POPULATION BIOLOGY OF TROPICAL INSECTS

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1 POPULATION BIOLOGY OF TROPICAL INSECTS

2 POPULATION BIOLOGY OF TROPICAL INSECTS ALLEN M. YOUNG Milwaukee Public Museum Milwaukee, Wisconsin PLENUM PRESS NEW YORK AND LONDON

3 Library of Congress Cataloging in Publication Data Young, Allen M. Population biology of tropical insects. Bibliography: p. Includes index. I. Insect populations - Tropics. 1. Title. QL Y '05248'0913 ISBN-13: e-isbn-13: : / AACR Plenum Press, New York Softcover reprint of the hardcover 1st edition 1982 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N.Y All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

4 "Internarse en un bosque es reconciliarse con la vida. El asombro resbala de hoja en hoja. La fecundidad telurica palpita y destella, como una libelula dorada, en los oblicuos rayos solares que se filtran por entre los densos ramajes."... From "Bosques Y Vagabundos," by Carlos Luis Altamirano La Nacion (seccion Ancora), San Jose, Costa Rica November 12, 1978

5 PREFACE In this book I have tried to bring together the major developments in the study of insect populations in tropical environments. In some ways, this task has been a difficult one because conceptually it is virtually impossible to limit a discussion of insect ecology to the tropics, since the same concepts, theories, and hypotheses concerning the mechanisms by which habitats support insect populations often apply both to temperate and to tropical regions. Thus one might argue effectively that a book such as Peter Price's Insect Ecology represents a more comprehensive treatment of insect ecology, including the tropical aspects. Yet because there has been a tremendous amount of new study on insects in the tropics in recent years, and because there has also been a strong historical interest in tropical insects, judging from early museum expeditions and medically and agriculturally oriented studies of insects in the New and Old World tropics, I believe there is a place for a book dealing almost exclusively with tropical insects. But logically so, such a book by necessity incorporates data and information from Temperate Zone studies, if for no other reason than because insights into the properties of tropical environments often emerge from compariso'ns of species, communities, or faunas between temperate and tropical regions. An understanding of insect populations in the tropics cannot be divorced from a consideration of Temperate Zone populations. The book is unabashedly biased to the New World tropical studies because of my interests and more proximal awareness of research on insects being conducted in this region. But a valid attempt is made to incorporate research studies from the Old World tropics. The basic mission of this book is to bring together various concepts and studies related to the issue of explaining the spatial and temporal patterns of insect diversity in the tropics, considering the breeding population of a species to be the major ecological unit beyond the individual within the population. The population is interacting with both biotic and physical components of the environment, within and between habitats, and regionally. Much of tropical insect population biology is therefore concerned in different ways with describing patterns of distribution of individual species in relation to resources and other environmental factors. There is also a need for detailed descriptive data on the natural history of vii

6 viii PREFACE individual species in particular habitats in the tropics. What are some of the major attributes underlying observed patterns of distribution? Can we recognize the roles of biotic control agents in influencing the populations of insect species as well as the role of abiotic factors? We recognize that the tropical regions of the world are extremely heterogeneous in terms of climatic factors and the patterns of fluctuation of these factors. The kinds of selection pressures affecting an insect species on a mountain top in the tropics may greatly differ from those affecting the same species or a closely related species in an adjacent lowland area. Within major climatic areas, varying subclimatic conditions will be arising from localized weather patterns and effects of topography and kind of vegetation cover. Selection pressures on many insect species may be very different in a strongly seasonal region than in a relatively nonseasonal tropical region. The fine tuning of insects and their resources, as exemplified by many plant-insect interactions in the tropics, may reflect coevolved interactions between insects and their resources, and in recent years there has been a great interest in defining the traits or properties of such associations. Emerging from such work is the concept of the individual green plant as an extremely heterogeneous resource for insects, in the sense that defense systems against such herbivores vary among different parts of the plant and as a result of highly localized environmental conditions at anyone point in time and changes in the environment through ecological and evolutionary time. Herbivore pressure may be a function of the ability of insect species to adapt to the defensive chemistry of plants selected as hosts. Herbivore pressure is also a function of (1) diurnal or seasonal cycles in defensive chemistry metabolism and (2) the degree to which herbivores synchronize feeding with phases of low concentration of defensive compounds in specific plant structures targeted as food. Much of our understanding of insect-plant interactions comes from studies in the tropics. Tropical insect studies also point to the need to consider the success of an insect species in a particular habitat or geographical region to be a function of behavioral and physiological flexibility instead of evolutionary adaptations, i.e., genetic variation in populations. Neotropical species of Drosophila may be capable of feeding on a variety of qualitatively different kinds of food not so much because populations of these flies are heterogeneous genetically, but because behavioral flexibility enables individual flies to effectively exploit the range of available food types or breeding sites. Understanding the occurrence of regions in the tropics of high species richness for insect taxa is a function of the analysis of historical (biogeographical) events and ecological factors. In recent years there has been increased study of forest refugia in Central and South America in relation to periods of recent glaciation, and such studies have increased our understanding of why pockets of high species richness are surrounded by areas of relatively low richness. Ecological factors, including the occurrence of natural and human-induced changes in habitats in the tropics-the latter through agriculture, lumbering, and population

