S., Leskey, T., Nielsen, A., Pinero, J., & Rodriguez-Saona, C. (2014). Use of pheromones in insect pest management, with special attention to weevil pheromones. In D. P. Abrol (Ed.), Integrated pest management current concepts and ecological perspective (pp. 141–168). Elsevier Inc. Wickman, P. O., & Rutowski, R. L. (1999). The evolution of mating dispersion in insects. Oikos, 84(3), 463–472. https://doi.org/10.2307/3546425

8

Seasonality in Insects

Abstract

Seasonal changes in the surrounding environment affecting growth and metabolism of insects and how insects cope up with these seasonal changes have been discussed in the chapter. Insects adapt to major strategies such as dormancy and migration to overcome the frequent seasonal changes. Dormancy types such as torpor, quiescence, hibernation, and aestivation are being explained. Keywords

Homodynamic · Heterodynamic · Migration · Dispersal · Dormancy · Torpor · Quiescence · Hibernation · Aestivation · Diapause

Seasonal changes are characteristic of the Earth’s climate, more pronounced at higher latitudes where “favorable” summers alternate with winters that are highly unfavorable to an organism’s growth, development, or reproduction. In response to such seasonality, insects, similar to many other organisms, have evolved mechanisms to counteract or synchronize with these events by using photoperiodic “clocks” to measure and respond to changes in day length (Saunders, 2002). It was earlier hypothesized that the seasonal changes are more pronounced in temperate regions, and hence insects of temperate regions have more adaptations as compared to that of tropical regions. Wolda (1988) was the first to review and hypothesize that the seasonal patterns in abundance, life-history development, appearance of reproductive activity, and dispersal also occurs in tropical insects and are not unique to temperate insects. Since then, several workers have reviewed about seasonality of tropical insects of which the review by Yamada and Itioka (2015) is worth mentioning.

# The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_8

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Insects, being “cold-blooded,” show temperature-dependent metabolic control of activity, development, and reproduction. On the basis of their dependency on seasonal changes, the insects can be classified as follows.

8.1

Homodynamic Insects or Life Cycles

The insect species survive under stable and favorable environmental conditions for continuous survival, growth, development, and reproduction throughout the year. Not much adaptations can be seen in these insects as they are thriving under stable environment. Most of the tropical conditions fall into this category. However, even in these tropical conditions rainfall and dry spell do impose selection pressure leading to temporary adaptations in living organisms including insects.

8.2

Heterodynamic Insects or Life Cycles

Insects that survive in temperate regions bound to have adaptation coinciding with the cyclic seasonal changes. Seasonal variations in such type of environmental conditions impose a corresponding seasonal change in the rate of development, alternating between periods of rapid growth and development and periods of rest or dormancy. Basic strategies in insects to withstand/overcome unfavorable climatic conditions are migration and dormancy.

8.3

Migration

Migration is the movement of a large number of species from one place to another, usually leaving no organisms in the original habitat. The word “migration” refers to movements of animals in a direction and for a distance over which they have control, and which result in a temporary or permanent change of habitat. In insects, such movements may cover a thousand miles or more, but in other cases the distances may be shorter. The numbers of individuals in a single movement may range from a few hundred up to thousands of millions. For example, locust migration, monarch butterfly. Dispersal: Spreading of individuals away from others mostly from parents or offspring. Dispersal leaves a portion of population behind in the original habitat.

8.3.1

Patterns of Migration

Migration moves species between two habitats, each of which is favorable at different times. True “migration” consisted of double movement or having return flight/migration and was primarily due to a breeding or genetic stimulus, while

8.3 Migration

79

“emigration” was the overflow from an overcrowded area and was attributable to an alimental or feeding stimulus. 1. Daily movement Most snails clump together in groups in humid microclimate during day. During night, they start foraging. Insects such as cutworms (larvae) congregate in soil crevices, shade, debris during daytime and becomes active during night and forage on plants. 2. Seasonal movement Many migratory insects have polymorphic forms. As seen in case of locust having solitary phase and gregarious phase. Insects migrate to avoid unfavorable climatic conditions or seasonal changes. Best example is monarch butterfly Lepidoptera. Monarch butterfly migrates from Southern Canada to Central Mexico during winter, where the adults spend their winter. During late winter or early spring, the adults leave Mexico, travel to north, mating takes place, and female deposits eggs on milkweed in Northern Mexico and Southern Texas. The caterpillars complete their life cycle on milkweed in North Mexico and Southern Texas, adults emerge and migrate towards Central Canada where the climatic conditions return to normal, and they continue there until next winter and again repeat the process. The following are examples of migratory insects under different orders . Orthoptera: Locust: Schistocerca gregaria makes regular seasonal movements in parts of Africa. Odonata: Libellula depressa Linnaeus, Libellula quadrimaculata Linnaeus, and Sympetrum striolatum (Charpentier). The migratory globe skimmer dragonfly (Pantala flavescens) migrates from India across the Indian Ocean to East Africa in the autumn, with a subsequent generation to return to India from East Africa the following spring. Their movements are assisted by wind. Coleoptera: Lady bird beetles such as Hippodamia convergens, Adalia bipunctata, and Coccinella undecimpunctata also have migratory capabilities. Hemiptera: Eurygaster integriceps Puton, Homoptera: Circulifer tenellus (Baker), Diptera: Syrphus lavendulae; Eristalis sylvaticus Meigen; Hymenoptera: Athalia rosae; the burrowing wasp, Sphex aegyptiacus Lepeletier, which uses locusts as food for its young. Lepidoptera: Danaus plexippus; Vanessa cardui; Catopsilia florella (Fabricius); Libythea labdaca; Phoebis sennae (Linnaeus), and Phoebis statira (Cramer); Agrotis infusa (Boisduval).

