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• Soil microorganisms can influence the development of complex plant phenotypes, including resistance to herbivores. This microbiome-mediated plasticity may be particularly important for plant species that persist in environments with drastically changing herbivore pressure, for example over community succession. • We established a 15-yr gradient of old-field succession to examine the herbivore resistance and rhizosphere microbial communities of Solidago altissima plants in a large-scale field experiment. To assess the functional effects of these successional microbial shifts, we inoculated S. altissima plants with microbiomes from the 2nd, 6th and 15th successional years in a glasshouse and compared their herbivore resistance. • The resistance of S. altissima plants to herbivores changed over succession, with concomitant shifts in the rhizosphere microbiome. Late succession microbiomes conferred the strongest herbivore resistance to S. altissima plants in a glasshouse experiment, paralleling the low levels of herbivory observed in the oldest communities in the field. • While many factors change over succession and may contribute to the shifts in rhizosphere communities and herbivore resistance we observed, our results indicated that soil microbial shifts alone can alter plants’ interactions with herbivores. Our findings suggest that changes in soil microbial communities over succession can play an important role in enhancing plant resistance to herbivores.

Patterns of herbivory may vary with fine-scale plant community structure: the degree of damage plants experience may depend on their neighbors (i.e., associational resistance or susceptibility). Differential herbivory, in turn, may facilitate a shift in plant community structure. We investigated these reciprocal effects of plant community structure and insect herbivory in a field mesocosm experiment with closely related, native Asteraceae that co-occur in early-successional habitats (old fields). After one year of establishment, we excluded or augmented insect herbivores for two years in equal-density communities of three types: goldenrod-dominated (Solidago spp.) or aster-dominated (Symphyotrichum spp.) congeneric communities and mixtures of the two genera. In manipulated outbreaks, overall and species-specific patterns of herbivory by the main herbivore, the leaf beetle Trirhabda virgata, varied dramatically with community composition. In both years, the preferred goldenrods suffered 25—70% higher defoliation in mixtures with the less-preferred asters (i.e., associational susceptibility), compared to when growing with congeners; in contrast, asters experienced lower damage in mixtures (i.e., associational resistance). Insect herbivory consistently reduced overall plant productivity, and promoted colonization by other oldfield species. Importantly, herbivory also initiated a shift in the structure of the plant communities, and this effect depended on the starting community composition, implying potential reciprocal effects. For instance, only in mixtures did elevated herbivory reduce the proportional abundance of the preferred host, and the old-field dominant, Solidago altissima. Our findings underscore the importance of plant community composition for variation in and impacts of herbivory and suggest the possibility of feedbacks between herbivory and local community structure as one of the mechanisms contributing to the maintenance of vegetation heterogeneity.

Plant-induced responses to multiple herbivores can mediate ecological interactions among herbivore species, thereby influencing herbivore community composition in nature. Several studies have indicated high specificity of induced responses to different herbivore species. In addition, there may be genetic variation for plant response specificity that can have significant ecological implications, by altering the competitive strength and hierarchical relationships among interacting herbivore species. However, few studies have examined whether plant populations harbor genetic variation for induction specificity. Using three distinct genotypes of Solidago altissima plants, we examined whether specialist herbivore species Dichomeris leuconotella, Microrhopala vittata, and Trirhabda virgata elicit specific induction responses from plants (specificity of elicitation), and whether induction differentially affects these herbivore species (specificity of effect). Results from bioassays and secondary metabolite analyses suggest that there is specificity of both elicitation and effect in the induced responses: D. leuconotella and M. vittata preferred and performed better on leaves damaged by conspecifics than heterospecifics, and induced qualitatively different secondary metabolite profiles. In contrast, T. virgata equally avoided but physiologically tolerated all types of damage. These patterns of specificity suggest that plant-induced responses mediate asymmetric competitive interactions between herbivore species, which potentially intensifies inter-specific relative to intra-specific competition. Plant genotypes widely differed in overall susceptibility to the herbivores and secondary metabolite production, yet we found no genotype-by-treatment interactions in insect performance, preference and plant secondary metabolite production. This lack of genetic variation for induction specificity suggests that competitive interactions between herbivore species on S. altissima are homogeneous across plant genotypes.

