Plant-herbivore-enemy interactions exhibit substantial variation across space and time, and understanding the sources of this variation is an important research goal. A potentially important source of spatiotemporal variation in plant-insect interactions is plant-mediated indirect effects between insect herbivores in different feeding guilds. For example, phloem-feeding herbivores can suppress induced plant resistance to leaf-chewing herbivores. This cross-guild induced susceptibility hypothesis predicts that phloem-feeders facilitate chewing herbivore feeding preference and decrease emission of induced volatiles that signal parasitoids of leaf-chewers. Landscape heterogeneity might also modify these plant-mediated interactions if it affects induced resistance traits of plants. For example, forest edges might promote stronger induction of phenolic compounds in trees relative to their counterparts in shaded forest interior because edge trees are less light- and carbon-limited. This hypothesis predicts that forest edge will strengthen cross-guild induced susceptibility effects for direct resistance traits, but not necessarily for induced volatiles, which are less metabolically costly. We studied white oak trees, their phloem-feeding treehopper and leaf-chewing caterpillar assemblages, and a naturally occurring community of parasitoids that attack these caterpillars in forest interiors and edges in Connecticut, USA. To test these hypotheses, we manipulated treehopper presence and assayed the plant’s direct and indirect resistance against caterpillars.
Results/Conclusions
Removal of treehoppers from tree branches at forest edges reduced feeding by caterpillars. However, in forest interiors, removal of treehoppers increased caterpillar feeding. These results offer preliminary support for the hypothesis that landscape heterogeneity modifies cross-guild induced susceptibility effects for direct resistance traits. With respect to indirect resistance, approximately 20 percent of caterpillars were parasitized on treehopper removal branches, whereas none of the caterpillars on treehopper replacement branches were parasitized. Parasitism of caterpillars varied independently of edge vs interior locations of experimental branches. Taken together, these findings suggest that plant-mediated indirect effects between insect herbivores in different feeding guilds might be an important source of spatiotemporal variation in tri-trophic interactions at a landscape scale.