Predicting novel trophic interactions in a non‐native world

Humans are altering the global distributional ranges of plants, while their co‐evolved herbivores are frequently left behind. Native herbivores often colonise non‐native plants, potentially reducing invasion success or causing economic loss to introduced agricultural crops. We developed a predictive model to forecast novel interactions and verified it with a data set containing hundreds of observed novel plant–insect interactions. Using a food network of 900 native European butterfly and moth species and 1944 native plants, we built an herbivore host‐use model. By extrapolating host use from the native herbivore–plant food network, we accurately forecasted the observed novel use of 459 non‐native plant species by native herbivores. Patterns that governed herbivore host breadth on co‐evolved native plants were equally important in determining non‐native hosts. Our results make the forecasting of novel herbivore communities feasible in order to better understand the fate and impact of introduced plants.     

Geographic variation in performance of a widespread generalist insect herbivore

1. Generalist herbivores are often widespread and occur in a variety of environments. Due to their broad distribution, it is likely that some populations of generalists will encounter host plants with geographic variation in traits that could affect the herbivore's growth and survival (i.e. performance). However, the geographic pattern of performance has rarely been studied for generalists, especially across large geographic ranges. 2. This study used one of the most generalist herbivore species known, the fall webworm (Hyphantria cunea Drury 1773, Erebidae, Lepidoptera), to experimentally test how the performance of a local population of fall webworms varies with increasing geographic distance of the host plant population from the local herbivore population. Specifically, a transplant experiment was used to compare the performance of one fall webworm population feeding on its local host plants with its performance on host populations from two other locations, 1300 and 2600 km away. 3. It was found that fall webworms performed better on their local host plant populations than on populations from other regions, with performance at its lowest when reared on hosts of the same species from the farthest region. It was also found that local fall webworms do not perform well on hosts commonly used by fall webworms at the other two, more distant sites. 4. This study helps to elucidate how the performance of generalist herbivores varies along their geographic range and suggests possible local adaptation to different sets of hosts across sites.     

The role of herbivore‐ and plant‐related experiences in intraspecific host preference of a relatively specialized parasitoid

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivorederived cues than plant-derived cues. Microplitis croceipes (Cresson)(Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.)(Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores:(i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. I terestingly, naive parasitoids attacked more soybeathan cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.     

The roles of geography and founder effects in promoting host‐associated differentiation in the generalist bogus yucca moth Prodoxus decipiens

There is ample evidence that host shifts in plant‐feeding insects have been instrumental in generating the enormous diversity of insects. Changes in host use can cause host‐associated differentiation (HAD) among populations that may lead to reproductive isolation and eventual speciation. The importance of geography in facilitating this process remains controversial. We examined the geographic context of HAD in the wide‐ranging generalist yucca moth Prodoxus decipiens. Previous work demonstrated HAD among sympatric moth populations feeding on two different Yucca species occurring on the barrier islands of North Carolina, USA. We assessed the genetic structure of P. decipiens across its entire geographic and host range to determine whether HAD is widespread in this generalist herbivore. Population genetic analyses of microsatellite and mtDNA sequence data across the entire range showed genetic structuring with respect to host use and geography. In particular, genetic differentiation was relatively strong between mainland populations and those on the barrier islands of North Carolina. Finer scale analyses, however, among sympatric populations using different host plant species only showed significant clustering based on host use for populations on the barrier islands. Mainland populations did not form population clusters based on host plant use. Reduced genetic diversity in the barrier island populations, especially on the derived host, suggests that founder effects may have been instrumental in facilitating HAD. In general, results suggest that the interplay of local adaptation, geography and demography can determine the tempo of HAD. We argue that future studies should include comprehensive surveys across a wide range of environmental and geographic conditions to elucidate the contribution of various processes to HAD.     

