Insect Herbivore Populations Research Articles

The role of age‐structure on the persistence and the dynamics of insect herbivore–parasitoid interactions

The role of top‐down (e.g. parasitism) and bottom‐up (e.g. resource competition) processes is of fundamental importance for the stability and persistence of insect herbivore populations. Although emphasis has often focused on single regulatory agents, the processes underpinning tri‐trophic interactions may actually be more pluralistic. Recently, further complexities involved in the regulation of tri‐trophic systems have been highlighted. In particular, life history characteristics may have a concomitant role when coupled with the regulatory effects of resource competition and/or parasitism. Here we present an age‐structured model to investigate the effects of larval development period, parasitism and resource competition on the stability and persistence of herbivore–parasitoid interactions. We show that the influence of weak density dependent parasitism is sufficient to stabilise the interaction when the period of host susceptibility to parasitism is short. For longer periods of host susceptibility, parasitism needs to be highly non‐linear to overcome the destabilising effects of the time delays. In systems where host development is protracted through the season we predict that resource competition is likely to be the dominant process for herbivore regulation. We use this age‐structured approach to explore the population dynamics of two field studies from temperate ecosystems. Predictions from these case studies show 1) that both the strength and type of competition and parasitism are important for the stability and persistence of the particular system, and 2) that the length of the developmental period of the vulnerable host is critical to understand the influence of different regulatory processes. Host demography is of overriding importance in determining whether herbivores show outbreaks, and which particular ecological processes and mechanisms are responsible for generating such overcompensatory dynamics.

The role of resources and natural enemies in determining the distribution of an insect herbivore population

Summary1. Both resources and natural enemies can influence the distribution of a herbivore. The ideal free distribution predicts that herbivores distribute themselves to optimise utilisation of resources. There is also evidence of herbivores seeking out refuges that reduce natural enemy attack (enemy‐free space). Which of these theories predominates in a thistle–tephritid Terellia ruficauda (Diptera: Tephritidae)–parasitoid interaction is examined.2. The plant, Cirsium palustre, had a contagious distribution approximated by the negative binomial distribution. Terellia ruficauda foraged preferentially and oviposited on isolated plants although its larvae gained neither nutritional benefit nor reduced natural enemy pressure from such behaviour.3. Parasitoids of T. ruficauda foraged and oviposited more frequently on isolated than on crowded T. ruficauda, resulting in inverse density‐dependent parasitoid attack at all spatial scales examined. Neither the herbivore nor natural enemies distributed themselves according to the predictions of the ideal free distribution and the herbivore did not oviposit to reduce natural enemy attack.4. Extrapolating from the theoretical predictions of the ideal free distribution and enemy‐free space to the field requires considerable caution. Terellia ruficauda and its parasitoids appear to select their oviposition sites to spread the risk of losses through factors (e.g. mammal herbivory) that may damage dense clusters of C. palustre.

SELECTION ON HERBIVORE LIFE-HISTORY TRAITS BY THE FIRST AND THIRD TROPHIC LEVELS: THE DEVIL AND THE DEEP BLUE SEA REVISITED

The timing of life-history events in insects can have important consequences for both survival and reproduction. For insect herbivores with complex life histories, selection is predicted to favor those combinations of traits that increase the size at metamorphosis while minimizing the risk of mortality from natural enemies. Studies quantifying selection on life-history traits in natural insect herbivore populations, however, have been rare. The purpose of this study was to measure phenotypic selection imposed by elements of the first and third trophic levels on variation in two life-history traits, the timing of egg hatch and pupal mass, in a population of oak-feeding caterpillars, Psilocorsis quercicella (Lepidoptera: Oecophoridae). Larvae were collected from the field throughout each of two generations per year for three years and reared to determine the effects of the date of egg hatch on both the risk of attack from parasitoids and the pupal mass of the survivors. The direction and strength of phenotypic selection attributed to aspects of the first and third trophic levels, as well as their combined effects, on the date of egg hatch was measured for each of the six generations. Heritabilities of and genetic correlations between pupal mass and the date of adult emergence from diapause (the life-history trait expected to have the largest influence on the timing of egg hatch, and thus larval development) were estimated from laboratory matings. In four of the six generations examined, significant directional selection attributed to the first trophic level was detected, always favoring early-hatching cohorts predicted to experience higher leaf quality than late-hatching cohorts. Directional phenotypic selection by the third trophic level was detected in only one of three years, and in that year the direction of selection was in opposite directions during the two successive generations. The combined effect of selection by both trophic levels indicated that the third trophic level acted to either reduce or enhance the more predictable pattern of selection attributed to the first trophic level. In addition, I found evidence of truncation selection acting to increase the mean and decrease the variance of pupal mass during the pupa-adult transition in the laboratory. Pupal mass and diapause duration were found to vary significantly among full-sibling families; upper bounds for heritability estimates were 0.57 and 0.30, respectively. Furthermore, these two traits were found to be positively genetically correlated (families with larger pupae had longer diapause durations), resulting in a fitness trade-off, because larger pupae enjoy higher survival through metamorphosis and female fecundity but emerge later, when average leaf quality for offspring is generally poorer.Corresponding Editor: J. Conner

