Do plant-mediated indirect interactions among herbivores affect parasitoid performance?

Individuals of different species interact in local communities and can influence each other’s fitness both directly or indirectly through intermediaries. This is particularly true for the interactions among plants, insect herbivores and the natural enemies of these herbivores. Plant traits, such a chemical defenses not only affect the performance of the herbivores, but also the performance of the natural enemies that feed on these herbivores. Because insect communities that feed on a particular plant species can vary considerably among different localities there will also be considerable variability in the induced plant responses that sequential herbivores are exposed to. We hypothesize that within each specific community herbivores and their natural enemies will have adapted to the plant defenses induced by earlier herbivores. As yet, interspecies effects mediated by plant defense responses have rarely taken into account the third trophic level.

We are working with a tritrophic system comprising several species of bean plants, bruchid beetles that feed on the beans, and the parasitoids that develop on the larvae of these beetles. Previous work in our lab has tested the hypothesis that the host plants affect the performance, behavior and genetic structure of bruchids and parasitoids and the plants mediate the interaction between these two upper trophic levels. The results from these previous studies provide mixed support for this hypothesis, mostly due to great differences across populations. These findings and the accumulating evidence for indirect interactions among sequential herbivores have led to the notion that variation in the occurrence and abundance of other herbivore species in this natural system may dictate the strength and nature of the interaction between bean plants, bruchids and their parasitoids. The proposed project will test this hypothesis.

Two dominating herbivores that attack bean plants before the bruchids are of particular interest, the leaf beetle Epilachna varivestis and the weevil Apion godmani. The co-occurrence of these herbivores with each other and the bruchids varies considerable among populations. Therefore the system lends itself exceedingly well for studies into the plant-mediated effects among sequential herbivores. We will test the above hypothesis by determining the extent to which damage on bean leaves and seeds by the leaf beetle and/or the weevil affects the performance and behavior of the bruchid. More importantly, we will study how such effects may cascade upwards to the next trophic level