Competition and facilitation in multispecies plant-herbivore systems of productive environments

We develop a multispecies plant-herbivore model to explore how plant competition for light and the selectivity of herbivores affect abundance patterns of plants and herbivores along productivity gradients. The model considers a small and a tall plant species, a generalist herbivore, and a selective herbivore. The selective herbivore feeds only on the small plant species. In the absence of the generalist herbivore, the tall plant species becomes increasingly dominant with increasing productivity, and the small plant and its selective herbivore disappear. The model shows that generalist herbivores can facilitate selective herbivores by suppressing competition for light. This favours the small plant species, and thereby the selective herbivores. The model predictions are qualitatively consistent with field studies of multispecies plant-herbivore systems.     

Species-specific herbivore–herbivore interactions and plant responses to sequential attack by multiple herbivores on a perennial woody plant

When plants are sequentially attacked by multiple herbivores, herbivore identity and host specialization can greatly influence the patterns of herbivore–herbivore and plant–herbivore interactions. However, how prior herbivory and the resulting induced plant responses potentially affect subsequent herbivores deserves further investigation. In this study, we conducted a common-garden experiment that manipulated sequential herbivory by the specialist caterpillar Gadirtha fusca Pogue (Lepidoptera: Nolidae) and the generalist caterpillar Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) on Chinese tallow, Triadica sebifera (L.) Small (Euphorbiaceae). We tested how prior exposure to herbivores with different levels of host specialization affected the performance of subsequently arriving con- and heterospecifics, as well as plant growth and defense responses under subsequent herbivory. We found that prior exposure to the specialist G. fusca facilitated the performance of subsequent conspecifics, resulting in a significant decrease in the growth (height and stem diameter at ground level) of tallow plants. However, prior exposure to the generalist S. litura did not affect the feeding of subsequent con- or heterospecifics or the growth of tallow plants. Sequential herbivory by specialist and generalist conspecifics resulted in lower levels of tannins and flavonoids, respectively, in leaves of tallow plants, whereas sequential herbivory by the two species did not affect the levels of tannins or flavonoids, compared to a single damage event. We conclude that herbivore species-specific plant responses appear to be more important than herbivore identity or specialization in determining herbivore–herbivore interactions and plant responses to sequential herbivore attack.     

Rapid evolution of Medicago polymorpha during invasion shifts interactions with the soybean looper

The Enemy Release Hypothesis posits that invasion of novel habitats can be facilitated by the absence of coevolved herbivores. However, a new environment and interactions with unfamiliar herbivores may impose selection on invading plants for traits that reduce their attractiveness to herbivores or for enhanced defenses compared to native host plants, leading to a pattern similar to enemy release but driven by evolutionary change rather than ecological differences. The Shifting Defense Hypothesis posits that plants in novel habitats will shift from specialized defense mechanisms to defense mechanisms effective against generalist herbivores in the new range. We tested these ideas by comparing herbivore preference and performance of native (Eurasia)‐ and invasive (New World)‐range Medicago polymorpha, using a generalist herbivore, the soybean looper, that co‐occurs with M. polymorpha in its New World invaded range. We found that soybean loopers varied in preference and performance depending on host genotype and that overall the herbivore preferred to consume plant genotypes from naïve populations from Eurasia. This potentially suggests that range expansion of M. polymorpha into the New World has led to rapid evolution of a variety of traits that have helped multiple populations become established, including those that may allow invasive populations to resist herbivory. Thus, enemy release in a novel range can occur through rapid evolution by the plant during invasion, as predicted by the Shifting Defense Hypothesis, rather than via historical divergence.     

Direct and indirect effects of marsupial browsing on a foundation tree species

Plant‐mediated indirect effects can be important ecological drivers in plant communities, especially in systems where extended genetic effects of foundation species can shape communities and influence ecosystem dynamics. Here we investigate the direct and indirect effects of uncontrolled browsing by marsupial herbivores including the common brushtail possum Trichosurus vulpecula, Bennetts wallaby Macropus rufogriseus and the red‐bellied pademelon Thylogale billardierii, in a Eucalyptus system known to have extended community and ecosystem genetic effects. In a common garden trial containing 525 full‐sib families from an incomplete diallel crossing program located in northeastern Tasmania, Australia, we assessed the genetic basis to herbivore preferences, the impact of a single and repeated marsupial browsing event on tree fitness and morphological traits and the associated indirect plant‐mediated effects on a subsequent herbivore, autumn gum moth Mnesampela privata. Marsupial browsing was not influenced by plant genetics, but spatial components instead affected the pattern of damage across the trial. Marsupial browsing had significant impacts on tree development, morphology and survival, resulting in reductions in survival, height and basal area, an increase proportion in multiple stems, delays in flowering as well as delays in phase change from juvenile to adult foliage. Fitness impacts were minimal in response to a once‐off browsing event, but effects were exacerbated when trees suffered repeated browsing. We demonstrate clear plant‐mediated indirect effects of marsupial browsing on subsequent tree use by an invertebrate herbivore, through induced changes in plant morphology. Such indirect effects have the potential to influence biotic community structure on a foundation species host‐plant, and the evolutionary interactions that occur between organisms and the host‐plant themselves.     

