Insect Herbivore Populations Research Articles

Response of white birch (Betula platyphylla Sukaczev) to temperature and precipitation in the mountain forest steppe and taiga of northern Mongolia

The mountain forest steppe and taiga in northern Mongolia have experienced a forest decline in area and quality since the end of the last century. Changes in land use, climate, fire frequency and pest occurrence are considered to be the main drivers of this vegetation shift and desertification. Because this region is the source for major rivers, is home to a unique flora and fauna and represents an important source of timber for Mongolia, the ability of different tree species to respond to these changes and regenerate is of increasing interest. Our contribution focuses on the climate-growth relationship of old and young birch trees from two valleys in the Mongolian province of Selenge Aimag.The research site Bugant, located in the Western Khentey Mountains, was the most important logging centre in Mongolia during socialist times. Today, the vegetation is dominated by succession forests of light taiga. The research site Altansumber, on the border of the Sant and Khushat soum, is dominated by light taiga and mountain forest steppe. Traditional nomads who depend on these forests for different reasons inhabit this area.Wood cores were sampled and chronologies of young and old birch trees at Bugant and Altansumber were created. Climate data were obtained from the Eroo station, which is known in the region for its long and reliable climate record. We analysed the climate-growth relationships of the chronologies from 1962 to 2009. At both sites and in both age classes, correlations with temperature were predominantly negative, particularly in April (Bugant, south- and east-facing slopes) and May (Altansumber, north-facing slopes). Precipitation of the late summer of the previous year (August/September) positively correlated with the growth of birch at Altansumber. We assume that the significant negative correlation between winter precipitation (December/January) and the growth of old birches at both sites is due to positive effects of snow cover on the survival rate of herbivorous insect populations. Our results indicate that during the early vegetation period, younger birch trees are more dependent on water availability than older ones. Negative pointer years were characterized by below-average precipitation during the current summer period and above-average spring temperatures. For the old trees, positive pointer years were characterized by above-average summer precipitation. We conclude that water availability is the most crucial factor for the growth of white birch in northern Mongolia.

Periodic solutions in a herbivore-plant system with time delay and spatial diffusion

Empirical studies indicate that many populations of herbivorous insects exhibit periodic outbreaks, but the intrinsic causes of this behavior are not well understood. Thus, in this study, we investigated a herbivore-plant system with time delay based on reaction-diffusion equations. Using normal formal theory and the center manifold theorem for partial functional differential equations, we show that this model exhibits the property of Hopf bifurcation. Therefore, interactions between the time delay and spatial diffusion will induce periodic outbreaks in herbivore populations. These results may suggest a new mechanism for herbivore outbreaks.

Changes in function and temporal variation in a guild of gall-parasitoids across a temperature gradient in Australian subtropical rainforest

Parasitoids play an important role in ecosystem functioning through their influence on herbivorous insect populations. Theoretical and experimental evidence suggest that increased species richness can enhance and stabilize ecosystem function. It is important to understand how richness-driven functional relationships change across environmental gradients. We investigated how temperature affected the relationship between parasitoid richness and parasitism rate in a guild of gall-parasitoids along an elevational gradient. We collected galls at 15 sites along five elevational gradients (between 762 m and 1145 m asl) on six occasions over a year. A total of 1902 insects, including 1593 parasitoids, were reared from 12 402 galls. Parasitism rate increased significantly with temperature on all sampling occasions, except December and February. We found a significant, positive richness–parasitism relationship. This relationship, however, was weaker at higher elevations which may be linked to decreased functional efficiency of parasitoids at lower temperatures. Temporal variability in parasitism rate and parasitoid richness were significantly related, regardless of temperature. A stable functional guild of this kind may provide a more reliable ecosystem service under environmental changes.

Pheromone Production by an Invasive Bark Beetle Varies with Monoterpene Composition of its Naïve Host.