Modelling herbivore movement and colonization: pest management potential of intercropping and trap cropping

Abstract1 Using a stochastic simulation model, we explored the effects of agroecosystem diversity on herbivore densities.2 Using parameters that included reproduction, colonization, and local movement rates, we simulated an insect herbivore population colonizing rows of plants in an agricultural setting.3 Plant rows were comprised of either principal crop, intercrop, or trap crop. Herbivore parameters varied for different plant types.4 Percent crop cover and movement rates were varied, and ensuing herbivore densities on crop rows were recorded.5 In trap cropping schemes, both percent crop cover and movement rates were critical in determining herbivore densities. Intercropping schemes were governed primarily by colonization rates.6 These results suggest that trap cropping schemes merit more attention than intercropping systems in the design and analysis of mixed cropping systems.

The Influence of Plant and Herbivore Characteristics on the Interaction between Induced Resistance and Herbivore Population Dynamics.

Induced plant resistance may contribute to regulating or driving fluctuations in insect herbivore populations. However, experimental demonstrations of induced resistance affecting long-term herbivore population dynamics are lacking, and few models find that induced resistance drives cycles in herbivore populations. Here a simulation model is used to explore the influence of characteristics of the plant-herbivore system on the likelihood that induced resistance can regulate or drive cycles in herbivore populations. Results of this model suggest that induced resistance may cause fluctuations in herbivore populations under more conditions than previously thought. The model incorporates parameters for the timing and strength of induced resistance and for herbivore mobility and host-plant selectivity. Results are presented for two configurations of the model: forest (many herbivore generations per plant generation) and crop (few herbivore generations per plant generation). In simulations of this model, induced resistance in the absence of other density-dependent factors can regulate herbivore populations. Induced resistance can also drive fluctuations in herbivore populations when there is a time lag between damage and the onset of induced resistance. The time lag required to cause fluctuations depends on characteristics such as the strength of induced resistance and the mobility of the herbivore.

The Influence of Plant and Herbivore Characteristics on the Interaction between Induced Resistance and Herbivore Population Dynamics

Induced plant resistance may contribute to regulating or driving fluctuations in insect herbivore populations. However, experimental demonstrations of induced resistance affecting long‐term herbivore population dynamics are lacking, and few models find that induced resistance drives cycles in herbivore populations. Here a simulation model is used to explore the influence of characteristics of the plant‐herbivore system on the likelihood that induced resistance can regulate or drive cycles in herbivore populations. Results of this model suggest that induced resistance may cause fluctuations in herbivore populations under more conditions than previously thought. The model incorporates parameters for the timing and strength of induced resistance and for herbivore mobility and host‐plant selectivity. Results are presented for two configurations of the model: forest (many herbivore generations per plant generation) and crop (few herbivore generations per plant generation). In simulations of this model, induced resistance in the absence of other density‐dependent factors can regulate herbivore populations. Induced resistance can also drive fluctuations in herbivore populations when there is a time lag between damage and the onset of induced resistance. The time lag required to cause fluctuations depends on characteristics such as the strength of induced resistance and the mobility of the herbivore.

ISOLATING A ROLE FOR NATURAL SELECTION IN SPECIATION: HOST ADAPTATION AND SEXUAL ISOLATION IN NEOCHLAMISUS BEBBIANAE LEAF BEETLES.