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8.4

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Seasonality in Insects

Dormancy

Through the process of evolution, insects have developed mechanism to overcome adverse climatic conditions such as high or low temperatures, moisture availability, or reduced availability of quality food. One such adaptation is dormancy through arrested growth and development until the favorable climatic conditions return. This arrested stage of development depends on nature and extent of deviation of environmental factor(s) from the optimum. Dormancy may intervene at embryonic, larval, pupal, or adult reproductive stages of the life cycle; the stage at which most types of dormancies occur is always characteristically fixed in species. A broader classification of dormancy in insects can be dealt under two different heads one diapauses and quiescence. On the basis of duration, dormancy can be broadly classified as quiescence and diapauses. Quiescence commonly refers to short periods of dormancy that are directly induced by adverse environmental conditions, principally low or high temperature. It also has the advantage of being quickly reversible upon the return of favorable conditions. In contrast, diapause is not directly induced, but is triggered by “token stimuli” and is a genetically programmed response to adverse climatic condition. Basically, diapause is an anticipatory mechanism to overcome an adverse climatic condition. This anticipatory mechanism allows time for substantial physiological changes prior to the arrival of adverse conditions. These changes include accumulation of lipid and glycogen reserves, deposition of cuticular lipids that enhance desiccation resistance, suppression of gametogenesis, decreased metabolic rate, increased tolerance of anoxia, and low temperature. Though abiotic factors such as temperature, moisture, and host-plant quality may serve as cues for the induction of diapause, photoperiod is regarded as the main factor that is responsible for induction of diapause. Several workers have given several definitions and types of dormancies of which the notable ones are Mansing, 1971 who has classified dormancy in addition to diapause as Athermopause (Diapause induced by factors other than temperature), oligopause (mild and long-term seasonal variations), high intensity and low intensity diapauses. However, for convenience and for easy comprehension, the classification given by Roberts, 1978 is used in the present discussion.

Summer Dormancy • Hear Torpor • Quiescence • Summar Diapause

/Aesvaon

Winter dormancy • Cold torpor • Quiescence • Winter

Diapause/Hibernaon

8.4 Dormancy

8.4.1

81

Difference Between Torpor and Quiescence

Torpor: Torpor and quiescence differ only in duration. Torpor is induced by sudden short spells of adverse weather, which although not severe enough to kill the insects, abruptly halts its activity and development. It is non-seasonal and may occur at any time of the year and may affect any stage of the insect. Quiescence: First described by Shelford (1929). Quiescence generally has longer duration in life cycle of many insects. Quiescence is directly induced and terminated by surrounding environmental conditions. Quiescence is influenced purely by extrinsic factors and acts directly on metabolic rate resulting in either slowing down or complete arrest of development. Summer quiescence: Larvae of the African chironomid midge, Polypedilum vanderplanki, inhabit temporary pools in hollows of rocks and become quiescent when the water evaporates. Dry larvae of this midge can “revive” when immersed in water, even after years of quiescence. The quiescent larva is in a state of cryptobiosis and tolerates the reduction of water content in its body up to 4%, surviving even brief exposure to temperatures ranging from 102  C to 270  C. Another example of summer quiescence is observed in quiescent eggs of the brown locust, Locustana pardalina, that survive in the dry soil of South Africa for several years until their water content decreases to 40%. When there is adequate rain, they absorb water, synchronously resume development, and hatch, resulting in an outburst of hopper populations.

8.4.1.1 Diapause Diapause in defined as dormancy state with the following characteristics Andrewartha (1952): “a stage in the development of certain animals, during which morphological growth and development are suspended or greatly decelerated.” Beck (1962) “state of arrested development in which the arrest is enforced by a physiological mechanism rather than by concurrently unfavorable environmental conditions.” Tauber et al. (1984) mentioned that “diapause is hormonally mediated state of low metabolic activity associated with reduced morphogenesis, increased resistance to environmental extremes and altered or reduced behavioral activity.” Tauber et al. (1986) again defined this term as “diapause is a neurohormonally mediated, dynamic state of low activity that occurs during a genetically determined stage(s) of metamorphosis, usually in response to environmental stimuli that precede unfavorable conditions.” Definitions of diapause have been framed.