The secondary chemistry of host plants can have cascading impacts on the establishment of new insect herbivore populations, their long-term population dynamics, and their invasion potential in novel habitats. Mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae) has recently expanded its range into forests of jack pine, Pinus banksiana Lamb., in western Canada. We investigated whether variations in jack pine monoterpenes affect beetle pheromone production, as the primary components of the beetle's aggregation pheromone, (-)-trans-verbenol and anti-aggregation pheromone (-)-verbenone, are biosynthesized from the host monoterpene α-pinene. Jack pine bolts were collected from five Canadian provinces east of the beetle's current range, live D. ponderosae were introduced into them, and their monoterpene compositions were characterized. Production of (-)-trans-verbenol and (-)-verbenone emitted by beetles was measured to determine whether pheromone production varies with monoterpene composition of jack pines. Depending on particular ratios of major monoterpenes in host phloem, jack pine could be classified into three monoterpenoid groups characterized by high amounts of (+)-α-pinene, 3-carene, or a more moderate blend of monoterpenes, and beetle pheromone production varied among these groups. Specifically, beetles reared in trees characterized by high (+)-α-pinene produced the most (-)-trans-verbenol and (-)-verbenone, while beetles in trees characterized by high 3-carene produced the least. Our results indicate that pheromone production by D. ponderosae will remain a significant aspect and important predictor of its survival and persistence in the boreal forest.

Indirect effects of nutrients in organic and conventional paddy field soils on the rice grasshopper, Oxya japonica (Orthoptera: Acrididae), mediated by rice plant nutrients

Current agricultural technology depends on synthetic fertilizers and enhances the growth of herbivorous insect populations. Some organic farming uses bio-based nutrients instead and is considered the most promising way to mitigate the negative effects of conventional farming. The rice grasshopper Oxya japonica (Thunberg) (Orthoptera: Acrididae) is the most abundant herbivore in both conventional and organic fields in northern Japan. This study investigated the effects of soil nutrients associated with conventional versus organic farming on rice plant quality,and the growth and performance of O. japonica in paddy fields at Sakata, Yamagata Prefecture. A laboratory experiment was also conducted to understand the effects, as mediated by host plant quality, of organic and conventional soil nutrients on the growth and feeding (as measured by the leaf area consumed) of O. japonica. Nymphs of O. japonica that fed on nitrogen-rich, carbon-poor plants cultivated in conventional soil grew and developed faster than those feeding on organic plants. The rice plants in conventional fields had greater biomasses, suggesting that they were more likely to tolerate grasshopper damage. These results support the importance of soil nutrient management as a factor that influences plant–herbivore interactions.

Plant‐mediated effects of host plant density on a specialist herbivore of Solanum carolinense

Host plant density can affect insect herbivore oviposition behaviour, which can, in turn, affect both plant and herbivore populations. Because clear generalisations about density effects on oviposition remain elusive, a better understanding of underlying mechanisms is needed. One such mechanism is plant‐mediated effects (i.e. changes in plant traits with density), which are often suggested but rarely isolated experimentally. In an experiment directly manipulating host plant density (Solanum carolinense) in the field, oviposition by a specialist herbivore (Leptinotarsa juncta) declined as plant density increased. A greenhouse cage experiment isolated the effects of plant‐mediated traits by removing neighbour plants before introducing L. juncta females. Oviposition declined as host plant density increased, supporting the importance of plant‐mediated effects in this system. To determine whether food quality contributed to plant‐mediated effects, larval growth rates were measured on leaves from both field‐ and greenhouse‐grown plants. In both the field and the greenhouse experiments, larval growth rate was not significantly affected by plant density. It is concluded that plant‐mediated effects contribute to, but do not completely explain, the strong influence of density on oviposition in the field experiment. These results suggest that considering plant‐mediated effects may help to explain variability in herbivore responses to plant density.

