Geography is more important than host plant use for the population genetic structure of a generalist insect herbivore.

Herbivore diet breadth is predicted to evolve in response to both bottom-up and top-down selective pressures, including host plant abundance, quality and natural enemy pressure. As the relative importance and strength of interactions change over an herbivore's geographic range, local patterns of host plant use should change in response, altering local diet breadths. Fall webworm (Hyphantria cunea) is a widespread, polyphagous moth species that feeds on hundreds of plant species worldwide. Populations of fall webworm in Colorado remain polyphagous, but their diet breadth is restricted compared to other populations and thus present an ideal opportunity to test the ecological drivers of host use by a polyphagous herbivore. We investigated how host abundance, larval performance, and parasitism affect host use for fall webworm to test how these selective pressures may act individually or in concert, as well as the role of any trade-offs among fitness components, to explain diet breadth and host use. We found that host abundance was a significant predictor of host use, which suggests a selective pressure to reduce search time for oviposition sites by adult females. We also detected an important trade-off between bottom-up and top-down selective pressures: higher quality host plants also had a greater proportion of larval mortality due to parasitism. Local patterns of host plant abundance appear to narrow the set of hosts used by fall webworms in Colorado, while the trade-off between host quality and risk of parasitism helps explain the maintenance of a generalized feeding strategy within this restricted set of hosts.

Why and how herbivorous insects choose to feed on some plant species and not others can be influenced by many factors; however, it is not always clear why herbivorous insects will choose to lay their eggs on some plants over others. The Hopkins' host selection principle (hereafter HHSP) hypothesizes that female insects prefer to lay eggs on host plants upon which they fed as larvae, but there are studies that both support and refute the hypothesis. Here, we test HHSP in a dietary generalist moth, fall webworm (Hyphantria cunea, hereafter FW). Previously, local host plant abundance has been the only factor found to determine FW host plant use; whether FW exhibit individual host plant preferences is unknown. We conducted preference tests with females of the two FW morphotypes (red-head and black-head), presenting females with four host plants: their one natal host plant, two other potential host plants, and one non-host plant for that morphotype that is a host plant for the other morphotype. Overwhelmingly, females of both morphotypes oviposited on a non-plant surface in the choice arena and those that did oviposit on a plant did not distinguish between host plants and non-host plants. Of the few FW females that oviposited on a host plant, only red-head females showed preference for their natal hosts. Our results support previous findings that HHSP is not a strong driver of host plant selection, suggesting that female FW are not selective in their choice of host plants, which may facilitate generalism in this species.

Abstract.1. The butterflyMelitaea cinxiauses two host plant species in the Åland Islands of south‐west Finland. Survey data show that host plant use is spatially variable and that the two species are not used (fed on byM.cinxialarvae) in proportion to their abundances. The pattern of host plant use byM.cinxiahas been attributed in part to plant distribution and spatial variation of butterfly oviposition preference.2. The additional roles that may be played by spatial variation in host plant quality and larval physiology (performance ability) were investigated. Seven years of field survey data and a series of laboratory experiments were used to demonstrate that neither of these variables contributed measurably to the observed pattern of host plant use.3. Specifically, while there was great variation among individuals in both the performance ability of caterpillars and host plant suitability, the two plant species appeared equally suitable, there was no within‐species geographic variation in plant suitability, and there was no evidence for adaptation of caterpillars to the locally used host plants.

Host plant use and availability were determined in early nymphal and adult-stage Schistocerca emarginata (=lineata) (Orthoptera: Acrididae) populations at six localities in Texas, USA. Early instar nymphal populations were feeding almost exclusively on either Ptelea trifoliata (Rutaceae) or Rubus trivialis (Rosaceae). This study represents the first demonstration of a geographic structure of host plant specificity in a polyphagous grasshopper. Recognizing this geographic structure required investigations of both developmental and geographical variation in host plant use. Nymphal diet breadths were significantly less than adult diet breadths at four of six localities and smaller overall when pooled nymphal and adult diet breadths were compared among sites. Neither restricted nymphal mobility nor host plant availability accounted for the observed differences in host plant use between developmental stages and among localities. Evidence suggests that the differences in host use among populations are due to host-plant-associated genetic differentiation.