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Characteristics of Diapause 1. It is species-specific stage. 2. Diapause is not induced in direct response to unfavorable conditions, but it is anticipatory, i.e., it occurs before the adverse climatic conditions begin. The factors responsible for diapause act well in advance of winter and provide time for the accumulation of the metabolites (lipids and proteins) needed during the dormant phase. 3. Photoperiod along with temperature is involved in the timing and induction of diapause. Diapause is under complex hormonal control (PTTH) prothoracic gland (ecdysteroid) that is involved in the regulation of larval and pupal (i.e., developmental) dormancies and corpus allatum (juvenile hormone) that is involved in adult (i.e., reproductive) dormancies (Denlinger, 1985), whereas quiescence is very simple function of temperature acting on metabolic function of an organism. On the basis of diapause, insects can be classified as follows: Long-day insects—Which grow and develop under long-day lengths, but diapause happens under short-day lengths. For example, Pectinophora gossypiella. Short-day insects—Which grow and develop under short-day lengths, but diapause happens under long-day lengths. For example, Mulberry silkworm. The induction of diapause is controlled by endogenous circadian system, whereas quiescence is controlled by exogenous system or direct effect of adverse climatic conditions.

8.4.1.2 Stages of Diapause Broadly divided into pre-diapause, diapause, and post-diapause stage. 1. Pre-diapause stage (a) Induction Phase “During this stage the cues or token stimuli from the environment are perceived and transduced into switching the ontogenetic pathway from direct development to diapause when the token stimuli reach some critical level (the response may be modified by other environmental factors).” (b) Preparation Phase “During this preparation phase behavioral and physiological preparations for diapause may take place such as food storage, behavioral changes and some changes in rate of development.” 2. Diapause Phase (a) Initiation Phase “Direct development stops or slows down, which is usually followed by regulated metabolic suppression. Mobile diapause stages may continue accepting food, building of energy reserves and seeking suitable microhabitat. Physiological preparations for the period of adversity may take place and intensity of diapause may increase.”

8.4 Dormancy

83

(b) Maintenance Phase “Endogenous developmental arrest persists while the environmental conditions are favorable for direct development. Specific token stimuli may help to maintain diapause (prevent its termination). Metabolic rate is relatively low and constant. Unknown physiological process leads to more or less gradual decrease of diapause intensity and increase of sensitivity to diapause terminating conditions.” (c) Termination Phase “Specific changes in environmental conditions stimulate (accelerate or resume) the decrease of diapause intensity to its minimum level and thus synchronize individuals within a population. By the end of the termination phase, a physiological state is reached, in which direct development may overtly resume (if the conditions are favorable) or covert potentiality for direct development is restored but not realized (if the conditions are not favorable).” 3. Post-diapause Phase “During Post-diapause quiescence, inhibition of development and metabolism was exogenously imposed, which follows the termination of diapause when conditions are not favorable for resumption of direct development.” It implies reorganization prior to full activity (Gill et al., 2017). For different theories of diapause and characteristics of different stages of diapause, the readers are requested to go through works of

8.4.1.3 Classification of Diapause Diapause can be divided into hibernation or aestivation depending on the season of inactivity. Differences between hibernation and aestivation is represented in Table 8.1. Table 8.1 Difference between hibernation and aestivation Hibernation Arrest of growth to overcome adverse conditions during winter Body temperature of hibernating insect is more than that of normal insect Respiratory movement become slow, irregular, and metabolic rate falls considerably

Aestivation During hot or summer months Shortage of food can also cause aestivation Homeostasis of organism remains unchanged. Respiration is slow and gradual In Southeastern Australia, the adults of the Bogong moth, Agrotis infusa, emerge in late spring to migrate from the plains to the mountains, where they aestivate, forming huge aggregations in rock crevices and caves The alfalfa weevil, Hypera postica (Gyllenhal), undergoes aestivation as adult

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Depending on the functionality, diapause can also be grouped as Obligate and Facultative diapause. Obligate diapause is the characteristic feature of univolatine species which is basically an inherent character. Facultative diapause is not inherent, depends on environmental characters, and is related to multivolatine species. Depending on the stage of the insect that undergoes diapause, they are classified as follows: (a) Egg diapause: For example, Bombyx mori, Hieroglyphus banian (b) Larval: Due to release of neurosecretory hormones from supra esophageal ganglia. For example, maggots—paddy gall midge; sorghum midge. Caterpillars: paddy stem bores, sorghum stem borer (c) Pupal diapause: Amsacta albistriga, Amsacta moorei, Euproctis lunata (d) Adult: White grub Holotrichia serrata, Holotrichia consanguinea