Chemical similarity between historical and novel host plants promotes range and host expansion of the mountain pine beetle in a naïve host ecosystem

Host plant secondary chemistry can have cascading impacts on host and range expansion of herbivorous insect populations. We investigated the role of host secondary compounds on pheromone production by the mountain pine beetle (Dendroctonus ponderosae) (MPB) and beetle attraction in response to a historical (lodgepole pine, Pinus contorta var. latifolia) and a novel (jack pine, Pinus banksiana) hosts, as pheromones regulate the host colonization process. Beetles emit the same pheromones from both hosts, but more trans-verbenol, the primary aggregation pheromone, was emitted by female beetles on the novel host. The phloem of the novel host contains more α-pinene, a secondary compound that is the precursor for trans-verbenol production in beetle, than the historical host. Beetle-induced emission of 3-carene, another secondary compound found in both hosts, was also higher from the novel host. Field tests showed that the addition of 3-carene to the pheromone mixture mimicking the aggregation pheromones produced from the two host species increased beetle capture. We conclude that chemical similarity between historical and novel hosts has facilitated host expansion of MPB in jack pine forests through the exploitation of common host secondary compounds for pheromone production and aggregation on the hosts. Furthermore, broods emerging from the novel host were larger in terms of body size.

Molecular characterization of trophic ecology within an island radiation of insect herbivores (Curculionidae: Entiminae: Cratopus)

The phytophagous beetle family Curculionidae is the most species-rich insect family known, with much of this diversity having been attributed to both co-evolution with food plants and host shifts at key points within the early evolutionary history of the group. Less well understood is the extent to which patterns of host use vary within or among related species, largely because of the technical difficulties associated with quantifying this. Here we develop a recently characterized molecular approach to quantify diet within and between two closely related species of weevil occurring primarily within dry forests on the island of Mauritius. Our aim is to quantify dietary variation across populations and assess adaptive and nonadaptive explanations for this and to characterize the nature of a trophic shift within an ecologically distinct population within one of the species. We find that our study species are polyphagous, consuming a much wider range of plants than would be suggested by the literature. Our data suggest that local diet variation is largely explained by food availability, and locally specialist populations consume food plants that are not phylogenetically novel, but do appear to represent a novel preference. Our results demonstrate the power of molecular methods to unambiguously quantify dietary variation across populations of insect herbivores, providing a valuable approach to understanding trophic interactions within and among local plant and insect herbivore communities.

Avifaunal disarray due to a single despotic species

AbstractAimReports of profound changes in species assemblages brought about by the influence of strongly interacting species are increasingly common. Where these strong interactors are sensitive to anthropogenic habitat changes, relatively small alterations in the environment can result in large and pervasive shifts in assemblages. We review the evidence for widespread assemblage‐level phase shifts across eastern Australia, triggered partly by anthropogenic habitat alteration and mediated by a native, despotic bird: the noisy miner Manorina melanocephala.LocationEastern Australia.MethodsBased on the literature, we developed conceptual models of factors affecting site occupancy by, and ecosystem‐level effects of, the noisy miner. We also analysed recent trends in the reporting rate of the noisy miner across its range.ResultsIndividuals of this species cooperate to aggressively exclude almost all smaller bird species from the areas they occupy. The noisy miner is advantaged by habitat fragmentation and structural simplification—habitat changes that facilitate detection and interception of potential competitors by miners. We report that the species is increasingly prevalent, particularly close to forest and woodland edges. Such edges have mainly been created by human land use. The evidence we reviewed showed: (1) strong causal links between the noisy miner and depressed richness and abundance of smaller birds, particularly nectarivores and insectivores; (2) moderate evidence of a positive association with larger bird species; (3) reduced tree condition stemming from impaired control of insect herbivore populations by smaller insectivores; and (4) a plausible negative effect on plant reproduction through reduced tree condition, altered pollination services and altered seed dispersal.Main conclusionsThis is the first synthesis to document the causes and likely ecological consequences of increasingly prevalent phase shifts catalysed by a despotic species on ecosystems at very large spatial scales (> 1 million km2). Native species affected by human activities can become agents that induce ecological dysfunction.

Cabbage Seasonal Leaf Quality Mediating the Diamondback Moth Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) Performance.