Herbivore species identity rather than diversity of the non‐host community determines foraging behaviour of the parasitoid wasp Cotesia glomerata

Extensive research has been conducted to reveal how species diversity affects ecosystem functions and services. Yet, consequences of diversity loss for ecosystems as a whole as well as for single community members are still difficult to predict. Arthropod communities typically are species‐rich, and their species interactions, such as those between herbivores and their predators or parasitoids, may be particularly sensitive to changes in community composition. Parasitoids forage for herbivorous hosts by using herbivore‐induced plant volatiles (indirect cues) and cues produced by their host (direct cues). However, in addition to hosts, non‐suitable herbivores are present in a parasitoid's environment which may complicate the foraging process for the parasitoid. Therefore, ecosystem changes in the diversity of herbivores may affect the foraging efficiency of parasitoids. The effect of herbivore diversity may be mediated by either species numbers per se, by specific species traits, or by both. To investigate how diversity and identity of non‐host herbivores influence the behaviour of parasitoids, we created environments with different levels of non‐host diversity. On individual plants in these environments, we complemented host herbivores with 1–4 non‐host herbivore species. We subsequently studied the behaviour of the gregarious endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) while foraging for its gregarious host Pieris brassicae L. (Lepidoptera: Pieridae). Neither non‐host species diversity nor non‐host identity influenced the preference of the parasitoid for herbivore‐infested plants. However, after landing on the plant, non‐host species identity did affect parasitoid behaviour, whereas non‐host diversity did not. One of the non‐host species, Trichoplusia ni Hübner (Lepidoptera: Noctuidae), reduced the time the parasitoid spent on the plant as well as the number of hosts it parasitized. We conclude that non‐host herbivore species identity has a larger influence on C. glomerata foraging behaviour than non‐host species diversity. Our study shows the importance of species identity over species diversity in a multitrophic interaction of plants, herbivores, and parasitoids.     

Host‐plant genotypic diversity and community genetic interactions mediate aphid spatial distribution

Genetic variation in plants can influence the community structure of associated species, through both direct and indirect interactions. Herbivorous insects are known to feed on a restricted range of plants, and herbivore preference and performance can vary among host plants within a species due to genetically based traits of the plant (e.g., defensive compounds). In a natural system, we expect to find genetic variation within both plant and herbivore communities and we expect this variation to influence species interactions. Using a three‐species plant‐aphid model system, we investigated the effect of genetic diversity on genetic interactions among the community members. Our system involved a host plant (Hordeum vulgare) that was shared by an aphid (Sitobion avenae) and a hemi‐parasitic plant (Rhinanthus minor). We showed that aphids cluster more tightly in a genetically diverse host‐plant community than in a genetic monoculture, with host‐plant genetic diversity explaining up to 24% of the variation in aphid distribution. This is driven by differing preferences of the aphids to the different plant genotypes and their resulting performance on these plants. Within the two host‐plant diversity levels, aphid spatial distribution was influenced by an interaction among the aphid's own genotype, the genotype of a competing aphid, the origin of the parasitic plant population, and the host‐plant genotype. Thus, the overall outcome involves both direct (i.e., host plant to aphid) and indirect (i.e., parasitic plant to aphid) interactions across all these species. These results show that a complex genetic environment influences the distribution of herbivores among host plants. Thus, in genetically diverse systems, interspecific genetic interactions between the host plant and herbivore can influence the population dynamics of the system and could also structure local communities. We suggest that direct and indirect genotypic interactions among species can influence community structure and processes.     