Muller (1942) and Mayr (1963) hypothesized that natural selection indirectly causes the evolution of reproductive barriers between allopatric populations by causing adaptive genetic divergence that pleiotropically promotes prezygotic or postzygotic incompatibility. Under this mechanism, herbivorous insect populations should be more prone to speciate if they are adapting to different host plants, because the evolution of reproductive isolation will be accelerated above the rate promoted by genetic drift and host-independent sources of selection alone. Although the Muller-Mayr hypothesis is widely accepted, little direct evidence has been collected in support of selection's role in allopatric speciation. This paper offers a method for isolating and evaluating the contribution of host plant-related natural selection pressures to the reproductive isolation between allopatric herbivore populations. The host-related selection hypothesis (HRSH) predicts that herbivore populations using different host plants should be more reproductively isolated than those using the same host, other things being equal. Here, I test this hypothesis using Neochlamisus bebbianae, an oligophagous leaf beetle with a geographically variable host range. In each of two sets of experiments (contrast I, contrast II), I compared two beetle populations (Georgia and New York) that use the same host (Acer) in nature and a third population that natively uses a different host (Betula in Oklahoma [CI], Salix in Ontario [CII]). Experiments showed that "different-host" populations were more strongly differentiated in host-use traits (oviposition, host fidelity, feeding response, larval performance) than were "same-host" populations and that each population most readily uses foliage from its native host. As predicted by the HRSH, sexual isolation was also greater between the adaptively divergent different-host populations (from Betula vs. Acer, from Salix vs. Acer) than between the same-host populations (from Acer), which were undifferentiated in host-use traits. Interpreting these results in a historical context provided by mtDNA sequences from test populations indicated: (1) that Acer- and Betula-associated N. bebbianae represent separate sibling species whose causal origins have been lost to history, and whose incomplete sexual isolation is fortified by host-associated ecological and "physiological" isolation; and (2) that incipiently speciating Acer- and Salix-associated populations are more closely related to each other than are the two Acer-associated populations, which is consistent with the HRSH. This study thus illustrates the consequences of host-related selection for both the origin and maintenance of reproductive isolation. More important, it provides evidence that the pleiotropic effects of natural selection promote allopatric speciation.

Impact of diet quality on demographic attributes in adult grasshoppers and the nitrogen limitation hypothesis

1. Various formulations of the nutrient stress hypothesis predict that insect herbivore populations will respond positively to increased nutrient quality of host plants, especially dietary nitrogen. Survival and reproduction by adult females of two grasshopper species [Acrididae;Melanoplus sanguinipes (Fabricius) and Phoetaliotes nebrascensis (Thomas)] were evaluated in response to defined diets that varied factorially in both total nitrogen (1–7%) and total soluble carbohydrate (4.3–26.7%). These grasshopper species coexist naturally but are typically shifted phenologically so that specific developmental stages normally encounter host plants of different nutritional quality under natural conditions.2. Demographic responses by adult females of both species varied according to diet quality, but not in the same fashion. Diet quality affected survival significantly in P. nebrascensis but not in M. sanguinipes. Survival in P. nebrascensis was greatest on diets containing the lowest nitrogen concentrations; carbohydrate level had no effect.3. Diet quality influenced reproduction significantly in both species. Egg production rate (eggs/day) in M. sanguinipes exhibited a negative linear response to increased carbohydrate, coupled with a significant quadratic response to nitrogen that reached a maximum at an intermediate level of about 4% total N. A significant quadratic response to total N for pod production rate (indicating the timing of reproduction) was also observed. Clutch size in M. sanguinipes exhibited a negative relationship with total carbohydrate in the diet, but no response to nitrogen. No interaction was observed between nitrogen and carbohydrate levels. For P. nebrascensis, response to diet quality was much weaker, with only a suggestive maximum at 4% total‐N for both egg production rate (eggs/day) and clutch size (eggs/pod) and a suggestive linear response for pod production rate as carbohydrate level increased. Female body weight did not contribute to any reproductive response as a covariate variable.4. Combined with a similar, previous analysis of demographic responses by the grass‐feeding grasshopper, Ageneotettix deorum, these results challenge the ability to draw generalizations about host plant nutritional quality and grasshopper demographic responses. These three grasshopper species respond quite differently to defined diets that vary in total nitrogen and carbohydrate levels. Thus, although host plant quality can contribute significantly to grasshopper population responses, a uniform explanation is not likely.