The evolution of increased competitive ability (EICA) hypothesis predicts that invasive plant species may escape their specialized natural enemies in their introduced range and subsequently evolve with a decrease in investment in anti-herbivore chemical defenses relative to native conspecifics. We compared the chemical profile of 10 populations of US native and 20 populations of European invasive Solidago gigantea. To test for differences in inducibility between native and invasive populations, we measured secondary chemistry in both damaged and undamaged plants. We also performed bioassays with three specialist and two generalist insect herbivores from four different feeding guilds. There was no evidence that invasive populations had reduced concentrations of sesquiterpenes, diterpenes, or short-chain hydrocarbons (SCH), although significant variation among populations was detected. Sesquiterpene and diterpene concentrations were not influenced by damage to the host plant, whereas SCH concentrations were decreased by damage for both native and invasive plants. Performance of the three specialist insects was not affected by the continental origin of the host plant. However, larval mass of the generalist caterpillar Spodoptera exigua was 37% lower on native plants compared to invasive plants. The other generalist insect, a xylem-tapping spittlebug that occurs on both continents, performed equally well on native and invasive plants. These results offer partial support for the defense predictions of the EICA hypothesis: the better growth of Spodoptera caterpillars on European plants shows that some defenses have been lost in the introduced range, even though our measures of secondary chemistry did not detect differences between continents. Our results show significant variation in chemical defenses and herbivore performance across populations on both continents and emphasize the need for analysis across a broad spatial scale and the use of multiple herbivores.

We extend the resource concentration hypothesis (herbivorous insects are more likely to find and stay in more dense and less diverse patches of their host plants) to plant communities. Specifically, whenever superior plant competitors spread to form dense stands, they will be found and attacked by their specialist insect enemies. This will decrease host plant abundance, causing a reduction in standing crop biomass, which will indirectly increase subordinate competitors and plant species richness. In this study, we found that a native, specialist chrysomelid beetle (Trirhabda virgata) in an old-field community decreased total standing crop biomass, leading to an increase in plant species richness. This reduction in biomass was due solely to a reduction in the biomass of the beetle's host plant, meadow goldenrod (Solidago altissima), which was the dominant plant species in this community. Our results demonstrate that when a superior competitor increases in density, the per-stem impact of herbivores increases due to a buildup of these herbivores in high-density host patches. Specifically, we found that as goldenrod density increased, the per-stem abundance of Trirhabda virgata also increased. In turn, species richness increased as the negative effect of insects on goldenrod biomass increased. Recent research suggests that litter accumulation could negate or cancel the effect of herbivorous insects on plant communities because litter accumulation increases with standing crop biomass, causing a decline in species richness. The litter accumulation hypothesis states that, in productive communities, the increase in the abundance of the superior competitor will lead to a dense accumulation of plant litter, causing a decline in species richness. Consistent with this hypothesis, we found that as the biomass of the dominant plant species increased, litter mass also increased. In turn, species richness decreased as the negative effects of litter on stem density increased. Interestingly, the effect of litter on stem density depended on whether insects were present. Our results suggest the potential for a general rule: specialist insect herbivores will function as classic keystone species in plant communities whenever host species form large, dense aggregations if host plants are dominant species.

Recent theoretical and empirical studies suggest that an animals's long-distance dispersal behavior can have a major influence on its population dynamics. In this study I examined the role of dispersal in the population dynamics of a goldenrod leaf beetle, Trirhabda virgata. I measured the long-distance emigration rates of this beetle in the field under different population densities. I manipulated densities of Trirhabda larvae early in the season to create crowded, heavily defoliated host patches and uncrowded, lightly defoliated patches. Newly emerged, teneral beetles were added to these two density treatments and their long-distance emigration rates were measured. Beetles flew from host patches that had high densities of adults and heavy damage caused by larvae. No flight was detected from low-density patches with little defoliation. Beetles that had developed in these crowded and uncrowded patches were placed on lush, undefoliated plants as teneral adults. Their emigration rates did not differ. In a final experiment, female beetles were placed on plants with or without mates. The plants were one of two types, either heavily defoliated or lush. On lush plants, females without mates showed a stronger tendency to leave than females that had access to mates. On defoliated plants, females left at high rates whether or not mates were present. These experiments indicate that long-distance emigration is triggered by conditions experienced during the teneral phase. Heavy host defoliation inflicted by high population densities appears to induce flight. This density-dependent emigration should reduce local population pressure and result in the spread of local outbreaks across a larger area. The influence of dispersal on population dynamics will vary depending on the pattern of beetle densities across the region.

