Variation in plant‐mediated intra‐ and interspecific interactions among insect herbivores: effects of host genotype

Abstract

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.

Pathways for plant‐mediated negative feedback to insect herbivores: accounting for non‐linear effects of larval density on plant quality and quantity

“…(Ortego et al., ) and were associated with reduced larval growth on multiple genotypes in our experiment. Second, in a previous study, we found that previous L. juncta larval damage to S. carolinense , at levels within the range of this experiment, significantly reduced the growth of conspecifics feeding on undamaged, isolated leaves on the same plant (McNutt & Underwood, ). Importantly, in these previous experiments, damaging larvae did not have access to leaves used for bioassays, so these negative effects on growth were unambiguously indirect and mediated by host plant systemic induced resistance.…”

“…Our study is consistent with the interpretation that reducing oviposition on damaged plants may help L. juncta to avoid exposing their offspring to plant induced responses that affect larval growth. Although the effects of larval density on larval performance were not particularly strong in this experiment, we have previously found that S. carolinense induced resistance to L. juncta damage can reduce larval growth by up to 42% over 48 h of feeding (McNutt & Underwood, ). The avoidance of damaged plants by L. juncta females may also be adaptive for other reasons.…”

“…We found that neither the strength of larval density feedbacks on L. juncta oviposition preference nor the larval density effect on growth rate differed among the small sample of plant genotypes in our study. This lack of genetic variation in density feedbacks via L. juncta growth rate is somewhat surprising, considering we have previously found genetic variation in S. carolinense induced resistance to L. juncta larvae (McNutt & Underwood, ). Although variation in induced resistance does thus exist, random samples of genotypes may not contain significant variation in their defense expression or effects on herbivores (as in this study, or an additional study in which we did not find genetic variation in the induction of some S. carolinense chemical and physical defenses, including tryPIs; McNutt, ).…”

“…Larval performance is a function of both growth and survival; in this study, we chose to focus on growth, because (1) in the absence of natural enemies (which applies to our greenhouse experiment), previous insect herbivore damage has been shown to affect growth (McNutt & Underwood, ) but not survival of L. juncta larvae (DW McNutt, unpubl. ), and (2) L. juncta larvae exhibit high survival rates across larval stages in laboratory experiments (DW McNutt, unpubl.).…”

Asymmetric Interaction between Aphis spiraecola and Toxoptera citricida on Sweet Orange Induced by Pre-Infestation

“…For example, conspecific pre-infestation increased the number of nymphs produced by Myzus persicae [ 9 ], while Bemisia tabaci pre-infestation reduced M. persicae fecundity [ 1 ]. Nevertheless, few studies have simultaneously investigated both intra- and inter-specific effects mediated by the same host plant [ 10 ]. Furthermore, while plant-mediated interactions between herbivorous insects have been widely reported for herbaceous plants, similar studies on woody plants have been largely overlooked.…”