Determinants of parasitoid communities of willow‐galling sawflies: habitat overrides physiology,host plant and space

Studies on the determinants of plant–herbivore and herbivore–parasitoid associations provide important insights into the origin and maintenance of global and local species richness. If parasitoids are specialists on herbivore niches rather than on herbivore taxa, then alternating escape of herbivores into novel niches and delayed resource tracking by parasitoids could fuel diversification at both trophic levels. We used DNA barcoding to identify parasitoids that attack larvae of seven Pontania sawfly species that induce leaf galls on eight willow species growing in subarctic and arctic–alpine habitats in three geographic locations in northern Fennoscandia, and then applied distance‐ and model‐based multivariate analyses and phylogenetic regression methods to evaluate the hierarchical importance of location, phylogeny and different galler niche dimensions on parasitoid host use. We found statistically significant variation in parasitoid communities across geographic locations and willow host species, but the differences were mainly quantitative due to extensive sharing of enemies among gallers within habitat types. By contrast, the divide between habitats defined two qualitatively different network compartments, because many common parasitoids exhibited strong habitat preference. Galler and parasitoid phylogenies did not explain associations, because distantly related arctic–alpine gallers were attacked by a species‐poor enemy community dominated by two parasitoid species that most likely have independently tracked the gallers’ evolutionary shifts into the novel habitat. Our results indicate that barcode‐ and phylogeny‐based analyses of food webs that span forested vs. tundra or grassland environments could improve our understanding of vertical diversification effects in complex plant–herbivore–parasitoid networks.     

DNA barcoding confirms polyphagy in a generalist moth,Homona mermerodes (Lepidoptera: Tortricidae)

Recent DNA barcoding of generalist insect herbivores has revealed complexes of cryptic species within named species. We evaluated the species concept for a common generalist moth occurring in New Guinea and Australia, Homona mermerodes, in light of host plant records and mitochondrial cytochrome c oxidase I haplotype diversity. Genetic divergence among H. mermerodes moths feeding on different host tree species was much lower than among several Homona species. Genetic divergence between haplotypes from New Guinea and Australia was also less than interspecific divergence. Whereas molecular species identification methods may reveal cryptic species in some generalist herbivores, these same methods may confirm polyphagy when identical haplotypes are reared from multiple host plant families. A lectotype for the species is designated, and a summarized bibliography and illustrations including male genitalia are provided for the first time.     

Leaf traits and herbivore selection in the field and in cafeteria experiments

Abstract Despite the vast diversity and complexity of herbivores, plants and their interactions, most authors agree that a small number of components of leaf quality affect preference by generalist herbivores in a predictable way. However, herbivore preference is determined not only by intrinsic plant attributes and herbivore biology but also by the environmental context. Within this framework, we aimed to analyse general interspecific trends in the association between herbivory and leaf traits over a wide range of angiosperms from central Argentina. We (i) tested for consistent associations between leaf traits, consumption in the field, and preference of generalist invertebrate herbivores in cafeteria experiments; (ii) assessed how well herbivore preferences in cafeterias matched leaf consumption in the field; and (iii) developed a simple conceptual model linking leaf traits, herbivore preference in cafeterias and consumption in the field. In general, we found that tender leaves with higher nutritional quality were preferred by herbivores, both in the field and in cafeteria experiments. According to our model, this relationship between field and cafeteria consumption and leaf quality is observed when generalist herbivores and plants of high accessibility are considered. However, differences between leaf consumption in the field and in cafeteria experiments can also be found. At least two reasons can account for this: (i) specialized plant–herbivore relationships often occur in the field, whereas cafeteria experiments tend to consider only one or a few generalist herbivores; (ii) different plant species growing in the field often differ in their degree of accessibility to herbivores, whereas in cafeteria experiments all species are equally accessible. Our results add new evidence to a growing consensus that, although herbivory in the field is determined by many factors, consistent patterns of differential susceptibility to foliar feeders can be found in leaves differing in nutritional quality and thus in resource‐use strategy.     

