Background/Question/Methods Herbivore species that share a host plant may indirectly influence the feeding and performance of each other by altering plant traits. To date, most studies have examined this phenomenon among insect herbivores; only a smaller subset of studies have shown the impact of vertebrate browsing on later arriving insect herbivores. Moreover, these studies have generally investigated plants that lack chemical defenses thought to be effective against vertebrate browsers. If induced, such defenses can also influence insect herbivores differentially, through positive effects on specialists and negative effects on generalist species. We examined the impact of white-tailed deer (Odocoileus virginianus) browsing on spicebush (Lindera benzoin L.), a non-preferred browse plant defended by a diverse group of terpenes in leaves and stem tissue, on native specialist and generalist insect herbivores of spicebush. Specifically we asked whether plant responses to deer browsing resulted in positive or negative effects on the specialist spicebush swallowtail (Papilio troilus L.) and a suite of generalist folivores naturally occurring on spicebush in the wild.
Results/Conclusions

Over 70% of spicebush plants at our study site had at least 50% of their twigs browsed early in the growing season, suggesting intense browsing on a plant that is generally ignored by deer. Spicebush plants responded by release of apical dominance, producing new growth that was higher in water content, less tough, and with altered nutrient and defensive chemistry. Deer browsing had generally ceased upon arrival of the first cohort of swallowtail caterpillars, and many spicebush plants had both “new” (post-browse) and “old” (pre-browse) leaves available to insect consumers. In laboratory choice feeding assays, caterpillars preferentially consumed and had greater performance on leaves from plants which had been browsed by deer over those from unbrowsed plants, consistent with the prediction that specialist herbivores might be attracted to and benefit from elevated levels of defensive chemistry induced by vertebrate browsing. In contrast, field surveys revealed that unbrowsed spicebush plants had significantly higher amounts of leaf damage from a suite of generalist folivores than did browsed plants of similar size. Overall, our results suggest that browsing by generalist vertebrates has the potential to indirectly facilitate specialist insects while indirectly suppressing generalist insects through herbivore-mediated changes in plant chemistry.