References Andrewartha, H. G. (1952). Diapause in relation to the ecology of insects. Biological Reviews, 27(1), 50–107. Beck, S. D. (1962). Photoperiodic induction of diapause in an insect. The Biological Bulletin, 122(1), 1–12. Denlinger, D. L. (1985). Hormonal control of diapause. In G. A. Kerkut & L. I. Gilbert (Eds.), Comprehensive insect physiology, biochemistry and pharmacology (Vol. 8, pp. 353–412). Pergamon Press. Gill, H. K., Gaurav, G., & Gurminder, C. (2017). Insect diapause: A review. Journal of Agricultural Science and Technology A, 7, 454–473. Mansing, A. (1971). Physiological classification of dormancies in insects. Canadian Entomologist, 103, 983–1009. Roberts, R. M. (1978). Seasonal strategies in insects. New Zealand Entomologist, 6(4), 350–356. Saunders, D. S. (2002). Insect clocks. Elsevier. Shelford, V. E. (1929). Laboratory and field ecology. Williams and Wilkins. Tauber, M. J., Tauber, C. A., & Masaki, S. (1984). Adaptations to hazardous seasonal conditions: Dormancy, migration and polyphenism. In C. B. Huffaker & R. L. Rabb (Eds.), Ecological entomology (pp. 149–183). Wiley-International Press. Tauber, M. J., Tauber, C. A., & Masaki, S. (1986). Seasonal adaptations of insects. Oxford University Press. Wolda, H. (1988). Insect seasonality: why? Annual Review of Ecology and Systematics, 19, 1–18. Yamada, K. K., & Itioka, T. (2015). How much have we learned about seasonality in tropical insect abundance since Wolda (1988). Entomological Science, 18, 407–419.

9

Characters of Population

Abstract

There are a set of characters of population of all living organisms from unicellular to multicellular organisms. These different characters define the structure and function of populations. Population density describes the availability of number of organisms determined by natality, mortality, immigration, and emigration. Depending on population growth, all living organisms are grouped under three different types of survivorship curves which have discussed here. The differences between exponential growth rate and logistic growth rate and the concept of environmental resistance and biotic potential have been the focal point of this chapter. Various distribution patterns of insects and the measure of distribution is outlined. Finally, on the basis of all these population characters all living organisms including insects are fitted into k and r life history strategies. Keywords

Density · Natality · Mortality · Maximum · Realized · Exponential growth · Logistic growth · Environmental resistance · Survivorship curves · k- and r-strategy · Distribution patterns and measures

9.1

Population Density

The number of organisms per unit geographical area as per m2, per ha etc., is known as population density. Population density determines the chances of getting sufficient food, space, and mate. The population density also determines the dispersion and distribution of a living organism. The initial population density also determines the behavior of resulting population as clearly seen in solitary and gregarious phase of desert locust Schistocerca grigaria.

# The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 K. V. H. Prasad, Insect Ecology: Concepts to Management, https://doi.org/10.1007/978-981-19-1782-0_9

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The solitary phase of desert locust is characterized by few nymphs and adults feeding independently as the environment becomes congenial the population increases and which forces the individual to crowd or aggregate that increases the level of serotonin in the thoracic ganglion paving the way for the solitary phase to transform into gregarious phase with changes in the morphology and behavior of these locust. These gregarious insects form into bands of several kilometers and devastate entire vegetation on their way and fly several 1000 km forming swarms (Uvarov, 1954). It has been estimated a critical density value around 2.45 hoppers m
2, above which gregarious hoppers were expected more that may lead to change of solitary phase to gregarious phase (Cisse et al., 2015).

9.2

Natality

Production of new individuals of any organism. Birth rate of any individual organism per unit time. Natality is of two types. Maximum natality: Also known as absolute or potential or physiological natality and is theoretical maximum production of new individuals under ideal conditions. Ecological or realized natality: It refers to population increase under an actual, existing population under specific condition. It takes into account all possible environmental conditions. N 1 ¼ N 0 þ ðB
DÞ þ ðI
E Þ where N0 ¼ Initial population; B ¼ No. of individual born; D ¼ No. of individual died; I ¼ No. of individual immigrate; and E ¼ No. of individuals emigrated. Factors that affect oviposition and fecundity in turn is reflected on natality of insects. Oviposition and fecundity are two different processes that determine the natality of insects. The fecundity of number of eggs/offspring produced by an insect depends on the availability, feeding, and assimilation of nutritionally rich diet during its early stage or active stage of development, i.e., larvae for holometabolous and nymphs in case of hemimetabolous insects. Insects fed on nutritionally rich diet during their active stage gives rise to bigger adult females and are able to have high fecundity though abiotic factors such as temperature also affects fecundity. Fecundity of any insect can be regarded more as physiological process and oviposition can be regarded more as behavioral process. Oviposition particularly in lepidopteran adult moths and butterflies are more affected by chemical cues either attractant or deterrent released by plants and perception of these chemical cues by ovipositing adults with the help of their mechano- and chemoreceptors present in the sensilla. Associate learning plays a very important role in oviposition behavior of many lepidopteran moths and butterflies. In general, it is presumed that adult insects oviposit on substrate that is a potential host for feeding by young ones in the subsequent generation. The role of semiochemicals in particular marking pheromone provides an additive edge to

9.5 Survivorship Curves

87

oviposition of adults mostly in parasitic Hymenoptera. For details in oviposition of lepidopteran insects, the reader is requested to go through excellent review by Renwick and Chew (1994) and for hymenopteran parasitoids (Gordh et al., 1999).

