Insect-Plant Interactions (1990)

Volume II

Elizabeth A. Bernays

• Publisher: CRC Press
• Year: 2017
• eBook ISBN: 9781351361439
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Insect-Plant Interactions is a series devoted to reviews across the breadth of the topic from cellular mechanisms to ecology and evolution. Articles are selected from areas of particular current interest or subjects that would especially benefit from a new review. It is hoped that the interdisciplinary selection in each volume will help readers to enter new fields of insect-plant interactions.

Volume II contains six very different articles.

1 Insect Herbivore Population Dynamics on Trees and Shrubs: New Approaches Relevant to Latent and Eruptive Species and Life Table Development

Peter W. Price, Neil Cobb, Timothy P. Craig, G. Wilson Fernandes, Joanne K. Itami, Susan Mopper and Ralph W. Preszler

Department of Biological Sciences

Northern Arizona University

Flagstaff, Arizona

TABLE OF CONTENTS

• I. Introduction
• II. Life Table Analysis in Retrospect
  • A. The Sampling Problem
  • B. The Plot Problem
  • C. The Experimental Problem
  • D. The Mortality-Natality Problem
• III. Latent Species — Galling Sawflies on Willows
• IV. Phylogenetic Constraints, Adaptive Syndromes, and Emergent Properties: Differences Between Latent and Eruptive Species
  • A. An Example of a Latent Species
  • B. An Example of an Eruptive Species
  • C. General Cases for Latent and Eruptive Species
• V. Reinterpretation of Life Table Studies
  • A. An Evolutionary Perspective
  • B. Female Behavior
  • C. Resource Quantity and Quality
  • D. Weather
  • E. Competition
  • F. Natural Enemies
  • G. Natality or Mortality?
• VI. Conclusions
• Acknowledgments
• References

I. Introduction

The development of life tables and their analyses so dominated the field of insect population dynamics in the 1960s that the approach may be regarded as a paradigm: the prevalent model in the field which frames the way in which we view the natural world. We examine, here, the validity of the life table approach, and what we have learned from several decades of its use. In 1989 the first life table for understanding natural populations of insects and their dynamics, developed by Morris and Miller,¹¹³ was 35 years old (see Reference 37 for earlier life table developments). Analytical approaches for a series of life tables began to appear in 1959.^110,167 This method for studying natural populations became widely adopted, and was a force which developed enormous research energy in the 1960s. Insect population dynamics became a major focus in ecology, and a flush of books soon reflected this interest (e.g., References 24, 38, 111, 159, and 169).

With the advantage of hindsight we can now evaluate the contribution this research has made to understanding insect population dynamics. This is important because the approach is still used for studying major insect pests in agriculture and forestry, for investigating herbivores to be used in biological weed control projects, and for the better conservation of rare and endangered species (e.g., References 48, 61, 84, and 170). In addition, millions of research dollars were expended on the often large projects which undertook life table construction for many insect generations, so it is appropriate for planners and research directors to understand the quality of research yielded and its long-term impact on progress of understanding. Finally, the large body of literature on life tables and analysis has shaped the way in which we now view nature and where we place research emphasis.

We develop the argument that life table construction emphasized mortality in populations, and the method dictated the kinds of results, and conclusions reached, in insect population dynamics studies. The important influences of natality and female oviposition behavior in response to plant quality were frequently overlooked. Female oviposition behavior is phylogenetically constrained and acts as an ultimate factor profoundly influencing the adaptive syndrome of the species, and the proximate ecological factors that are commonly studied in insect life table analysis. Therefore, a radical shift in perspective is needed to place the evolutionary history of a species in a central causal position in order to understand the emergent ecological phenomena involved with population change. Such a shift also requires a revision of life tables to better evaluate natality effects, female oviposition behavior, and the role of plant quality. We use these approaches to identify the fundamental differences between latent and eruptive species of insect herbivores.

We emphasize in this review the gathering of the empirical data and the construction of life tables. We do not stress the theoretical basis of forest insect population dynamics which has been reviewed effectively in the recent past (e.g., References 8, 12, 15, 16, 18, 66, and 92).

We use an historical perspective to consider what we know about insect population dynamics, and what contributions the field of evolutionary ecology can make. We then discuss modifications or alternative approaches that might increase our understanding of population dynamics and foster new research directions or emphases. We argue that a synthesis of several areas of evolutionary ecology with a modified life table approach may change our perceptions of how insect population dynamics should be studied and the major factors driving population change.

Our perspective is colored by the organisms we have studied over the past 10 years. Of particular interest is the shoot-galling sawfly, Euura lasiolepis, which attacks arroyo willow, Salix lasiolepis, and the sawfly’s relatives. This sawfly is hardly regarded as a pest; it is not eruptive or an outbreak species, and has shown remarkably stable populations over the last decade in our study sites. It is perhaps an apparent paradox that we claim to shed new light on pest insect population dynamics by having considered species with stable populations. However, we argue that a strong comparative approach among species with stable populations and those with eruptive populations can further our understanding of the mechanisms involved with both kinds of insect populations.