1. Specialisation in host plant use is strongly correlated with speciation in many plant‐feeding insects. Specialised taxa, however, could be restricted in host range due to limits in ecological availability of host plant species rather than trade‐offs in using alternate host species.2. Moths in the genus Prodoxus are extreme specialists on Yucca and speciation is closely tied to host plant shifts. However, many Yucca ranges are allopatric. This study examined whether the bogus yucca moth Prodoxus decipiens is limited in host range because of biogeographic factors or due to differences in the characteristics of host plant species.3. In a common garden, local P. decipiens moths that use Yucca filamentosa were exposed to individuals of five Yucca species, two that are known hosts of P. decipiens in other parts of its range and three that are used by its sister species, Prodoxus quinquepunctellus.4. Local moths were attracted to flowers of all Yucca species and females attempted oviposition in the flowering stalks of all species. However, larvae successfully completed development to diapause in only one of the five host plant species. Larval development on non‐natal Yucca species was significantly reduced compared with the local host.5. The results suggest that differences in host plant characteristics among Yucca species would result in strong natural selection during a host shift. Thus, specialisation in host plant use is probably due to trade‐offs involved with using novel host plant species as well as ecological availability.

Dietary generalist herbivorous insects are widespread and often occur in a variety of environments. Across their geographic range, herbivorous insects may encounter variable plant traits as they feed on high‐quality or low‐quality plants. Herbivorous insect larvae experience both bottom‐up (host plant) and top‐down (parasitoid) factors that affect survival. Host plant quality may affect larval growth and survival in that larvae feeding on low‐quality plants often suffer reduced fitness. However, herbivores on different host plants are also subject to different levels of parasitism. High‐quality plants confer stronger larval performance (higher survival, more offspring), but larvae may also face higher parasitism. In some herbivore species, diet mediates larval immune response. The generalist insect herbivore fall webworm (FW), Hyphantria cunea Drury (Lepidoptera: Erebidae), is a moth native to North America, and its larvae have considerable variance in their performance when reared on different host plants. We investigated whether diet affects the immune response in FW larvae when they are reared on different host plant species known to vary in food quality. We measured immune response by melanization of a nylon filament. We found significant differences in immune response across host plants, indicating that diet mediates immune response in FW larvae. Our study helps elucidate the factors that cause variation in immune response in a generalist herbivore.

Herbivore diet breadth has been hypothesised to be driven by plant quality (physiological efficiency hypothesis) or natural enemies (enemy‐free space hypothesis). These hypotheses on their own are now known to be insufficient explanations for the range of herbivore diet breadths and more integrative approaches consider how trade‐offs and ecological contingencies shape host plant use by herbivores. I carried out laboratory experiments to test whether physiological efficiency and defence against natural enemies trade off when larvae of the ornate bella moth,Utetheisa ornatrix(L.) (Lepidoptera: Erebidae), a genus‐specialist pod‐boring herbivore, feeds on leaves and pods of fourCrotalariaspecies (Fabaceae). Different components ofU.ornatrixphysiological efficiency traded off amongst each other and with defence against adult predation by the spiderLycosa erythrognathaLucas (Araneae: Lycosidae).Crotalaria micansLink provided the highest survival and fast development but low pupal weight and defence.Crotalaria ochroleucaG. Don provided poor survival, slow development, and low defence, but high pupal weight.Crotalaria pallidaAiton provided slow development and intermediate survival and pupal weight, but the best defence.Crotalaria vitellinaKer Gawl. provided high survival, fast development, and efficient defence, but low pupal weight. Compared to leaves, feeding on green seeds provided greater defence against predation, and developmental benefits, independent of host plant species, but there was a large cost in boring into the pod to reach the seeds. I therefore provide evidence that the trade‐offs among physiological efficiency components and between physiological efficiency and defence could maintain multiple host plant use in specialist herbivores. I also show that feeding on green seeds incurs a cost through pod‐boring but gives the moth benefits in adult defence. Therefore, the evolution of host plant use and pod‐boring in insect herbivores could be driven by defence as much as by physiological efficiency.