9.3

Mortality

The number of individuals that are dying per unit time due to old age or senescence or old age or natural death and also termed as potential or physiological mortality which is different from ecological or realized mortality which is actual loss of individuals of a population at a given period of time due to biotic and abiotic factors. As with any other living organism, every insect on this world has some natural enemies viz., parasitoids, predators; some pathological diseases that in turn is affected by several abiotic factors.

9.4

Population Growth

Both natality and mortality determine the population growth of any living organism of which insect is of no exception. Population growth r can be calculated by the equation. r ¼ ðN þ I Þ
ðM þ E Þ, where N ¼ Natality, I ¼ Immigration, M ¼ Mortality, E ¼ Emigration Natality and mortality here are related to realized or ecological natality and mortality. When natality is equal to mortality with no migration, then r ¼ 0 (population ceases to grow). When birth rate is more than death rate, then “r” is positive and population increases. When death rate is more than birth rate, “r” is negative and population decreases.

9.5

Survivorship Curves

Graphical representation of mortality of a living organism over a period of time. Lotka (1925) pioneered the comparison of survivorship curves among populations, by plotting the log of number or percent of living individuals against time. Type I: The curve is convex and population mortality rate is low until end of the life span. For example, bigger animals, human beings. This type of survivorship curve involves greater degree of parental care. Type II: This curve describes a situation where no particular life stage significantly shows higher mortality than any other, i.e., probability of death at each stage is constant giving a linear curve. This pattern, i.e., constant mortality is more characteristic of birds/lizards.

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9 Characters of Population

Type I

Type I: Low death rate Many individuals live to old age.

1000

100 Survivors

Type II 10 Type III 1

0.1 Type III: High death rate Many individuals die young, and few live to old age.

Age

Type II: Moderate death rate Individuals die at all ages

Fig. 9.1 Survivorship curve

Type III: Population whose mortality rate is high during growth and developmental stage. For example, smaller animals/insects/sea oyster/fish. These organisms have high rates of natality and high rates of mortality during the early stages of development and low mortality during later life stages (Fig. 9.1) The work of Cornell and Hawkins (1995) is worth mentioning when studying the life tables of insects. These workers have reviewed patterns to be found in 530 life tables on 124 holometabolous, herbivorous insects. A general observation from their work is endophytes or internal borers tend to have more survival rate in the early life stages as their eggs or larvae are protected within plant tissues whereas the external feeders have 5–10% greater mortality compared to internal feeders. External feeders suffer greater mortality from natural enemies than internal feeders (51% vs. 44%) and are less impacted by plant resistance factors than internal feeders (2% vs.15%) Price (1997) who studied the work of Cornell and Hawkins (1995) have opined that the first instar larvae of insect is the critical stage that may influence the population trend. According to him, there is a contrast in the survival of two insects, i.e., cabbage maggot Erioischia brassicae and olive fruit fly, Dacus oleae. The cabbage maggot female lays eggs at the collar of the young cabbage plant at soil level, and the tiny larvae must burrow down to roots before they can feed, with great loss of individuals through desiccation, difficulty burrowing, perhaps predation, and difficulty establishing a feeding site by tunneling through the plant cuticle. The resulting survivorship curve is strongly concave.

9.7 Age Pyramids

89

In contrast, the olive fruit fly lays eggs directly through the skin of the olive fruit, placing the eggs into the flesh on which the larvae will feed. The larva is protected from desiccation, hard to find by natural enemies, and surrounded by food. Thus, the survivorship is strongly convex in most cases. From these studies one can hypothesize that if under given favorable environmental conditions and with no dearth of food and space most of the internal borers/ feeders may tend to have Type I or convex survivorship curve compared to external feeders where the survivorship curve tend to be concave or Type III.

9.6

Age Structure

The ratio of various age groups in a population determines the current reproductive status of the population and thus anticipates its future there are three ecological age structure i.e., pre-reproductive, reproductive, and post-reproductive.

9.7

Age Pyramids

Age pyramid or structure gives indication of number of individuals at different stages of life of an organism. Provided the space and food is of no constraint, the growing population generally will have more individual in young age and declining population generally will have few individuals at the young age class. Insects living in tropical regions generally have shorter life cycles, have overlapping generations and mostly affected by fluctuating temperature and rainfall. Insects from temperature regions generally have discrete cyclic generations and have a typical diapausing stage coinciding with unfavorable cold weather. In general, most of the insects have long pre-reproductive period and short reproductive and post-reproductive period except few insects such as Magi ciccada spp. which has broods of 13–17 years and the nymphs spend 13–17 years as pre-reproductive period and emerge as adults after 13–17 years. Bodenheimer, 1938 has proposed three age pyramids (Fig. 9.2) (a) Triangular shape: High birth rate, high population growth. For example, houseflies/Indian population, Mexican, Pakistan, etc. (b) Bell-shaped polygon/stable population: Moderate proportion of young to old. Pre Reproductive (PR) and Reproductive (R) are equal Post Reproductive (POR) is small. For example, highly industrialized nations such as Sweden and Norway. (c) Urn/pot-shaped declining population. Low % of young. If birth rate is drastically reduced, PR group becomes smaller compared to R and POR. It indicates declining population, for example, the USA/European.

