Background/Question/Methods   Determinants of dietary specialization in insect herbivores may involve plant-herbivore (bi-trophic) or plant-herbivore-carnivore (tri-trophic) interactions. Although they represent a minority of insect herbivore species, dietary generalists offer a relatively unexplored opportunity to address the role of bi-trophic and tri-trophic interactions as determinants of diet breadth. We hypothesize that trade-offs in fitness components among different host-plant species favor polyphagy by equalizing average fitness among hosts. Such trade-offs can be bi-trophic (host-plant quality vs availability) or tri-trophic (host-plant quality vs enemy-free space). Here we address this trade-off hypothesis with a comparative approach to evolutionary ecology, using numerous species of polyphagous caterpillars on several different host-plant species to test hypothetical determinants of dietary generalism. We test predictions of bi-trophic and tri-trophic trade-off hypotheses among all combinations of the following measured ecological factors: host-plant quality, host-plant abundance, and enemy-free space. The polyphagous caterpillars in this study are spring-feeding caterpillars of macro-moths using the same set of deciduous forest tree species in the northeastern U.S.A.

Results/Conclusions   We observed that these host-plant species offer varied food quality, have different abundances, and provide different degrees of enemy-free space for these herbivores. We found that this polyphagous caterpillar assemblage experienced a strong trade-off between food quality and enemy-free space across eight different host-plant species, with no evidence for other tested trade-offs. That is, plants offering the highest food quality and supporting the best caterpillar growth tended to expose caterpillars to the greatest degree of bird predation, as measured by a bird exclusion field experiment. The strength of bird predation was dependent on caterpillar density and host-plant species identity. These results suggest that tri-trophic trade-offs among host-plant species maintain dietary generalism in this caterpillar assemblage.