Host plant use in Lepidoptera has been a primary focus in studies of ecological specialization, and multiple factors are likely to be involved in shaping the evolution of diet breadth. Here, we first describe the Salient Aroma Hypothesis, suggesting that the availability of chemical information, particularly host-associated aromas, plays a critical role in shaping dietary specialization. According to the Salient Aroma Hypothesis, herbivores active during periods when chemical information is abundant, particularly during the daytime hours when plant aromas are hypothesized to be more prevalent, are more likely to evolve specialized diets. First, with meta-analysis, we show that plants release more diverse and abundant volatile compounds during daylight hours, increasing the availability of chemical information. We found that diurnal Lepidoptera tend to have specialized diets, while nocturnal species are more generalized, consistent with the prediction of the Salient Aroma Hypothesis. We further observed that morphological differences in the antennae of female Lepidoptera are correlated with variation in diet breadth and diel activity patterns, indirectly supporting the Salient Aroma Hypothesis. While multiple factors influence host plant specialization, the Salient Aroma Hypothesis offers a useful framework linking chemical information availability (e.g. plant volatiles) and ecological specialization.

There is a great number of Charaxinae (Lepidoptera: Nymphalidae) species in the tropics whose larvae feed on several plant families. However the genus Anaea is almost always associated with Croton species (Euphorbiaceae). This work describes patterns of host plant use by immature and adult abundance on different vertical strata of sympatric Anaea species in a forest of Southeastern Brazil. Quantitative samples of leaves were taken in April/1999 and May/2000 to collect eggs and larvae of four Anaea species on C. alchorneicarpus, C. floribundus and C. salutaris in a semideciduous forest. Sampled leaves were divided into three classes of plant phenological stage: saplings, shrubs and trees. The results showed that the butterfly species are segregating in host plant use on two scales: host plant species and plant phenological stages. C. alchorneicarpus was used by only one Anaea species, whereas C. floribundus was used by three species and C. salutaris by four Anaea species. There was one Anaea species concentrated on sapling, another on sapling/shrub and two others on shrub/tree leaves. Adults of Anaea were more frequent at canopy traps but there were no differences among species caught in traps at different vertical positions. This work supplements early studies on host plant use among Charaxinae species and it describes how a guild of closely related butterfly species may be organized in a complex tropical habitat.

Molecular phylogenetic analyses were conducted to determine relationships and to investigate character evolution for the Troidini/Aristolochia interaction, in an attempt to answer the following questions: (1) what is the present pattern of use of Aristolochia by these butterflies; (2) is the pattern we see today related to the phylogeny of plants or to their chemical composition; (3) can the geographical distribution of Aristolochia explain the host plant use observed today; and (4) how did the interaction between Troidini and Aristolochia evolve? Analyses of character optimization suggest that the current pattern of host plant use of these butterflies does not seem to be constrained by the phylogeny of their food plants, neither by the secondary chemicals in these plants nor by their geographical similarity. The current host plant use in these butterflies seems to be simply opportunistic, with species with a wider geographical range using more species of host plants than those with a more restricted distribution.