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Fig. 9.2 Age pyramids Fig. 9.3 Exponential growth

dN rN =

dT

Population

Time

9.8

Rate of Growth of a Population

The simplest model of population growth is generally known as exponential growth model or geometric growth model. This type of growth occurs in an environment where resources are unlimited and is density-independent growth. Exponential growth rate was described by T.R. Malthus (Fig. 9.3).

9.8 Rate of Growth of a Population

91

Exponential growth rate rN ¼

dN dT

rN ¼ rate of change per individual, dN ¼ change of population over time, dt ¼ change of period or time. This is generally used when the intervals are too small. However, if we have to calculate the population over long intervals, then we used Exponential growth rate N t ¼ N 0 ert N0 ¼ Initial population; r ¼ Intrinsic rate of increase or exponential growth rate; t ¼ time or period; e ¼ base of the natural log ¼ 2.71828 When describing cohort life table data r¼

log eR0 T

R0 ¼ Replacement rate; T ¼ Generation time. Unfortunately, exponential growth curve did not adequately describe how population increases in reality. Under natural condition, food supplies always have an upper limit.

9.8.1

Logistic Growth

Pearl and Reed (1920) used logistic equation developed by Verhulst (1838) and derived a model to describe the growth which they called logistic growth (Fig. 9.4). dN dT

= rN (

K–N K

) K Carrying capacity

Population

Time Fig. 9.4 Logistic growth rate

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9 Characters of Population

Exponential growth K Carrying capacity

Environmental resistance Population

Logistic growth

Time Fig. 9.5 Environmental resistance

In logistic growth, the initial growth is very rapid and the rate of growth increases. As the population reaches a natural limit (K), the rate of growth decreases. Logistic growth rate ¼


 dN K
N ¼ rN dT K

r ¼ rate of increase or intrinsic rate of increase; N ¼ population; K ¼ carrying capacity of the environment and to calculate population density at time “t”. Nt ¼

K 1 þ be
rt

Nt ¼ Population density at time “t”; K ¼ Carrying capacity of the environment; r ¼ Intrinsic rate of increase; t ¼ Period of time; b ¼ Constant b¼

K
N0 N0

N0 ¼ Initial population density; K ¼ Carrying capacity when N ¼ K, then dN dT ¼ 0. If K (resource) is depleted, the population levels off at carrying capacity and a steady state is maintained. As N approaches K, K
N K will decrease and the rate of population growth will decline. Logistic model is also called Verhulst-Pearl equation. The difference between exponential growth and logistic growth represents the interaction between the biotic potential of the population and the food supply, and this difference is called as environmental resistance (Fig. 9.5).

9.9 Population Dispersal

9.9

93

Population Dispersal

Movement of individual into or out of the population is called population dispersal. Dispersal contributes to drift/infusion of new genetic material into different populations resulting in an increase in genetic heterogeneity enhances the capacity of the population to adapt to changing conditions. Dispersion takes place because of the following factors/reasons:

9.9.1

Life History Strategies

Species characterizing relatively stable infrequently disturbed habitats tends to dispense slowly (e.g., forest insects) when compared to species characterizing frequent disturbances. For example, Aphids. r-strategy (opportunistic) “r”-selected population is the one in which the maximum rate of increase “r” is important. An “r”-selected population can take advantage of a favorable situation by hang the ability to increase population size rapidly. These species are opportunists that quickly colonize new resources, but they are poor competitors and cannot persist when competition increases in stable habitats. This means many offspring which under normal circumstances die before reaching maturity but which may survive if circumstances favor. Hence, “r” selection is associated with Type III of survivorship curves. K-strategy (Equilibrium) By contrast, the K-strategy is characterized by low rates of natality and dispersal, but high investment of resources in storage and in individual offspring ensure their survival. These species are adapted to persist under stable conditions, where competition is intense, but reproduce and disperse too slowly to be good colonizers. “k”-selected population is associated with a steady carrying capacity k. “k”-selected population are less able to take advantage of particular opportunities to expand than r-selected population. K-strategists: These species rarely become pests, occur in more stable habitats, tend to have lower reproductive rates, less dispersible, and are bigger compared to that of r-strategists’ insects. Populations of k-strategists do not have wide fluctuations and usually are regulated by density-dependent mortality factors. Tsetse flies and carpenter bees are examples for k-selected insects (Matthew and Kitching 1984). K-strategists can often be managed through modification of the environment (changes in agronomic practices, destruction of alternative hosts), disruption of reproduction (e.g., sterile male technique), or through precisely targeted spray applications used in monitoring since they do not adapt quickly to change. They are in general more stable and less likely to suffer high mortality rates of immature individuals. They resemble Type I or II survivorship curves. In the nature, r-strategist is controlled by density-independent factors while k-strategist is controlled by density-dependent factors. While working with a tropical