Novel chitinolytic enzymes with biological activity against herbivorous insects

The soil bacteria, Streptomyces albidoflavus, secretes endochitinases and chitobiosidases that are active over a broad range of pH (4–10). Ingestion of this mixture of chitinolytic enzymes significantly reduced the growth and development of Trichoplusia ni and significantly reduced survival of Myzus persicae, Bemisia argentifolii, and Hypothenemus hampei. Perfusion chromatography was used to separate endochitinases from chitobiosidases. The endochitinases had significantly greater biological activity against Bemisia argentifolii than the chitobiosidases. The utility of chitinolytic enzymes as regulators of populations of herbivorous insects is discussed.

Higher-order predators and the regulation of insect herbivore populations.

Empirical research has not supported the prediction that populations of terrestrial herbivorous arthropods are regulated solely by their natural enemies. Instead, both natural enemies (top-down effects) and resources (bottom-up effects) may play important regulatory roles. This review evaluates the hypothesis that higher-order predators may constrain the top-down control of herbivore populations. Natural enemies of herbivorous arthropods generally are not top predators within terrestrial food webs. Insect pathogens and entomopathogenic nematodes inhabiting the soil may be attacked by diverse micro- and mesofauna. Predatory and parasitic insects are attacked by their own suite of predators, parasitoids, and pathogens. The view of natural enemy ecology that has emerged from laboratory studies, where natural enemies are often isolated from all elements of the biotic community except for their hosts or prey, may be an unreliable guide to field dynamics. Experimental work suggests that interactions of biological control agents with their own natural enemies can disrupt the effective control of herbivore populations. Disruption has been observed experimentally in interactions of bacteria with bacteriophages, nematodes with nematophagous fungi, parasitoids with predators, parasitoids with hyperparasitoids, and predators with other predators. Higher-order predators have been little studied; manipulative field experiments will be especially valuable in furthering our understanding of their roles in arthropod communities.

Estimating the relative roles of top-down and bottom-up forces on insect herbivore populations: a classic study revisited.

Although most ecologists agree that both top-down and bottom-up forces (predation and resource limitation, respectively) act in concert to influence populations of herbivores, it has proven difficult to estimate the relative contributions of such forces in terrestrial systems. Using a combination of time-series analysis of population counts recorded over 16 years and experimental data, we present the first estimates of the relative roles of top-down and bottom-up forces on the population dynamics of two terrestrial insect herbivores on the English oak (Quercus robur). Data suggest that temporal variation in winter moth, Operophtera brumata, density is dominated by time-lagged effects of pupal predators. By comparison, spatial variation in O. brumata density is dominated by host-plant quality. Overall, top-down forces explain 34.2% of population variance, bottom-up forces explain 17.2% of population variance, and 48.6% remains unexplained. In contrast, populations of the green oak tortrix, Tortrix viridana, appear dominated by bottom-up forces. Resource limitation, expressed as intraspecific competition among larvae for oak leaves, explains 29.4% of population variance. Host quality effects explain an additional 5.7% of population variance. We detected no major top-down effects on T. viridana populations. An unknown factor causing a linear decline in T. viridana populations over the 16-year study period accounts for most of the remaining unexplained variance. We discuss the observed differences between the insect species and the utility of time-series analysis as a tool in assessing the relative importance of top-down and bottom-up forces on herbivore populations.

The impact of weed diversity on insect population dynamics and crop yield in collards, Brassica oleraceae (Brassicaceae).

Vegetational diversity within agricultural fields is often suggested as a means to reduce insect herbivore populations and to increase their natural enemies. In this paper we compare population densities of herbivores, predators, and parasitoids on collards in monocultures and on collards interplanted with two different groups of weeds, one with weed species from the same plant family as the collards (Brassicaceae) and one with weed species from unrelated plant families (non-Brassicaceae). The collards in the Brassicaceae weed polyculture had higher densities (number of herbivores/mean leaf area (cm2) per plant) of specialist herbivores than collards in the non-Brassicaceae weed polyculture and in collard monoculture. The "resource concentration" hypothesis is supported by the observation of higher populations of Phyllotreta spp., acting as facultative polyphages, in the Brassicaceae weed polyculture than in the non-Brassicaceae weed polyculture where Phyllotreta spp. are facultative monophages. Population densities of natural enemies (mostly coccinellids, carabids, and staphylinids) were higher in the polycultures than in the monoculture: carabid and staphylinid predators may be responsible for larval mortality in the imported cabbage worm, Pieris␣rapae, and in the diamondback larvae, Plutella xylostella. In spite of differences in densities of specialist herbivores across treatments, crop yield, leaf area (cm2), the proportion of leaf area damaged, and the number of leaves undamaged did not differ. These findings suggest that plant competition may interfere with attempts to reduce herbivore damage. We conclude that the use of weedy cultures can provide effective means of reducing herbivores if the crop and weed species are not related and plant competition is prevented.

