Abstract. Many studies show that indirect interactions between insect herbivores via shared host plants are common and often mediated by plant-induced responses to damage. However, we lack some types of data that will be needed to model how plant-mediated interactions on individual plants contribute to the population dynamics of interacting herbivores. Specifically, there are few simultaneous characterizations of both the intra-and interspecific effects that are mediated by the host plant, as well as host plant constitutive resistance. Additionally, as herbivores are likely to move among plants that differ in quality, we must consider how this set of intra-and interspecific effects differs among plant genotypes-that is, how plantmediated effects genetically vary or covary. We examined how the set of intra-and interspecific indirect effects involving the insect folivores Leptinotarsa juncta and Manduca sexta varies across different genotypes of a shared host plant, Solanum carolinense. We damaged 12 plant genotypes using both herbivore species, then measured effects on the growth of both con-and heterospecifics, as well as constitutive resistance to each herbivore. We then tested for genetic variation and covariation in plant-mediated effects and constitutive resistance among plant genotypes. We found that on average, there were significant negative intraspecific plant-mediated effects on the growth rate of both herbivores, as well as asymmetric negative interspecific effects of M. sexta on L. juncta. Both intra-and interspecific effects varied across plant genotypes. For example, the interspecific effect of M. sexta on L. juncta ranged from significantly negative to significantly positive. Additionally, there were strong correlations among the individual effects mediated by S. carolinense, particularly between constitutive resistance and both intra-and interspecific effects. We find that these genetic correlations might limit the types and strength of interactions that take place across multiple genotypes of the same plant species. Our results suggest that future models of plant-mediated interactions between herbivores should account for patterns of genetic variation and covariation when scaling from individual interactions to population-level processes.

Tolerance to herbivory (the degree to which plants maintain fitness after damage) is a key component of plant defense, so understanding how natural selection and evolutionary constraints act on tolerance traits is important to general theories of plant-herbivore interactions. These factors may be affected by plant competition, which often interacts with damage to influence trait expression and fitness. However, few studies have manipulated competitor density to examine the evolutionary effects of competition on tolerance. In this study, we tested whether intraspecific competition affects four aspects of the evolution of tolerance to herbivory in the perennial plant Solanum carolinense: phenotypic expression, expression of genetic variation, the adaptive value of tolerance, and costs of tolerance. We manipulated insect damage and intraspecific competition for clonal lines of S. carolinense in a greenhouse experiment, and measured tolerance in terms of sexual and asexual fitness components. Compared to plants growing at low density, plants growing at high density had greater expression of and genetic variation in tolerance, and experienced greater fitness benefits from tolerance when damaged. Tolerance was not costly for plants growing at either density, and only plants growing at low density benefited from tolerance when undamaged, perhaps due to greater intrinsic growth rates of more tolerant genotypes. These results suggest that competition is likely to facilitate the evolution of tolerance in S. carolinense, and perhaps in other plants that regularly experience competition, while spatio-temporal variation in density may maintain genetic variation in tolerance.

Changes in plant traits induced by herbivore damage can produce a negative feedback to increasing herbivore densities. Several aspects of these plant-mediated feedbacks are predicted to influence herbivore population dynamics, but the degree to which feedbacks are non-linear, whether their strength varies among plant genotypes, and whether they occur via changes in the quality of plant tissue or the amount of tissue available have rarely been examined. In this study, we damaged five genotypes of the perennial weed Carolina horsenettle, Solanum carolinense L. (Solanaceae), with eight densities of the false potato beetle, Leptinotarsa juncta (Germar) (Coleoptera: Chysomelidae, Chrysomelini). To account for plant quantity available to beetles, we measured the leaf area of each plant after imposing larval density treatments. We then measured the oviposition preference of L. juncta adults for damaged vs. undamaged plants and the relative growth rate (RGR) of L. juncta larvae on each plant, as well as plant trypsin proteinase inhibitor (tryPI) expression. We found that L. juncta females strongly preferred to oviposit on undamaged S. carolinense, and that preference for a plant decreased as the density of damaging larvae increased. In addition, we found decreased larval RGR and increased production of tryPIs with greater initial larval density. Effects of larval density on insect preference, performance, and tryPI expression were linear and did not vary among plant genotypes. Larval density effects were not solely due to reduced plant quantity, as plant leaf area had no effect on oviposition preference or larval performance. This suggests that effects were most likely due to changes in the quality of plant tissue. Thus, negative feedback on increasing larval density can be mediated by the effects of induced resistance on both oviposition preference and larval performance. Published models suggest that the linear, quality-mediated feedbacks observed in this experiment are predicted to stabilize herbivore population dynamics.

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