Divergence in host use ability of a marine herbivore from two habitat types

We studied ecological divergence of host use ability in a generalist marine herbivore living in two distinct host plant assemblages. We collected Idotea balthica isopods from three populations dominated by the brown alga Fucus vesiculosus and three dominated by the seagrass Zostera marina. In two reciprocal common garden feeding experiments for adult and laboratory‐born juvenile isopods, we found that isopods from both assemblages performed better with their sympatric dominant host species than did isopods allopatric to this host. This indicates parallel divergence of populations according to the sympatric host plant assemblage. Furthermore, initial body size and body size‐dependent mortality differed between populations from the two assemblages. In nature, this may result in lower fitness of immigrants compared with that of residents and consequently reinforce divergence of the populations. Finally, we discuss how phenotypic plasticity and maternal and random effects may associate with the results.     

Selection Mosaic Exerted by Specialist and Generalist Herbivores on Chemical and Physical Defense of Datura stramonium

Selection exerted by herbivores is a major force driving the evolution of plant defensive characters such as leaf trichomes or secondary metabolites. However, plant defense expression is highly variable among populations and identifying the sources of this variation remains a major challenge. Plant populations are often distributed across broad geographic ranges and are exposed to different herbivore communities, ranging from generalists (that feed on diverse plant species) to specialists (that feed on a restricted group of plants). We studied eight populations of the plant Datura stramonium usually eaten by specialist or generalist herbivores, in order to examine whether the pattern of phenotypic selection on secondary compounds (atropine and scopolamine) and a physical defense (trichome density) can explain geographic variation in these traits. Following co-evolutionary theory, we evaluated whether a more derived alkaloid (scopolamine) confers higher fitness benefits than its precursor (atropine), and whether this effect differs between specialist and generalist herbivores. Our results showed consistent directional selection in almost all populations and herbivores to reduce the concentration of atropine. The most derived alkaloid (scopolamine) was favored in only one of the populations, which is dominated by a generalist herbivore. In general, the patterns of selection support the existence of a selection mosaic and accounts for the positive correlation observed between atropine concentration and plant damage by herbivores recorded in previous studies.     

A meta‐analysis of insect pest behavioral manipulation with plant volatiles

Many insect pests utilize plant volatiles for host location and untangling the mechanisms of this process can provide tools for pest management. Numerous experimental results have been published on the effect of plant volatiles on insect pests. We used a meta‐analysis to summarize this knowledge and to look for patterns. Our goal was to identify herbivore and plant traits that might explain the herbivores’ behavioral response to plant volatiles in field applications. We scored a total of 374 unique plant volatile‐insect herbivore interactions obtained from 34 published studies investigating 50 herbivore pest species. Attractants had a significant effect on insect herbivore abundance but repellents did not; this latter result could be a result of the comparatively small number of field studies that tested plant volatiles as repellents (3%). Females were significantly more attracted to plant volatile baits than males. The diet breadth of herbivores was independent of a behavioral response to plant volatiles, but more case studies show effects of volatiles on chewers, followed by wood‐borers and sap‐feeders. There are more demonstrations of attraction to plant volatiles in Lepidoptera than in Thysanoptera. The method of plant volatile application had a significant effect on herbivore abundance and increasing the number of chemicals in individual baits attracted more herbivores. The magnitude of the response of herbivores to plant volatiles in forest and agricultural habitats was similar. We explore consistent patterns and highlight areas needing research in using plant volatiles to manage insect pests.     