Evolution of host-selection behaviour in insect herbivores: genetic variation and covariation in host acceptance within and between populations of Choristoneura rosaceana (Family: Tortricidae), the obliquebanded leadfoller

Populations of insect herbivores exploiting habitats that differ in host species composition may experience selection for divergent host-selection behaviour. To assess potential genetic constraints on the evolution of host-selection behaviour in such populations, we estimated genetic variation and covariation in larval acceptance of different host species within and between populations of the obliquebanded leafroller, Choristoneura rosaceana. Host-acceptance behaviour was analysed using the threshold model of quantitative genetics. According to this model, there is a continuously distributed variable describing the motivational/physiological state of individuals in the population, and a threshold of acceptance representing the acceptability of a host species. Individuals in which this variable exceeds the threshold accept the host while individuals below the threshold reject it. Parent–offspring regressions, a selection experiment, and comparisons of full-sibs in three pairs of populations revealed significant additive genetic variation in host acceptance. Genetic correlations between the responses to different hosts were either positive or not significantly different from zero, suggesting that local change in host acceptance will not favour behavioural specialization in populations of C. rosaceana. The pattern of the reaction norms for host response in the pairs of populations confirmed that divergence in host-selection behaviour does not involve behavioural specialization in the obliquebanded leafroller.

The effects of crop diversification on herbivorous insects: a meta‐analysis approach

Abstract. We used a meta‐analysis to evaluate the hypothesis that diversified crops are subject to lower density of herbivorous insects. This method consists of integrating findings of independent studies by calculating the magnitude of treatment effects (effect size). A literature review covering a period of over 10 years yielded twenty‐one studies that contained sufficient information for the meta‐analysis. The data were analysed according to three criteria: yearly replications of the same study were considered independent data so that research repeated over time would have greater weight; a second meta‐analysis was conducted with results of only 1 year because of possible lack of independence between results of the same study; the third approach was to remove studies involving Brassica spp. from the analysis because they are redundant from the standpoint of testing a general hypothesis of crop diversification. The three analyses resulted in effect sizes of 0.50, 0.35 and 0.27, respectively, and all were statistically significant. These results correspond to 69%, 63% and 60% differences between average insect densities for treatment (diversified crops) and control groups. We considered the last criterion for data analysis as the most appropriate because of lowest bias from non‐independence between samples. Therefore we concluded that crop diversity caused moderate reduction of herbivorous insect populations. The implications and limitations of meta‐analyses are discussed.

Effects of Resource Distribution on Animal-Plant Interactions

M.D. Hunter and P.W. Price, Introduction: Plants as a Variable Resource Base for Animals. M.C. Rossiter, The Impact of Resource Variation on Population Quality in Herbivorous Insects: A Critical Aspect of Population Dynamics. R.S. Ostfeld, Small Mammal Herbivores in a Patchy Environment: Individual Strategies and Population Responses. A.E. Weis and D.R. Campbell, Plant Genotype: A Variable Factor in Insect-Plant Interaction. B.J. Rathcke, Nectar Distributions, Pollinator Behavior, and Plant Reproductive Success. P.W. Price, Plant Resources as the Mechanistic Basis for Insect Herbivore Population Dynamics. J.C. Schultz, Factoring Natural Enemies into Plant Tissue Availability to Herbivores. T. Ohgushi, Resource Limitation on Insect Herbivore Populations. J.R. Karr, M. Dionne, and I. Schlosser, Bottom-Up versus Top-Down Regulation of Vertebrate Populations: Lessons from Birds and Fish. M.D. Hunter, Interactions Within Herbivore Communities Mediated by the Host Plant: The Keystone Herbivore Concept. D.W. Roubik, Loose Niches in Tropical Communities: Why Are There So Few Bees and So Many Trees? T.H. Fleming, How Do Fruit-and-Nectar Feeding Birds and Mammals Track Their Food Resources? T. Inoue and M. Kato, Inter-and Intraspecific Morphological Variation in Bumblebee Species, and Competition in Flower Utilization. J.M. Scriber and R.C. Lederhouse, The Thermal Environment as a Resource Dictating Patterns of Feeding Specialization of Insect Herbivores. Each chapter includes references. Index.