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9 Characters of Population

helicon butterfly Heliconius ethilla, Ehrlich & Gilbert, 1973 found that H. ethilla fits the general description of a “K-Strategist” as the insects live in fairly constant environment in which resources are limiting, the insects have a Type I survivorship curve as adults, maintain a nearly constant population size, and have delayed reproduction and a long length of life for a butterfly. This is in contrast to butterflies such as Euphydryas editha, which more generally fit the description of an r-strategist. Another example of k-strategy is observed in morabines grasshopper. The population of this species contains only a few individuals (low reproduction rate) but with the ability to survive in the environment by using mimicry/camouflage on the plants where they live (Matthews & Kitching, 1984). Tsetse flies, carpenter bees, and some forest lepidoptera are examples of K-strategist species (Speight et al., 1999). Insects also tend to have strategies changing between r and k. The best studied of flexible r–K-strategy are Tribolium castaneum and Tribolium confusum (King & Dawson, 1972). These beetles have intrinsic density-regulating mechanism, including egg and pupal cannibalism by larvae and adult, reduced fecundity and fertility in aged population, and the secretion of an inhibitory quinone gas in crowded situation. Different strategies adapted by r- and k-strategist insects are outlined in Table 9.1.

9.9.2

Crowding

Crowding increases the competition for resources and may interfere with foraging or mating activity, thereby encouraging individuals to seek less crowded conditions. Crowding affects insects’ tendency to disperse. It may also stimulate morphological or physiological transformation that facilitates dispersal. Crowding in some cases may stimulate cannibalism leading to dispersal. Crowding also stimulates the morphological changes that lead to dispersal, for example, locusts.

9.9.3

Nutritional Status

The amount of food reserves (particularly fat bodies) within insect body determines the dispersal behavior. If the reserve food is depleted due to non-availability of food in the habitat, insects tend to disperse to gain more food reserves.

9.9.4

Habitat Condition

The likelihood that an insect will find a suitable patch depends strongly on patch size and proximity to insect population source. Dispersal generally takes place in three ways as follows:

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95

Table 9.1 Strategies of r and k insects Body size Population size

R-Strategist Body size small Variable

K-Strategist Body size large Constant

Short generation time High reproductive rate and often reach overshoot

Long generation time Low reproductive rate and rarely reach overshoot High survival rate, especially reproductive stages Population in equilibrium Type I or II

Population rarely able to persist successfully for long period in certain habitat

Survivorship curves Mortality Dispersal

Competition Environment

Population usually below K Type III The mortality is density-independent High level of dispersal Good colonization ability

Emigration common and recolonization high Tend to be poor competitors They live in unpredictable environment No food specialization Able to quickly discover new habitats or nomadens Examples: grasshoppers (Chortoicetes terminifera), wasp (Scelio fulgidus), bush fly Musca vetustissima

The mortality is densitydependent Low level of dispersal The population only contains a few of individuals Recolonization uncommon Good competitors or keen They usually live in a stable environment Usually, they restrict just to few plants

Examples: Morabine grasshoppers (Geckomima sp.)

1. Emigration: Movement away from source of birth. 2. Immigration: Movement into new areas or colonization of vacant areas which can be technically called as IAS (Invasive Alien Species). 3. Migration: It involves mass movement of entire population from one area to another to overcome the unfavorable condition in the native place and returning of entire population to the native place when once normal conditions return, for example, locust, monarch butterfly. Insects having a wider diet breadth or insects that are polyphagous generally tend to have more dispersal capacity as compared to the insects having shorter diet breadth or monophagous/oliphagous. Insects that are regularly prone to dispersal habits tend to produce separate morphs that exercise the option of dispersal such as winged morphs of aphids and macropterous forms of brown plant hopper, i.e., Nilaparvata lugens. The same mechanism is also observed in locust which has

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9 Characters of Population

solitary phase which feed sparsely and are not migratory which under crowded condition changes to gregarious phase capable of long-distance migration leading to locust plague. Possibility that insects disperse randomly may not be able to find a suitable habitat patch and as such most insects having migration behavior depends upon plant cues and orient themselves towards a suitable habitat patch.

9.10

Population Structure

When a population of an organism disperses to a new locality, three types of distribution of these organisms take place in the new habitat. Distribution of insects is dealt in next chapter.