Seasonal variation in plant quality may be intense enough to generate predictable patterns in insect herbivore populations. In order to explain seasonal oscillations in neotropical populations of the diamondback moth Plutella xylostella (L.), we tested the following: (1) if nutritional quality of cabbage (Brassica oleraceae var. capitata), a primary host plant of diamondback moth, adversely affects the performance of this insect in late spring and early summer, when populations decline and go extinct, and (2) if nutritional features of cabbage change with the seasons. We measured the performance of diamondback moth reared on leaves of cabbages grown during the four seasons of the year. Summer plants proved to be worse for the survival of the immature stages and subsequent adult fecundity, but there were no significant differences between the remaining seasons. Our results support the hypothesis that short-lived plants, grown in different seasons of the year in the tropics, have different nutritional and defensive attributes. We analyzed nutritional quality of cabbage leaves from the four seasons, but only total lipids were reduced in summer plants. Neotropical populations of diamondback moth collapse before plant quality decay in the summer. If the diamondback moth is well adapted to the seasonal deterioration of the habitat, including the reduction in the quality of host plants, it is expected that emigration happens before the mortality increases and natality decreases during the summer.

Looking for a similar partner: host plants shape mating preferences of herbivorous insects by altering their contact pheromones

The role of phenotypical plasticity in ecological speciation and the evolution of sexual isolation remains largely unknown. We investigated whether or not divergent host plant use in an herbivorous insect causes assortative mating by phenotypically altering traits involved in mate recognition. We found that males of the mustard leaf beetle Phaedon cochleariae preferred to mate with females that were reared on the same plant species to females provided with a different plant species, based on divergent cuticular hydrocarbon profiles that serve as contact pheromones. The cuticular hydrocarbon phenotypes of the beetles were host plant specific and changed within 2 weeks after a shift to a novel host plant species. We suggest that plant-induced phenotypic divergence in mate recognition cues may act as an early barrier to gene flow between herbivorous insect populations on alternative host species, preceding genetic divergence and thus, promoting ecological speciation.

Winter predation by insectivorous birds and consequences for arthropods and plants in summer

Top-down effects of predators can have important consequences for ecosystems. Insectivorous birds frequently have strong predation effects on herbivores and other arthropods, as well as indirect effects on herbivores' host plants. Diet studies have shown that birds in temperate ecosystems consume arthropods in winter as well as in summer, but experimental studies of bird predation effects have not attempted to quantitatively separate winter predation impacts from those in summer. To understand if winter foraging by insectivorous birds has consequences for arthropods or plants, we performed a meta-analysis of published bird exclusion studies in temperate forest and shrubland habitats. We categorized 85 studies from 41 publications by whether birds were excluded year-round or only in summer, and analyzed arthropod and plant response variables. We also performed a manipulative field experiment in which we used a factorial design to exclude birds from Quercus velutina Lam. saplings in winter and summer, and censused arthropods and herbivore damage in the following growing season. In the meta-analysis, birds had stronger negative effects on herbivores in studies that included winter exclusion, and this effect was not due to study duration. However, this greater predation effect did not translate to a greater impact on plant damage or growth. In the field experiment, winter exclusion did not influence herbivore abundance or their impacts on plants. We have shown that winter feeding by temperate insectivorous birds can have important consequences for insect herbivore populations, but the strength of these effects may vary considerably among ecosystems. A full understanding of the ecological roles of insectivorous birds will require explicit consideration of their foraging in the non-growing season, and we make recommendations for how future studies can address this.

Not just the usual suspects: Insect herbivore populations and communities are associated with multiple plant nutrients