The eruptive and noneruptive species are only two ends of a continuum of variation in natural insect herbivore populations, but for simplicity we will concentrate on these extremes. Therefore, a few definitions are needed to clarify the terms we use.

A latent species refers to a species with typically latent populations, and here is defined as an insect herbivore species that remains at steady population densities, varying between one or two orders of magnitude, and usually incapable of increasing explosively to cause heavy damage to the host plant population (synonyms; nonout-break,^89,171 nonpest, noneruptive). The key characters are steady densities with low levels of damage to host plant populations. Species are not necessarily rare, or at low densities.

An eruptive species refers to a species with eruptive populations, and here is defined as an insect herbivore species that has both an endemic phase of low density and low damage to the host-plant population, and an epidemic phase in which populations become dense and damaging, with fluctuations ranging over three to five orders of magnitude (synonyms: cyclic,^17,118 irruptive,¹⁷¹ outbreak,¹⁷¹ pest).

Endemic — the low-density, nondamaging phase in an eruptive species often associated with populations locally distributed in pockets or foci in particularly favorable host plant groups or populations.

Epidemic — the high-density, damaging phase in an eruptive species frequently associated with either rapid spread of populations or an apparent spread resulting from rapid in situ increases from very low populations.

We prefer to use the terms latent and eruptive species for several reasons. Latent and eruptive are descriptors of species type and therefore more helpful than such designations as nonoutbreak vs. outbreak, or nonpest and pest. The term latent implies the potential to increase to high densities, a possibility we will discuss later, but in most places for most of the time we argue that such species are so constrained that the latent state will prevail. In our term eruptive we include all of the outbreak types recognized by Berryman and Stark¹⁷ and Berryman,¹³ so our use is much broader simply because we wish to emphasize extremes in population types, while they were concerned with classifying kinds of outbreaks. These terms also permit the classical epidemiological usage of endemic and epidemic for phases in an eruptive species’ population change.

Use of the term steady population density in the definition of a latent species needs some clarification. We imply here changes in density of only one or two orders of magnitude between the highs and lows in population size over several years. This contrasts with many eruptive species which commonly show density changes of three to five orders of magnitude (e.g., References 100, 118). We do not imply stability in the long term, over decades, which is usually used in ecology to denote a stable equilibrium with low variance around an average population size.⁹⁵

Important contributions to understanding differences between latent and eruptive species have been made recently by Rhoades,¹⁴⁸ Nothnagle and Schultz,¹²⁰ Mason,⁸⁹ Wallner,¹⁷¹ and Myers.¹¹⁸ Each had a slightly different focus with Rhoades being very general, Mason emphasizing latent forest Lepidoptera, Wallner contrasting latent and eruptive species, and Myers, and Nothnagle and Schultz concentrating on eruptive forest Lepidoptera. These publications mark an important surge in synthesis, and together with Barbosa and Schultz⁸ may well mark a new phase in reaching generalizations about latent and eruptive species.

On a less positive note these authors do not claim any great success in reaching a synthesis. They reinforce the view that we still have much to learn, generalities are hard to achieve, and little really definitive understanding of insect herbivore population dynamics is available. Mason (Reference 89, p. 52) declared that “There is a surprising lack of basic biological information on the nonoutbreak defoliators, and virtually nothing is known about their population dynamics.” Wallner¹⁷¹ categorized nonoutbreak species as K-selected and rare (his Table 1) and the extreme form of outbreak species as r-selected and eruptive, but Nothnagle and Schultz¹²⁰ maintained that current knowledge was too incomplete to assess such a categorization. They said, “Although much is known about some irruptive pest species, the majority of the nonirruptive species remain virtually uncharacterized in any meaningful way. Hence, comparing the biological traits of pest and nonpest species is very difficult” (p. 60). Myers (Reference 118, p. 231) focused on disease as a major factor in cycles while admitting “I may simp...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Insect Herbivore Population Dynamics on Trees and Shrubs: New Approaches Relevant to Latent and Eruptive Species and Life Table Development
• Chapter 2 The Role of Experience in Host Plant Choice by Phytophagous Insects
• Chapter 3 Phenolglucosides and Interactions at Three Trophic Levels: Salicaceae — Herbivores — Predators
• Chapter 4 Learning and Flower Use in Butterflies: Hypotheses from Honey Bees
• Chapter 5 The Mechanisms of Nutritional Compensation by Phytophagous Insects
• Chapter 6 Mother Doesn’t Know Best: Selection of Hosts by Ovipositing Insects
• Index