9.11

Distribution Patterns

Spatial arrangement of a biological taxon is known as distribution. When a population of an organism disperses to a new locality, three types of distribution of these organisms take place in the new habitat. Spatial distribution can be termed as the by-product of environmental heterogeneity and reproductive population growth acts on random processes of movement and mortality. The information on distribution patterns of an insect will be useful in formulating an efficient sampling procedure on the basis of which an efficient management system can be adopted. (a) Distribution patterns 1. Random distribution/Poisson distribution In this case of distribution, the species are randomly distributed and there is no clear pattern, no competition and the species will occur randomly. Randomness also describes the total independence of an individual where one individual in no way influences the existence of other individual and this type of distribution is rather uncommon. Pseudo randomness is a condition in which owing to the non-availability of sufficient data the distribution is designated as pseudo random. In a randomly dispersed population, individuals neither space themselves apart nor are attracted to each other. The occurrence of one individual in a sample unit has no effect on the probability that other individuals will occur in the same sample unit. Sample densities show a skewed (Poisson) distribution. Random distribution, also called unpredictable spacing, is the least common of all types of distribution that occurs in nature. This type of distribution is generally found in homogeneous environment in which position of individual is independent of other individuals and they neither attract nor repel each

9.11

Distribution Patterns

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other. This generally happens by lack of any strong social interaction between species. 2. Uniform/Regular distribution/Competitive distribution This occurs where competition between individuals for food and space is severe, for example, forest trees. “Regularity” is a spatial condition with more equal spacing of individuals than occurs at random. Repulsion is implied by regularity. A regular (uniform) dispersion pattern occurs when individual organism occurs at an equal distance from each other most probably to reduce the competition. For example, bark beetles, antlion larvae, Helicoverpa larvae, etc. Such spacing reduces competition for resources. Also known as even distribution or evenly spaced distribution. In this case, the distance between the individuals is maximized in order to reduce the competition for space shelter and food. In case of plants, this is generally achieved by means of allelopathy. And in case of higher animals, it is generally achieved through territory marking. 3. Clumped/Aggregated/Contagious/Clustered/Social distribution The organisms are clumped or aggregated. “Aggregation” is the condition that describes a spatial condition in which density is more locally condensed than at random. Aggregation in insects can happen due to protection from natural enemies, proximity for the availability of food, or any other behavior that is useful to the insects that are aggregating. Gregarious sawfly larvae, gregarious early instar of tobacco caterpillars, hairy caterpillars, and tent caterpillars are examples of aggregated distribution. Aphids may be aggregated as a result of rapid, parthenogenetic reproduction, as well as host and habitat preferences. In clumped distribution, the distance between neighboring individuals is minimized. This type of behavior results due to • Patchy resources. • Inability of the offspring to independently move from their habitat. • Protection from natural enemies in case of herbivores insects/community hunting in case of carnivorous animals, for example, lions, hyenas, aphids, and mealybugs.

Poisson distribution represents random occurrence of event at a given time, and this model generally describes random distribution of an organism. Positive binomial distribution model best describes the regular or uniform distribution of an organism. The pattern of distribution of an organism may not be constant throughout the life cycle and may change from time to time depending on several factors such as availability of food, space, and sex of the insect species. Naylor (1961) while working with different densities of Tribolium reported that the pattern of distribution of female changed with the density and as the density of insect per unit volume of the flour increased the distribution of female started as aggregate changed to random and finally settled as uniform. This uniform distribution was attributed to the repulsive

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behavior of the female insects by the smell of grubs in the flour. However even though the same smell of grubs in the flour was repulsive to male insects, they tend to aggregate. The distribution pattern of insects also depends upon the age and developmental stage of the insect. Aphids during the nymphal stage generally have clumped or aggregate distribution. However, this clumped distribution changes to random distribution when the aphids are migrating from one place to another as these migrating aphids utilize atmospheric energy which is omnipresent (Taylor, 1984). Random distribution also occurs when migrating insects encounter a suitable patch where they alight at random on the edge of these suitable patches and start colonizing the patch and transforming themselves into aggregate distribution. For more insights into distribution patterns, the reader is advised to go through works of Taylor (1984).

9.11.1 Distribution Parameters and Their Calculations Relation of distribution parameter estimates and actual distribution of insects as expressed by various scientists is given in Table 9.2 P fx (a) Mean X ¼ n : F ¼ number of plants/branch/leaf/having x number of insects (frequency), x ¼ number of insects per plant/branch/leaf, n ¼ Total number of plants   ð∑fxÞ2 ∑f x2 2 n 2 (b) Variance S = n21 2

S (c) Variance mean ratio ¼ VMR ¼ Mean

S2 X 2

S (d) Index of David and Moore (IDM) ¼ Mean
1 pffiffiffiffi qffiffiffiffiffiffiffiffi 2 2 S S (e) Index of Lexis ¼ Mean χ¼ X qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi pffiffiffiffiffiffiffiffiffi ðS2
MeanÞ S2
X (f) Charlier coefficient ¼ Mean X

Table 9.2 Relation between distribution parameter and distribution of insects

Variance (s2) Variance mean ratio (VMR) IDM Index of Lexis Charlier Coefficient

Aggregate or clumped Variance > Mean 1

Uniform or regular Variance < Mean