The relationship between plant nutrient content and insect herbivore populations and community structure has long interested ecologists. Insect herbivores require multiple nutrients, but ecologists have focused mostly on nitrogen (an estimate of plant protein content), and more recently phosphorus (P); other nutrients have received little attention. Here we document nutrient variation in grass and forb samples from grassland habitats in central Nebraska using an elemental approach; in total we measured foliar concentrations of 12 elements (N and P, plus S, B, Ca, Mg, Na, K, Zn, Fe, Mn, and Cu). We detected significant variability among sites for N, P, Mg, Na, K, and Cu. We next used a model selection approach to explore how this nutritional variation and plant biomass correlate with grasshopper densities (collectively and at the feeding-guild level), and principal component analysis to explore nutrient correlations with grasshopper community species composition. When all grasshoppers were pooled, densities varied among sites, but only P was associated with abundance of the elements shown to vary between sites. Different responses occurred at the feeding-guild level. For grass specialists, densities were associated with N, plus P, Mg, and Na. For forb specialists, N and P were often associated with density, but associations with Na and K were also observed. Finally, mixed-feeder abundance was strongly associated with biomass, and to a lesser extent P, Mg, Na, and Cu. At the community level, B, Ca, Zn, and Cu, plus biomass, explained > 30% of species composition variation. Our results confirm the positive association of N and P with insect herbivore populations, while suggesting a potential role for Mg, Na, and K. They also demonstrate the importance of exploring effects at the feeding-guild level. We hope our data motivate ecologists to think beyond N and P when considering plant nutrient effects on insect herbivores, and make a call for studies to examine functional responses of insect herbivores to dietary manipulation of Mg, Na, and K. Finally, our results demonstrate correlations between variation in nutrients and species assemblages, but factors not linked to plant nutrient quality or biomass likely explain most of the observed variation.

Climate response of tree-ring width in Larix sibirica growing in the drought-stressed forest-steppe ecotone of northern Mongolia

Siberian larch (Larix sibirica) has its southern distribution limit in Mongolia in an area of rapidly rising temperatures. Direct effects of climate on tree-ring formation due to drought stress or indirect effects via the control of insect herbivore populations are little studied. The hypotheses were tested that stem increment of Siberian larch is reduced by (1) drought during the growing season and (2) high snow cover, as the latter is thought to protect hibernating herbivores, including gypsy moth (Lymantria dispar). Tree-ring width increases with decreasing summer temperature, increasing precipitation during the growing season and decreasing winter precipitation. The susceptibility of stem wood formation to drought during the growing season suggests that a future climate warming will decrease productivity of Siberian larch, thus affecting its existence within the forest-steppe ecotone of Mongolia. Narrow tree rings in years following winters with low snowfall support the hypothesis that winter precipitation exerts an indirect effect on the growth of Siberian larch by controlling the survival rates of gypsy moth eggs.

Areawide Suppression of European Corn Borer with Bt Maize Reaps Savings to Non-Bt Maize Growers

Transgenic maize engineered to express insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has become widely adopted in U.S. agriculture. In 2009, Bt maize was planted on more than 22.2 million hectares, constituting 63% of the U.S. crop. Using statistical analysis of per capita growth rate estimates, we found that areawide suppression of the primary pest Ostrinia nubilalis (European corn borer) is associated with Bt maize use. Cumulative benefits over 14 years are an estimated $3.2 billion for maize growers in Illinois, Minnesota, and Wisconsin, with more than $2.4 billion of this total accruing to non-Bt maize growers. Comparable estimates for Iowa and Nebraska are $3.6 billion in total, with $1.9 billion for non-Bt maize growers. These results affirm theoretical predictions of pest population suppression and highlight economic incentives for growers to maintain non-Bt maize refugia for sustainable insect resistance management.

Divergent host plant adaptation drives the evolution of sexual isolation in the grasshopper Hesperotettix viridis (Orthoptera: Acrididae) in the absence of reinforcement

Early stages of lineage divergence in insect herbivores are often related to shifts in host plant use and divergence in mating capabilities, which may lead to sexual isolation of populations of herbivorous insects. We examined host preferences, degree of differentiation in mate choice, and divergence in cuticular morphology using near-infrared spectroscopy in the grasshopper Hesperotettix viridis aiming to understand lineage divergence. In Kansas (USA), H. viridis is an oligophagous species feeding on Gutierrezia and Solidago host species. To identify incipient mechanisms of lineage divergence and isolation, we compared host choice, mate choice, and phenotypic divergence among natural grasshopper populations in zones of contact with populations encountering only one of the host species. A significant host-based preference from the two host groups was detected in host-paired feeding preference studies. No-choice mate selection experiments revealed a preference for individuals collected from the same host species independent of geographic location, and little mating was observed between individuals collected from different host species. Female mate choice tests between males from the two host species resulted in 100% fidelity with respect to host use. Significant differentiation in colour and cuticular composition of individuals from different host plants was observed, which correlated positively with host choice and mate choice. No evidence for reinforcement in the zone of contact was detected, suggesting that divergent selection for host plant use promotes sexual isolation in this species.

