We examined distributions of 33 gall forming insect species on parent species and three hybrid phenotypes in two Eucalyptus hybrid zones in Australia. Variation in insect abundance among hybrid classes was greater than variation between species suggesting that hybrid zones are sites of dynamic interactions between plants and herbivores. For instance, of 25 galling species in Victoria, 52% showed significant differences in abundance among hybrid classes, whereas only 24% differed between pure host species. While some components of hybrid use were very predictable, others were not. Based on galler use of pure species, we could accurately predict which hybrid phenotype would be most used. Our data show that most gall species concentrate on the hybrid class that is most similar to the parent species on which it is most abundant. However, species related taxonomically or by feeding guild did not exhibit consistent responses to hybrid and parent hosts. Thus pooling either hybrid classes or insect species for statistical analysis may mask underlying patterns. Overall, galler responses to three hybrid phenotypes are consistent with the hypothesis that plant hybrid zones and especially backcross hybrids are centers of insect species richness and abundance. Furthermore, galler response to hybrids plants suggests that the narrow host specificity characteristic of gallers may be less strongly influenced by plant developmental processes than generally thought.

Our observations support the hypothesis that genetic differences among host plants in hybrid zones underlies patterns of insect host use. They also suggest that other mechanisms might be involved. Hybrid zone studies may have much to tell us about the ecology and evolution of plant-herbivore interactions.