Plant genetic differences influence herbivore community structure: evidence from a hybrid willow system.

To determine the influence of plant genetic variation on community structure of insect herbivores, we examined the abundances of 14 herbivore species among six genetic classes of willow: Salix eriocephala, S. sericea, their F(1) and F(2) interspecific hybrids, and backcross hybrids to each parental species. We placed 1-year-old plants, grown from seeds generated from controlled crosses, in a common garden. During the growing season, we censused gall-inducing flies and sawflies, leaf-mining insects, and leaf-folding Lepidoptera to determine the community structure of herbivorous insects on the six genetic classes. Our results provided convincing evidence that the community structure of insect herbivores in this hybrid willow system was shaped by genetic differences among the parental species and the hybrid genetic classes. Using MANOVA, we detected significant differences among genetic classes for both absolute and relative abundance of herbivores. Using canonical discriminant analysis, we found that centroid locations describing community structure of the insect herbivores differed for each genetic class. Moreover, the centroids for the four hybrid classes were located well outside of the range between the centroids for the parental species, suggesting that more than additive genetic effects of the two parental species influenced community formation on hybrid classes. Line-cross analysis suggested that plant genetic factors responsible for structuring the herbivore community involved epistatic effects, as well as additive and dominance effects. We discuss the ramifications of these results in regard to the structure of insect herbivore communities on plants and the implications of our findings for the evolution of interspecific interactions.