Spatial population dynamics of a specialist aphid parasitoid, Lysiphlebus hirticornis Mackauer (Hymenoptera: Braconidae: Aphidiinae): evidence for philopatry and restricted dispersal

Within insect communities, the population ecology of organisms representing higher trophic levels, for example, hymenopterous parasitoids, may be influenced by the structure of their insect hosts. Using microsatellite markers and ecological data, we investigated the population structure of the specialist braconid wasp parasitoid, Lysiphlebus hirticornis Mackauer attacking Metopeurum fuscoviride, a specialist aphid feeding on tansy, Tanacetum vulgare. Previous studies revealed that M. fuscoviride has a classic metapopulation structure with high subpopulation turnover. In this study, up to 100% of ramets within a host plant genet colonized by aphids were colonized by the parasitoid, yet plants with aphids but no parasitoids were also observed. Genetic differentiation measured by F(ST), actual differentiation (D) and relative differentiation (G(ST)) indicated highly structured parasitoid population demes, with restricted gene flow among and between parasitoid subpopulations at the various sites. Interestingly, both field data and population assignment analysis showed that the parasitoid is highly philopatric. Thus, despite the frequent local extinctions of the aphid host, the parasitoid continuously exploits its aphid host and contributes to the demise of local aphid subpopulations, rather than spreading its genes over many aphid populations. F(ST) values for the haplodiploid parasitoid were similar to those found in an independent study of the diploid aphid host, M. fuscoviride, hence supporting the view that an insect herbivore's population structure directly influences the ecology and genetics of the higher trophic level, in this case the wasp parasitoid.

Landscape diversity enhances biological control of an introduced crop pest in the north‐central USA

Arthropod predators and parasitoids provide valuable ecosystem services in agricultural crops by suppressing populations of insect herbivores. Many natural enemies are influenced by non-crop habitat surrounding agricultural fields, and understanding if, and at what scales, land use patterns influence natural enemies is essential to predicting how landscape alters biological control services. Here we focus on biological control of soybean aphid, Aphis glycines Matumura, a specialist crop pest recently introduced to the north-central United States. We measured the amount of biological control service supplied to soybean in 26 replicate fields across Michigan, Wisconsin, Iowa, and Minnesota across two years (2005-2006). We measured the impact of natural enemies by experimentally excluding or allowing access to soybean aphid infested plants and comparing aphid population growth over 14 days. We also monitored aphid and natural enemy populations at large in each field. Predators, principally coccinellid beetles, dominated the natural enemy community of soybean in both years. In the absence of aphid predators, A. glycines increased significantly, with 5.3-fold higher aphid populations on plants in exclusion cages vs. the open field after 14 days. We calculated a biological control services index (BSI) based on relative suppression of aphid populations and related it to landscape diversity and composition at multiple spatial scales surrounding each site. We found that BSI values increased with landscape diversity, measured as Simpson's D. Landscapes dominated by corn and soybean fields provided less biocontrol service to soybean compared with landscapes with an abundance of crop and non-crop habitats. The abundance of Coccinellidae was related to landscape composition, with beetles being more abundant in landscapes with an abundance of forest and grassland compared with landscapes dominated by agricultural crops. Landscape diversity and composition at a scale of 1.5 km surrounding the focal field explained the greatest proportion of the variation in BSI and Coccinellidae abundance. This study indicates that natural enemies provide a regionally important ecosystem service by suppressing a key soybean pest, reducing the need for insecticide applications. Furthermore, it suggests that management to maintain or enhance landscape diversity has the potential to stabilize or increase biocontrol services.