7 PREFACE ix expansions-also shape the distribution of insect species and the kinds of resources they exploit in a particular area. An attempt is made to examine the properties of insect populations in such man-made environments as well as the effects of natural succession on species. Basic research studies on the mechanisms of insect-plant interactions in the tropics provide mankind with new information about the unique and regulatory properties of natural products, data that have the potential to be translated into applied use through biotechnology as related to the development of naturally based repellents, insecticides, and fungicides. Tropical floras represent a millieu of novel and diverse mechanisms of chemical defense against herbivores, and the insects exploiting such resources have developed detoxification enzyme systems and other means of countering the defenses of plants. Such interactions provide a basis for the development of new and effective methods of control for pests and pathogens associated with Temperate Zone crops and forests. The study of tropical insects and their interactions with their resources has great potential to the application of natural products to new research in agriculture, horticulture, pharmacology, etc. But the first step in reaching such applications is to determine what is out there, to examine the patterns of distributions of species or groups of species and genera, to find out what causes the observed patterns, and in the case of insect-plant interactions to elucidate the mechanisms holding such relationships together and their genetic, physiological, or behavioral basis. I have attempted to summarize a bulk of the pertinent literature on tropical insect population biology, with an emphasis on research done within the last 50 years. This book can serve as a helpful starting point for undergraduate and firstor second-year graduate students with interests in ecology or environmental biology. I wish to thank all of the biologists who consented to copyright permissions from their publications. Mary Joan Young offered encouragement during the preparation of the manuscript. Dr. Theresa A. Noeske provided painstaking technical assistance. Karen Heerhold did an excellent job of typing the final draft of the manuscript. The guidance and cooperation of Ellis Rosenberg and his staff at Plenum are also appreciated. Gerald R. Noonan reviewed Chapter 10. I am grateful for the encouragement and strong vote of confidence given to me by my good friend Oskar R. Zaborsky with the initial plan to write this book. My own field work experience in the tropics covers a 12-year period, with support from the Organization for Tropical Studies, Inc., the Associated Colleges of the Midwest and Lawrence University, the National Science Foundation, Friends of the Milwaukee Public Museum, and the American Cocoa Research Institute. I owe a great deal to these people and sources of funding for making my field work possible. Milwaukee Allen M. Young

8 CONTENTS 1. The Faunistic Richness of Insects in the Tropics: A Brief Overview 1 2. Individual and Population Responses to Environments Ecological Response Natural Selection Some Properties of Populations Ecological Diversity and the Tropics Some Mechanisms Underlying Tropical Insect Density Strategies of Response to the Environment Natural Selection and Properties of Populations Environmental Predictability and Response Patterns Machinery of Environmental Response Mechanisms in Insects: Key to Evolutionary and Ecological Diversification Notes on Insect Diversity Comments on Insect Physiology Integration of the Insect Phenotype by Hormonal Systems: A Brief Review Diapause and Migration in Insect Populations: Escape in Time and Space Temperature, Photoperiod, and Other Factors Related to Insect Activity: Further Comments Notes on Mating and Communication in Insects Rhythmicity in Tropical Insects Sociality and Defense in Social Insects Density-Related Behavior in Insects Notes on the Physiology of Feeding in Insects 114 xi

9 xii CONTENTS 4. Ecological Aspects of Plant Defenses against Insects Plant Defenses and Coevolution: An Overview Evolution of Unpalatability and Implications for Food Chains Notes on Defoliation in the Tropics Notes on Seed Predation in Tropical Habitats Notes on Plant-Ant Interactions in the Tropics Distribution Patterns of Insects in Tropical Habitats The Concept of Local Adaptation Examples of Habitat Differentiation in the Tropics Insect Communities in Tropical Habitats Plant Resources and Habitat Selection in Tropical Insects Disturbed and Undisturbed Habitats: Some General Effects Population Responses to the Environment in Tropical Insects Adaptation to Habitats Demography and Fitness in PopUlations: Implications for Tropical Insects Further Notes on Population Regulation Some Examples of Census Histories in Tropical Insects Population Responses to Environmental Heterogeneity in Tropical Insects Competition as a Selective Force in PopUlation Responses Spatial Environmental Heterogeneity and Demes in the Tropics Effects of Seasonality on Insect Populations in the Tropics Temperate Zone and Tropical Seasonality Some Regional Patterns of Seasonality in the Tropics Some Seasonal Distributions of Tropical Insects Molding of Ecological Traits in Tropical Insects by the Dry Season

10 CONTENTS xiii 8. Dynamics of Organization of Insect Communities in Tropical Ecosystems Notes on Tropical Ecosystems Patterns of Organization in Tropical Forests: Some Comments Notes on the Dynamics of Tropical Insect Communities ,4. Some Ecological Factors Generating Patchy Distributions of Insects in Tropical Habitats Insect Species in Agricultural Habitats in the Tropics Plant-Insect Interactions and Tropical Agricultural Habitats: Some Comments Insect Pollination in an Agricultural Habitat in the Tropics: The Case of Cacao Grazing and Related Activities by Insects in Agricultural Habitats in the Tropics Tropical Insect Pest Populations Biogeographical and Regional Evolutionary-Ecological Effects on the Maintenance of Tropical Insect Faunas: A Brief Perspective Historical Events and Ecological Processes Altitudinal and Other Macrogradients in Tropical Regions Notes on Altitudinal Gradients in the Tropics Biogeographical Patterns of the Origins of Tropical Insect Faunas Some Mechanisms of Ecological and Evolutionary Differentiation Some Examples of Differential Adaptation to Tropical Lowland and Montane Areas by Insects References Index 495

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