Population Dynamics Of Insect Herbivores Research Articles

Leaf N:P stoichiometry overrides the effect of individual nutrient content on insect herbivore population dynamics in a Tibetan alpine grassland

Based on resource availability hypothesis, increasing exogenous nutrient inputs may favour herbivores by improving quality and quantity of host plants. Such plant-herbivore interactions have been demonstrated under nitrogen (N) addition, but less examined in the context of simultaneous inputs of N and phosphorus (P). Here, we conducted a multiple-nutrient addition experiment (Control, N addition alone, P addition alone, both N and P additions) in a Tibetan alpine grassland, to explore how N addition interacts with P to affect the larval density of the grassland caterpillar Gynaephora menyuanensis. We showed that P addition alone did not obviously affect the larval density, but P suppressed N-induced increases in larval density when N and P were simultaneously added. We further found that changes in leaf N:P of host plants induced by nutrient additions regulated larval density more strongly than leaf N or P content. Both increased plant productivity and shifted species composition under nutrient additions contributed less to the larval density. Interactive effects of N and P suggest that the simultaneous inputs of multiple nutrients may boost the carrying capacity of grasslands and reduce the risk of herbivorous insect outbreaks. The work also reveals that the resource availability hypothesis does not well predict the response of insect herbivore to all nutrient inputs, implying the necessity of incorporating N:P stoichiometry in this hypothesis.

Effect of elevated CO2 and temperature on biochemistry of groundnut and inturn its effect on development of leaf eating caterpillar, Spodoptera litura fabricius

Climate change in terms of elevated CO2 (eCO2) and temperature may have host mediated effects which could affect the survival, growth and development, and population dynamics of insect herbivores. The present study aimed to examine the growth and development of leaf feeding Spodoptera litura (Fabricius) (Noctuidae: Lepidoptera) reared on groundnut (Arachis hypogaea L.) grown under different climate change treatments under open top chambers (OTC’s) at University of Agricultural Sciences, Raichur, Karnataka. Significantly lower leaf nitrogen, higher carbon, C: N ratio, phenols and tannins was observed in the groundnut foliage grown under eCO2 conditions. This alteration in food quality in elevated conditions significantly affected the growth parameters of S. litura in the form of increased food consumption, increased larval weight and more faecal matter production due to extended larval and pupal duration. This resulted in reduced fecundity, particularly in the population raised under eCO2 conditions compared to ambient conditions. Further, the insect larva showed increased approximate digestibility and relative consumption rate under eCO2 condition coupled with reduced efficiency of conversion of ingested food. As a result, the relative growth rate was decreased under eCO2 conditions. In nutshell, it can be concluded that eCO2 concentrations altered the quality of groundnut foliage as it was noticed by the changes in biochemical constituents of the foliage and has the negative effect on the growth and development of S. litura.

Insect herbivory in a mature Eucalyptus woodland canopy depends on leaf phenology but not CO2 enrichment.