We used European geometrid moths (>630 species) as a model group to investigate how life history traits linked to larval host plant use (i.e., diet breadth and host-plant growth form) and seasonal life cycle (i.e., voltinism, overwintering stage and caterpillar phenology) are related to adult body size in holometabolous insect herbivores. To do so, we applied phylogenetic comparative methods to account for shared evolutionary history among herbivore species. We further categorized larval diet breadth based on the phylogenetic structure of utilized host plant genera. Our results indicate that species associated with woody plants are, on average, larger than herb feeders and increase in size with increasing diet breadth. Obligatorily univoltine species are larger than multivoltine species, and attain larger sizes when their larvae occur exclusively in the early season. Furthermore, the adult body size is significantly smaller in species that overwinter in the pupal stage compared to those that overwinter as eggs or caterpillars. In summary, our results indicate that the ecological niche of holometabolous insect herbivores is strongly interrelated with body size at maturity.

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.

ABSTRACT. Tent caterpillars are generalists across their full host range, but display local host plant preferences. We present evidence for a new host plant record, wax currant (Ribes cereum), for western tent caterpillars (Malacosoma californicum) along the Colorado Front Range. We tested the suitability of wax currant as a host plant for western tent caterpillars as compared to chokecherry (Prunus virginiana), an abundant and commonly used host plant. We measured the density of tent caterpillar tents in areas where both host plants occur to assess host plant use. We reared tent caterpillar larvae on both host plants and measured fitness effects due to host plant quality (survival, pupal mass) and natural enemies (parasitism). We did not find a relationship between host plant abundance and use by tent caterpillars and found no evidence for a preference for either host plant. We found that western tent caterpillars do not differ in pupal mass when reared on chokecherry and on wax currant in a laboratory s...

Intraspecific specialization by insect herbivores on different host plant species contributes to the formation of genetically distinct "host races," but the effects of plant virus infection on interactions between specialized herbivores and their host plants have barely been investigated. Using three genetically and phenotypically divergent pea aphid clones (Acyrthosiphon pisum L.) adapted to either pea (Pisum sativum L.) or alfalfa (Medicago sativa L.), we tested how infection of these hosts by an insect-borne phytovirus (Bean leafroll virus; BLRV) affects aphid performance and preference. Four important findings emerged: 1) mean aphid survival rate and intrinsic rate of population growth (Rm) were increased by 15% and 14%, respectively, for aphids feeding on plants infected with BLRV; 2) 34% of variance in survival rate was attributable to clone × host plant interactions; 3) a three-way aphid clone × host plant species × virus treatment significantly affected intrinsic rates of population growth; and 4) each clone exhibited a preference for either pea or alfalfa when choosing between noninfected host plants, but for two of the three clones tested these preferences were modestly reduced when selecting among virus-infected host plants. Our studies show that colonizing BLRV-infected hosts increased A. pisum survival and rates of population growth, confirming that the virus benefits A. pisum. BLRV transmission affected aphid discrimination of host plant species in a genotype-specific fashion, and we detected three unique "virus-association phenotypes," with potential consequences for patterns of host plant use by aphid populations and crop virus epidemiology.

Plants produce an array of chemical and mechanical defences that provide protection against many herbivores and pathogens. Putatively defensive compounds and structures can even occur in floral rewards: for example, the pollen of some plant taxa contains toxic compounds or possesses conspicuous spines. Yet little is known about whether pollen defences restrict host-plant use by bees. In other words, do bees, like other insect herbivores, tolerate the defences of their specific host plants while being harmed by non-host defences? To answer this question, we compared the effects of a chemical defence from Lupinus (Fabaceae) pollen and a putative mechanical defence (pollen spines) from Asteraceae pollen on larval survival of nine bee species in the tribe Osmiini (Megachilidae) varying in their pollen-host use. We found that both types of pollen defences reduce larval survival rate in some bee species. These detrimental effects were, however, mediated by host-plant associations, with bees being more tolerant of the pollen defences of their hosts, relative to the defences of plant taxa exploited by other species. This pattern strongly suggests that bees are adapted to the pollen defences of their hosts, and that host-plant use by bees is constrained by their ability to tolerate such defences.