Oecologia (2004) 138: 547–557 DOI 10.1007/s00442-003-1472-4 PLANT ANIMAL INTERACTIONS Cris G. Hochwender . Robert S. Fritz Plant genetic differences influence herbivore community structure: evidence from a hybrid willow system Received: 23 July 2003 / Accepted: 26 November 2003 / Published online: 15 January 2004 # Springer-Verlag 2004 Abstract 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 commu- nity 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. Keywords Bottom-up effects . Community genetics . Hybridization . Plant-herbivore interactions . Salix Introduction Host plant quality, and hence insect abundance, is commonly affected by plant genotype (Karban 1992). As a consequence, plant genetic variation may be projected upward onto the next trophic level, making plant genetics a foundation upon which communities of insect herbivores are structured (Fritz and Price 1988; Fritz 1992; Morrow et al. 1994; Whitham et al. 1999; Dungey et al. 2000). For example, the proportions of four sawfly species differed consistently among clonal genotypes in a common garden experiment across 2 consecutive years (Fritz and Price 1988). Even though plant genetic variation may influence the community structure of insect herbivores, interpreting how genetic differences affect community structure has not been feasible because plant genotypes are typically sampled at random to estimate population-level variation. Hybrid genetic classes differ from each other in known ways, so plant hybrid systems can be used to clarify the role of genetic variation on community structure of insect herbivores. Consider F 1 and F 2 hybrids, which both inherit half of their genes from each parental species. If only additive effects of genes regulate differences between these genetic classes, the structure of the insect herbivore communities on these two hybrid classes should not differ. F 2 hybrids, however, are a recombinant hybrid class, whereas F 1 hybrids are not. Because recombination between the chromosomes of the two parental species occurs when F 2 hybrids are produced, greater variation due to epistasis is expected in F 2 hybrids compared to F 1 hybrids, but only half of the dominance variation is expected (Mather and Jinks 1982). If dominance effects and/or epistatic interactions alter the community structure C. G. Hochwender (*) . R. S. Fritz Department of Biology, Vassar College, Poughkeepsie, NY 12604, USA e-mail: ch81@evansville.edu Tel.: +1-812-4792005 Fax: +1-812-4881039 Present address: C. G. Hochwender Department of Biology, University of Evansville, 1800 Lincoln Ave., Evansville, IN 47722, USA