BackgroundClimate change factors such as elevated atmospheric carbon dioxide concentrations (e[CO2]) and altered rainfall patterns can alter leaf composition and phenology. This may subsequently impact insect herbivory. In sclerophyllous forests insects have developed strategies, such as preferentially feeding on new leaf growth, to overcome physical or foliar nitrogen constraints, and this may shift under climate change. Few studies of insect herbivory at elevated [CO2] have occurred under field conditions and none on mature evergreen trees in a naturally established forest, yet estimates for leaf area loss due to herbivory are required in order to allow accurate predictions of plant productivity in future climates. Here, we assessed herbivory in the upper canopy of mature Eucalyptus tereticornis trees at the nutrient-limited Eucalyptus free-air CO2 enrichment (EucFACE) experiment during the first 19 months of CO2 enrichment. The assessment of herbivory extended over two consecutive spring—summer periods, with a first survey during four months of the [CO2] ramp-up phase after which full [CO2] operation was maintained, followed by a second survey period from months 13 to 19.ResultsThroughout the first 2 years of EucFACE, young, expanding leaves sustained significantly greater damage from insect herbivory (between 25 and 32 % leaf area loss) compared to old or fully expanded leaves (less than 2 % leaf area loss). This preference of insect herbivores for young expanding leaves combined with discontinuous production of new foliage, which occurred in response to rainfall, resulted in monthly variations in leaf herbivory. In contrast to the significant effects of rainfall-driven leaf phenology, elevated [CO2] had no effect on leaf consumption or preference of insect herbivores for different leaf age classes.ConclusionsIn the studied nutrient-limited natural Eucalyptus woodland, herbivory contributes to a significant loss of young foliage. Leaf phenology is a significant factor that determines the level of herbivory experienced in this evergreen sclerophyllous woodland system, and may therefore also influence the population dynamics of insect herbivores. Furthermore, leaf phenology appears more strongly impacted by rainfall patterns than by e[CO2]. e[CO2] responses of herbivores on mature trees may only become apparent after extensive CO2 fumigation periods.Electronic supplementary materialThe online version of this article (doi:10.1186/s12898-016-0102-z) contains supplementary material, which is available to authorized users.

Variability in plant nutrients reduces insect herbivore performance.

The performance and population dynamics of insect herbivores depend on the nutritive and defensive traits of their host plants. The literature on plant-herbivore interactions focuses on plant trait mean values, but recent studies showing the importance of plant genetic diversity for herbivores suggest that plant trait variance may be equally important. The consequences of plant trait variance for herbivore performance, however, have been largely overlooked. Here we report an extensive assessment of the effects of within-population plant trait variance on herbivore performance using 457 performance datasets from 53 species of insect herbivores. We show that variance in plant nutritive traits substantially reduces mean herbivore performance via non-linear averaging of performance relationships that were overwhelmingly concave down. By contrast, relationships between herbivore performance and plant defence levels were typically linear, with variance in plant defence not affecting herbivore performance via non-linear averaging. Our results demonstrate that plants contribute to the suppression of herbivore populations through variable nutrient levels, not just by having low average quality as is typically thought. We propose that this phenomenon could play a key role in the suppression of herbivore populations in natural systems, and that increased nutrient heterogeneity within agricultural crops could contribute to the sustainable control of insect pests in agroecosystems.

Bottom-Up Mechanisms Generate the Same Temporal Pattern of Attack by a Specialist and a Generalist Caterpillar on Short-Lived Plants.

The local population dynamics of insect herbivores in ephemeral patches of short-lived plants are poorly known. We investigated whether a specialist and a generalist caterpillar exhibit contrasting temporal patterns of attack during plant development and also assessed bottom-up forces related to plant ontogeny that govern such population trends. Immature stages of the polyphagous Trichoplusia ni (Hübner) and the oligophagous Plutella xylostella (L.) were sampled throughout the development of cabbage (Brassica oleracea L. var. capitata L.) crops. We measured protein and glucosinolate contents and insect performance with regard to plant age and leaf strata. The populations of both caterpillar species changed in close parallel throughout plant development, and a nonlinear temporal pattern of egg laying was reproduced in sequential population patterns of the larval stages until pupation. Reduced protein availability and insect performance coincided with a decline in egg laying and subsequent larval abundance in mature plants. By standardizing the plant size, we found that young and nutritious plants support proportionately more insects than large and mature plants. In our models of the population oscillations, the interaction between plant size and quality provided a strong causal explanation for the densities of both oligophagous and polyphagous caterpillars. Patches of fast-growing herbaceous plants are very common worldwide in the form of crop fields, and a generalized temporal pattern of attack may be widespread among caterpillars, regardless of their feeding specialization. Our results highlight the role of bottom-up forces in shaping the population dynamics of caterpillars in such systems.