Coastal insect herbivore populations are strongly influenced by environmental variation

Abstract. Pissonotus quadripustulatus is a brachypterous planthopper that feeds on the clonal salt marsh plant, sea oxeye daisy, Borrichia frutescens. Asphondylia borrichiae is a macropterous gall fly that creates galls on the apical meristems. Most Borrichia occurs in isolated patches consisting of more than one genotype. Densities of Pissonotus and Asphondylia do not significantly differ within patches but they do differ between patches. I tried to find out why some patches of Borrichia support consistently higher densities of Pissonotus and Asphondylia than others. Reciprocal transplants of Borrichia between patches showed that for both Pissonotus and Asphondylia recipient site (local environment) had a strong effect on population densities. However, the best sites for Asphondylia were generally the worst sites for Pissonotus and vice versa. Donor site (local plant genotypes) was not significant for Pissonotus but was marginally significant for Asphondylia. However, for Asphondylia recipient site had a greater effect. Local plant genotype effects may be greater for gall‐forming insects, which tend to be specialists, than for more generalist sapsucking insects.

Intra- and Interspecific Competition in Adults of Two Abundant Grasshoppers (Orthoptera: Acrididae) from a Sandhills Grassland

Recognizing the existence and importance of intra- and interspecific competition in natural populations of insect herbivores remains problematical. We performed field-cage experiments to detect the presence of both intra- and interspecific competition among adults of two common, grass-feeding grasshoppers, Ageneotettix deorum (Scudder) and Amphitornus coloradus (Thomas). Based on increased mortality and lowered fecundity at higher densities, strong evidence for intraspecific competition exists for both species. Interspecific competition was weaker than intraspecific competition in this study. When interspecific competition existed, it was asymmetric in its effect, with the larger species ( A. coloradus ) negatively impacting the smaller one ( A. deorum ). Although there was little evidence for interspecific effects on mortality, egg production was impacted in a size-dependent, asymmetric manner between the two species. Significant density dependent depression of available food was observed for A. coloradus , the larger species. Experiments were nm at high densities (10–30 individuals per square meter), levels only irregularly reached under natural conditions (either in sporadic patches or overall about every 10–12 yr). Because naturally occurring densities are typically below the competitive threshold, interspecific competition between these two species may occur only intermittently. Other forces may greatly impact adult mortality at this site, including predation.

Successional Trends in Insect Herbivore Population Densities: A Field Test of a Hypothesis

Theory suggests that plants typical of early and late succession should possess qualitatively different anti-herbivore chemical defences. Differences in the structure, mode of action and effect of these chemicals should result in greater herbivore population levels on plants of early succession. This theory was tested for three separate insect taxa over a successional gradient in southern England. All taxa displayed greater population densities on short-lived plants of early succession than on birch, a plant of late succession. Although the results support theories which suggest a dichotomy of effect of the chemical defences of early and late successional plants, difficulties remain in reconciling the theory of plant chemical defences with existing data

Do Birch Trees (Betula pendula) Grow Better if Foraged by Wood Ants?

1. Populations of insect herbivores in three main guilds were studied on experimental saplings and natural birch (Betula pendula) trees which were either foraged or not foraged by wood ants (Formica rufa) on the Gait Barrows NNR. 2. The effects of the ants on the insects, and hence on the damage by the herbivores to the trees were measured. 3. Growth parameters of foraged and unforaged trees were measured. 4. Of the seven aphid species feeding on Betula pendula, two had mutualistic relationships with the ants and were increased in numbers by them. The remaining five species were decreased in numbers

Darwinian Methodology and the Theory of Insect Herbivore Population Dynamics

In his approach to the development of theory, Darwin set an example from which we can learn in our endeavors to develop a theory of insect herbivore population dynamics. Darwin was an accomplished naturalist; he relied on empirical observations and developed a factually based theory, building from the detection of pattern in nature to hypotheses on mechanisms accounting for pattern. Emulating Darwin's approach, patterns in plant, insect herbivore, and carnivore relationships, based on empirical observations, are summarized, arguing for recognition of strong influences passing up through trophic webs, from plants to herbivores to carnivores, and of pattern imposed by a landscape represented by all stages of ecological succession. Hypotheses accounting for linkage among the empirical patterns are erected in the form of flow diagrams of influence. Unanswered questions and the need for further testing of hypotheses are discussed in the hope of fostering more concentrated effort in the development of a theory on insect herbivore population dynamics.