Seasonality and abundance of Metamasius callizona (Coleoptera: Dryophthoridae), an invasive insect herbivore, on two species of Tillandsia (Bromeliaceae) in Florida

Seasonality and abundance of an insect herbivore population are regulated by a complex array of forces from the bottom up, from the top down, and from competition and the environment. This paper examines the effect that two host plants have on an insect herbivore in the absence of top‐down regulation or competition. Metamasius callizona is an invasive bromeliad‐eating weevil in south Florida that has escaped its natural parasitoid in its home range. A 4‐year field study shows the weevil to be present and active year‐round, but to exhibit different patterns of seasonality and abundance on two of its host plants. It is argued that this demographic variability is generated by the differences in leaf type and growth habit that exist between the two host plants.

A Coevolutionary Arms Race: Understanding Plant-Herbivore Interactions

Plants and insects share a long evolutionary history characterized by relationships that affect individual, population, and community dynamics (Gatehouse, 2002). Plant-herbivore interactions are a prominent feature of this evolutionary history; it is by plant-herbivore interactions that energy is transferred from primary producers to the rest of the food web (Agrawal, 2004). On average, more than 10% of plant production is consumed by herbivores annually (Coley et al., 1985). This initial interaction between plants and their herbivores establishes the amount of energy available to the rest of the community. Consequently, the nature and extent of plant- herbivore interactions can significantly impact community structure, diversity, and function (see Silliman et al., 2005). Not only are plant-herbivore food webs important to community dynamics, but these webs also constitute a significant portion of the world's biodiversity. A recent study estimated that plant-herbivore food webs represent more than 40% of the global terrestrial biodiversity (Novotny & Basset, 2005). Consequently, it is important to understand the concepts central to these interactions as well as the processes that might influence the evolution of plant and herbivore species. Back Off Bugs! Plant Defenses Plant-herbivore relationships pit the insect against the plant in a race to maximize fitness. Maximizing fitness is the general goal for both plants and insects; however, in the case of plant-herbivore interactions, the insect maximizes its fitness by consuming plant tissues. Reduced biomass and tissue damage can negatively impact plant growth and fecundity, which ultimately decrease plant population growth rate. Individual plants that can somehow prevent or tolerate herbivory have a higher fitness than their more vulnerable neighbors; thus, these individuals contribute more genes to future generations. Not all plant-herbivore interactions are equal; rather, the type and amount of herbivore damage varies greatly among plant species and populations (Coley et al., 1985). This variation in consumption is likely a reflection of variation in plant palatability. Plants employ a variety of mechanical and chemical defenses that affect palatability and deter herbivores (Lerdau et al., 1994; Mauricio, 1998; Gatehouse, 2002; Siemens et al., 2002; Kliebenstein, 2004). Leaf toughness and trichome production are two ways in which plants mechanically protect themselves from herbivores (Feeny, 1970; Mauricio, 1998). These defenses act as physical barriers, preventing or slowing herbivores from accessing and handling plant tissue. In contrast, chemical defenses make it difficult for herbivores to process plant material once it is ingested. Chemicals, including glucosinolates, monoterpenes, and tannins, are generally produced through secondary metabolism (Lerdau, 1994; Mauricio, 1998; Gatehouse, 2002; Siemens et al., 2002; Kliebenstein, 2004). These compounds may have several functions that make them important to the plant's survival. In terms of defense, these compounds are often toxic or lethal to herbivores, causing a suite of problems that deter herbivores from foraging on the defended plant (Gatehouse, 2002). Operation Adaptation: Herbivore Offenses Plant defenses are important to insect herbivore populations because of their effect on insect growth and reproduction (Agrawal, 2004). The direction and magnitude of these effects depend on the evolutionary relationship of the insect herbivore to the plant; in particular, the effects of plant defenses on herbivore populations depend on whether the insect is a specialist or generalist herbivore on a specific host plant (Traw & Dawson, 2002a,b). In general, specialist herbivores tend to be unaffected by chemical defenses, many of which are lethal to generalist herbivores (Agrawal et al., 1999; Agrawal, 2000; Traw & Dawson, 2002). In some cases, chemical defenses are actually positively correlated with feeding by specialist herbivores, indicating that specialist herbivores use the chemical defenses as a cue to locate their preferred prey or exploit the defense in their own interactions with predator species (Ehrlich & Raven, 1964; Agrawal et al. …