A case of habitat complementation in forest pests: Pine processionary moth pupae survive better in open areas

Little attention has been given to the relevance of habitat complementation concept to the population dynamics of insect herbivores. Late instar larvae of the pine processionary moth (PPM) Thaumetopoea pityocampa move in late winter from pine stands, their feeding habitat, to neighbouring habitats where they pupate until next summer. They search for sunny exposed soil which they can find in open areas. We investigated the effect of both forest cover and soil origin, with three matching types (pine stand, broadleaved stand and open area) on the survival of PPM pupae. The microclimatic soil variables which significantly differentiated cover types and soil origins were the maximum temperature and the mean relative humidity in spring, soon after pupation has occurred. A significant effect of the cover type, but not of the soil origin, was detected on the proportion of dead, emerged or diapausing pupae. Open areas were more suitable for pupae survival than forest covers (pine or broadleaved stands), due to warmer and more humid soil conditions. In this study, we provide one of the first examples of habitat complementation for an insect herbivore, as PPM population can benefit from the combination of pine habitats for the feeding of larvae with open habitats for pupation. The presence of broadleaved stands next to pine stands might also represent an ecological trap for PPM pupae, as broadleaved habitats may turn to be less suitable for pupae survival in spring when the apparition of leaves creates cooler conditions.

Relative importance of biotic and abiotic soil components to plant growth and insect herbivore population dynamics.

BackgroundPlants are affected by several aspects of the soil, which have the potential to exert cascading effects on the performance of herbivorous insects. The effects of biotic and abiotic soil characteristics have however mostly been investigated in isolation, leaving their relative importance largely unexplored. Such is the case for the dune grass Ammophila, whose decline under decreasing sand accretion is argued to be caused by either biotic or abiotic soil properties.Methodology/Principal FindingsBy manipulating dune soils from three different regions, we decoupled the contributions of region, the abiotic and biotic soil component to the variation in characteristics of Ammophila arenaria seedlings and Schizaphis rufula aphid populations. Root mass fraction and total dry biomass of plants were affected by soil biota, although the latter effect was not consistent across regions. None of the measured plant properties were significantly affected by the abiotic soil component. Aphid population characteristics all differed between regions, irrespective of whether soil biota were present or absent. Hence these effects were due to differences in abiotic soil properties between regions. Although several chemical properties of the soil mixtures were measured, none of these were consistent with results for plant or aphid traits.Conclusions/SignificancePlants were affected more strongly by soil biota than by abiotic soil properties, whereas the opposite was true for aphids. Our results thus demonstrate that the relative importance of the abiotic and biotic component of soils can differ for plants and their herbivores. The fact that not all effects of soil properties could be detected across regions moreover emphasizes the need for spatial replication in order to make sound conclusions about the generality of aboveground-belowground interactions.

<I>Insect Herbivore–Host Dynamics: Tree-Dwelling Aphids</I>

Literature on the population dynamics of insect herbivores tends to favour a top-down regulation of abundance, owing much to the action of natural enemies. Originally published in 2005, this volume challenges this paradigm and argues that tree-dwelling species of aphids, through competition for resources, regulate their own abundance. The biology of tree-dwelling aphids is examined, particularly their adaptation to the seasonal development of their host plants. When host-plant quality is favourable, aphids, by telescoping generations, can achieve prodigious rates of increase which their natural enemies are unable to match. Using analyses of long-term population censuses and results of experiments, this book introduces students and research workers to insect herbivore-host dynamics using the interaction between aphids and trees as a model.

Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores

Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g−1), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully‐grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing‐induced changes in food quality.

Plant defense and density dependence in the population growth of herbivores.

Long-standing theory has predicted that plant defensive and nutritional traits contribute to the population dynamics of insect herbivores. To examine the role of plant variation in density dependence, I took a comparative approach by conducting density manipulation experiments with the specialist aphid, Aphis nerii, on 18 species of milkweed (Asclepias spp.). The strength of density dependence varied on the plant species. Variation in plant secondary compounds (cardenolides), trichomes, leaf carbon and nitrogen concentrations, and seed mass of the milkweed species predicted the R(max) of aphid populations, while specific leaf weight, carbon concentration, latex, water content, and trichome density were significant predictors of the strength of density dependence. Thus, plant traits that probably evolved for primary and defensive functions contribute to the ecological dynamics of herbivore populations.

