Several hypotheses exist to explain host use in polyphagous insect herbivores. The Physiological-Efficiency Hypothesis emphasizes the food quality of the host, whereas the Enemy-Free-Space Hypothesis identifies safety from enemies as an important driver of host use. Our studies of host use by the fall webworm caterpillar (Hyphantria cunea) in Middlesex County, Connecticut test three possible bases of host use: relative host plant availability, host plant quality, and parasitism risk. H. cunea provides an ideal study system for our purpose because it is a broad dietary generalist and experiences high rates of parasitism. We measured relative host plant availability in the field and then reared webworms on natal hosts in the laboratory to determine herbivore performance (pupal mass) and frequencies of parasitism on different host plants. Results/Conclusions Host use was positively correlated with relative availability, however, high quality hosts were chosen with greater frequency than predicted by their availability. We found no evidence that parasitism was driving host use, or of a tradeoff between host quality and parasitism risk. However, the inverse relationship that we observed between performance and availability suggests that a tradeoff exists between host quality and host availability in H. cunea. Tradeoffs of this kind can be explained by foraging constraints on ovipositing females, such as rarity of high quality hosts or time limitations on oviposition due to predation risk. The latter seems likely because high quality hosts do not appear to be rare and the risk of bat predation on adults is likely to be severe.