The tri-trophic interactions hypothesis: interactive effects of host plant quality, diet breadth and natural enemies on herbivores

Several influential hypotheses in plant-herbivore and herbivore-predator interactions consider the interactive effects of plant quality, herbivore diet breadth, and predation on herbivore performance. Yet individually and collectively, these hypotheses fail to address the simultaneous influence of all three factors. Here we review existing hypotheses, and propose the tri-trophic interactions (TTI) hypothesis to consolidate and integrate their predictions. The TTI hypothesis predicts that dietary specialist herbivores (as compared to generalists) should escape predators and be competitively dominant due to faster growth rates, and that such differences should be greater on low quality (as compared to high quality) host plants. To provide a preliminary test of these predictions, we conducted an empirical study comparing the effects of plant (Baccharis salicifolia) quality and predators between a specialist (Uroleucon macolai) and a generalist (Aphis gossypii) aphid herbivore. Consistent with predictions, these three factors interactively determine herbivore performance in ways not addressed by existing hypotheses. Compared to the specialist, the generalist was less fecund, competitively inferior, and more sensitive to low plant quality. Correspondingly, predator effects were contingent upon plant quality only for the generalist. Contrary to predictions, predator effects were weaker for the generalist and on low-quality plants, likely due to density-dependent benefits provided to the generalist by mutualist ants. Because the TTI hypothesis predicts the superior performance of specialists, mutualist ants may be critical to A. gossypii persistence under competition from U. macolai. In summary, the integrative nature of the TTI hypothesis offers novel insight into the determinants of plant-herbivore and herbivore-predator interactions and the coexistence of specialist and generalist herbivores.     

Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores

Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra‐ and interspecific), most studies have analyzed these effects separately. We compared the effects of genotypic diversity of big‐leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large‐scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of fifty‐nine 21 × 21‐m plots, with 64 tree saplings each (3‐m spacing between plants). Plots were either mahogany monocultures or species polycultures of four species (including mahogany) and – within each of these two plot types – mahogany was represented by either one or four genotypes. Throughout a five‐month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no effect of mahogany genotypic diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had significant effects on both specialists but neither of the generalist herbivores. Specifically, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such effects were not mediated by effects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not influence spider abundance. This study is one of few to directly compare sources of plant diversity, and uniquely compares such effects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity effects outweigh those of genotypes, and our results suggest that such effects are stronger on specialist than generalist herbivores.     

Larval feeding induced defensive responses in tobacco: comparison of two sibling species of <Emphasis Type="Italic">Helicoverpa</Emphasis> with different diet breadths

Plants respond differently to damage by different herbivorous insects. We speculated that sibling herbivorous species with different host ranges might also influence plant responses differently. Such differences may be associated with the diet breadth (specialization) of herbivores within a feeding guild, and the specialist may cause less intensive plant responses than the generalist. The tobacco Nicotinana tabacum L. is the common host plant of a generalist Helicoverpa armigera (Hübner) and a specialist H. assulta Guenée (Lepidoptera, Noctuidae). The induced responses of tobacco to feeding of these two noctuid herbivores and mechanical wounding were compared. The results showed that the feeding of the specialist H. assulta and the generalist H. armigera resulted in the same inducible defensive system, but response intensity of plants was different to these two species. Inductions of jasmonic acid (JA), lipoxygenase (LOX), and proteinase inhibitors (PIs) were not significantly different concerning these two species, but H. assulta caused the less intensive foliar polyphenol oxidase (PPO) increase, more intensive nicotine and peroxidase (POD) increases in tobacco than H. armigera. The defensive response of plant to herbivores with different diet breadth seems to be more complicated than we expected, and the specialist does not necessarily cause less intensive plant responses than the generalist.     

Testing host‐associated differentiation in a quasi‐endophage and a parthenogen on native trees

Host‐associated differentiation (HAD) is the formation of genetically divergent host‐associated sub‐populations. Evidence of HAD has been reported for multiple insect herbivores to date, but published studies testing more than one herbivore for any given host‐plant species pair is limited to herbivores on goldenrods. This limits the number of pair‐wise comparisons that can be made about insect life‐history traits that might facilitate or inhibit host‐race development in general. Two traits previously proposed to facilitate HAD include endophagy and parthenogenesis. We tested for HAD in two herbivores, a quasi‐endophagous caterpillar and a parthenogenetic aphid, feeding on two closely related species of hickories. We found that the quasi‐endophage is panmictic, whereas the parthenogen exhibits HAD on their sympatric host plants, pecan and water hickory, at a geographic mesoscale. This is an important first step in the characterization of HAD in multiple insect herbivores using North American hickories, a host‐plant system with many shared parthenogens.