Impact of avian and arthropod predation on lepidopteran caterpillar densities and plant productivity in an ephemeral agroecosystem

Abstract. 1. Most studies evaluating the combined impact of spiders and other predators on herbivore densities in agroecosystems have focused primarily on their trophic connections with invertebrate predators (e.g. carabids, chrysopids); however linkages among spiders and vertebrate predators may also help structure the population dynamics of insect herbivores. A field experiment was conducted to examine the impact of avian and spider predation on lepidopteran caterpillar densities and plant productivity within a Brassica agroecosystem.2. Arthropod abundance, leaf‐chewing damage, and final plant productivity associated with broccoli, Brassica oleracea L. (var. italica), were recorded for four treatments: (1) bird present but spiders removed; (2) both birds and spiders present; (3) birds excluded, spiders present; and (4) birds and spiders both excluded.3. Densities of Artogeia rapae L. (Lepidoptera: Pieridae) and Trichoplusia ni Hübner (Lepidoptera: Noctuidae) large caterpillars and post feeding stages were reduced significantly by bird predation. The abundance of large caterpillars was also reduced on spider‐inhabited plants during early plant growth; however the assemblage of birds and spiders did not suppress caterpillar densities more significantly than either predator alone.4. Plants protected by birds, spiders, and birds plus spiders sustained less folivory attributable to leaf chewing caterpillars than check plants. Plant productivity was also greater for predator‐protected plants than check plants.5. Although spiders and parasitoids were responsible for some of the mortality inflicted upon lepidopteran caterpillars, it was concluded that in this study system, birds are the most important natural enemies of folivores.

Landscape structure, habitat fragmentation, and the ecology of insects

Landscape structure, habitat fragmentation, and the ecology of insects

Comparing the Consequences of Induced and Constitutive Plant Resistance for Herbivore Population Dynamics

Although it has been suggested that induced and constitutive plant resistance should have different effects on insect herbivore population dynamics, there is little experimental evidence that plant resistance can influence herbivore populations longer than one season. We used a density-manipulation experiment and model fitting to examine the effects of constitutive and induced resistance on herbivore dynamics over both the short and long term. We used likelihood methods to fit population dynamic models to recruitment data for populations of Mexican bean beetles on soybean varieties with no resistance, constitutive resistance, or induced resistance. We compared model configurations that fit parameters for resistance types separately to models that did not account for resistance type. Models representing the hypothesis that the three resistance types differed in their effects on beetle dynamics received the most support. Induced resistance resulted in lower population growth rates and stronger density dependence than no resistance. Constitutive resistance resulted in lower population growth rates and stronger density dependence than induced resistance. Constitutive resistance had a stronger effect on both short-term beetle recruitment and predicted beetle population dynamics than induced resistance. The results of this study suggest that induced and constitutive resistance can differ in their effects on herbivore populations even in a relatively complex system.

Host plant species can influence the fitness of herbivore pathogens: the winter moth and its nucleopolyhedrovirus.