Consumers can mediate the composition of plant communities and alter ecosystem processes. Although herbivores usually increase N availability in the short term, they might decrease it in the long term. I investigated the long-term effect of insect herbivores on leaf tissue quality and soil N availability in goldenrod (Solidago altissima) fields using two approaches: (1) I compared plots from which herbivores had been excluded for 17 years with adjacent plots that had experienced normal levels of herbivory, and (2) I examined a chronosequence of nine goldenrod fields representing three successional stages: early, middle, and late. These parallel approaches showed that, in the long term, herbivores decrease the quality of leaf litter and soil N availability in goldenrod fields. These long-term effects appear to compensate for various short-term effects that increase N availability in the soil (e.g., added frass, increased light penetration). Furthermore, herbivores decrease leaf litter quality and N availability by reducing the quality of leaf tissue within the same species. This pattern may result from insect herbivores preferentially grazing on plants with a high N content thereby increasing the amount of recalcitrant litter over the course of succession.

In central New York, two chrysomelid beetles, Trirhabda virgata and T. borealis, frequently cause severe defoliation of tall goldenrod, Solidago altissima. This plant is also the primary host of Publilia concava (Membracidae), a sap-feeding treehopper that is tended by ants, especially Formica spp. Staged encounters indicate that Formica ants attack adult Trirhabda beetles on goldenrod stems bearing membracids. Such stems escape defoliation by Trirhabda, and show greater mean height increment and seed production than their nearest neighbors without ants. The degree of plant protection depends on the duration of Formica presence. During Trirhabda outbreaks, only stems bearing Formica ants for most of the season are likely to produce flowers and seeds. While Formica ants do not exclude Trirhabda larvae from goldenrod stems, they do deter feeding; plants with Formica ants experience significantly less defoliation by larvae than neighboring stems without ants. Two smaller, less aggressive an species (Prenolepis imparis and Myrmica sp.) do not affect either larval or adult beetle densities.

The leaf beetles Trirhabda borealis and T. virgata are specialist herbivores of meadow goldenrods, Solidago spp., in central New York. Upon emergence, both larvae and adults must actively search for food plants, and must choose among the five goldenrod species (S. canadensis, S. gigantea, S. graminifolia, S. juncea and S. rugosa) that co-occur in old fields. The relationship between Trirhabda foraging decisions and plant quality was examined by comparing food preferences in the field with the performance of beetles caged on each host. Trirhabda adults were highly selective in their use of food plants. Adults of T. borealis preferred a single host, S. canadensis, while T. virgata adults were most common on S. canadensis and S. gigantea. These preferences were not strictly related to variation in plant quality. In the laboratory, T. borealis performed equally well on four goldenrods (but completely failed to reproduce when fed S. graminifolia), and T. virgata performed equally well on all five hosts. Larval feeding preferences in each beetle species were broader, and were more in accord with subtle variation in plant quality. Newly-hatched T. borealis larvae readily colonized four hosts and rejected only S. graminifolia, which conferred the lowest survivorship and the slowest growth. Larvae of T. virgata accepted each host, even though growth rates were somewhat slower on S. graminifolia and S. juncea. Ontogenetic differences in host preference and host tolerance may have evolved because of the different host-finding abilities of each beetle stage. During host search, the sluggish, newly-emerged larvae may be more food-limited and accept even marginally inferior food plants. The relatively mobile adults are more discriminating and use a subset of suitable hosts. The intermingled dispersion of Solidago species in old fields results in frequent larval colonization of hosts seldom used by adults. In diverse plant communities, ultimate patterns of food plant choice can be a complex function of at least three factors: intrinisic plant quality, local plant dispersion, and the host-search abilities of the insect forager.

The goldenrods Solidago bicolor and S. nemoralis are scarce in the old-field habitat of a goldenrod leaf beetle, Trirhabda virgata LeConte, in central New York. Two additional species, S. caesia and S. flexicaulis, are wholly restricted to woodlands. In the laboratory only S. bicolor supported complete development and reproduction by T. virgata, but this plant was an inferior host compared to five common meadow goldenrods that had been tested previously. Nine Solidago spp. can be ranked in four categories of suitability for T. virgata. Host quality is not well correlated with taxonomic affinities of the plants.