Plants can have a significant impact on the fitness and efficacy of natural enemies. These interactions are widespread and suggest that the influences on the population dynamics of insect herbivores cannot be simply divided into "bottom up" and "top down". Several questions remain little studied in this field. Firstly, to what extent can plants affect the interactions between insects and their pathogens? Secondly, what are the effects of variation within natural enemy species on host/enemy/plant interactions? Finally, if plant/pathogen interactions can occur, do pathogens have increased fitness on the locally abundant food plant of their host? This study explored the influence of three host plant species of the polyphagous winter moth, Operophtera brumata, on infections caused by two geographic isolates of the winter moth nucleopolyhedrovirus (NPV) collected from distinct winter moth habitats. Insects were infected on excised leaf tissue of common oak, Quercus robur, Sitka spruce, Picea sitchenis, and heather, Calluna vulgaris. Parameters fundamental to the basic reproductive rate of the pathogen were estimated: these being infectivity, speed of kill and the yield of virus per insect. Leaf nitrogen and phenolic content were measured as indicators of host plant quality for the three plant species: oak had the highest levels of nitrogen and also the highest levels of phenolic compounds. Heather had higher levels of phenolic compounds than Sitka spruce. Host plant did not affect the infectivity of either isolate but insects that ingested virus on oak foliage died sooner and yielded more virus than insects that ingested virus on Sitka spruce or heather. The effect of host plant species on pathogen yield varied between the two isolates of the NPV but not as predicted by our adaptive hypothesis. The interactions between virus and food plant are discussed in relation to host and pathogen population dynamics.

Is the Biological Control of Insects a Natural Phenomenon?

Classical biological control (natural enemy introductions) has long served as a paradigm for the role of predators and parasitoids in insect herbivore population dynamics, and it is widely held that there is no fundamental difference between successful biological control and the action of native natural enemies (natural control'). We ask if biological control is really a natural phenomenon. To address this, we focus on two aspects of biological control that potentially influence the ability of natural enemies to exert top-down control on victim populations: (1) control is most often attempted against insect herbivores occupying habitats that have been substantially simplified in diversity and structure, and (2) the enemy-herbivore-plant food web in biological control systems is often composed entirely of exotic species that share few evolutionary or ecological links with the native biota, further simplifying the web in which the enemies operate. Using the insect life table literature, we found no differences in the frequencies of all types of natural enemies acting as key factors in the dynamics of exotic and native insect populations. However, top-down control, when it occurs, is more frequently due to a single parasitoid species for exotic insect herbivores on exotic plants in cultivated habitats. whereas for native insect herbivores on native plants in natural habitats it is more frequently due to a suite of generalist predators. We also found from the historical record of biological control that success rates are substantially greater in exotie. simplified, managed habitats than in natural habitats, particularly when involving parasitoids. We suggest that biological control is not strictly a natural' phenomenon, because it overestimates the extent to which parasitoids exert top-down control on insect populations, and it results most often from the formation of a single strong link in simplified food webs, in contrast to the natural control' that results from multiple links in more complex food webs.

Influence of form and quantity of selenium on the development and survival of an insect herbivore

Even plants classified as ‘nonaccumulators’ can sequester concentrations of sodium selenate, sodium selenite, selenocystine and selenomethionine that can strongly influence insect development and survival. These forms of selenium (Se), tested in diet-incorporation bioassays, proved toxic to larvae of a generalist insect herbivore at relatively low levels. Sodium selenite was the most toxic form tested against Spodoptera exigua (Hübner), with an LC 50 of 9.14 μg g −1 wet wt (21.11 μg g −1 dry wt). Selenocystine was intermediate with an LC 50 of 15.2 μg g −1 wet wt. The least toxic forms, sodium selenate and selenomethionine, had LC 50s below 50 μg g −1 dry wt, the upper level for tissues of plants classified as nonaccumulators. Ingestion of some forms of Se also affected growth and development. Increasing concentrations of sodium selenate and sodium selenite decreased pupal weight and added significantly to the time needed for development to the pupal and adult stages. The time required to complete the larval stage increased by over 25% and the time from egg to adult emergence was extended by 22% to nearly 30%. Selenocystine and selenomethionine did not significantly increase developmental times, even at concentrations that killed 90% or more of the test populations. Analyses of relative growth rate, relative growth index, and an analysis of covariance technique for measuring growth indicated that the form of Se affected growth rates, growth inhibition responses of the larvae, and toxicological effects. Thus, quantity and the form of Se accumulating in plants grown on Se-contaminated sites are likely to influence the population dynamics of insect herbivores. The implications of these results for the ecology of contaminated sites 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.

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.