In a previous study it was demonstrated that a major selection pressure responsible for seasonal changes in the searching and oviposition preferences of the pipevine swallowtail butterfly, Battus philenor (Lepidoptera: Papilionidae), is a seasonal increase in the disappearance rate of early—instar larvae from one of their two host plants in east Texas, Aristolochia reticulata (Aristolochiaceae). The primary objective of this study was to determine the causes, both proximate and ultimate, of that increased disappearance rate. The results may be summarized as follows: 1) The seasonal increase in disappearance rates of larvae feeding on A. reticulata plants is due primarily to a decrease in the size at which larvae disperse from their original host plant. No seasonal increase in the intensity of predation on larvae was detected. 2) Larvae readily consume the young foliage of A. reticulata plants but seldom accept mature foliage. 3) The amount of A. reticulata foliage that is acceptable to larvae decreases between early April and late May. This decrease explains the seasonal decrease in the size at which larvae disperse from their original host plant. 4) Growth rates, corrected for variation in consumption rates, are much lower for larvae fed mature leaves than for larvae fed young leaves. 5) Poor larval growth on mature leaves appears to be due to two effects of sclerophyllization: low nitrogen content and increased toughness. No effects of digestibility—reducing plant secondary compounds were detected. 6) For A. reticulata plants in the field, the probability that a leaf will be eaten once it is discovered by a larva is lower for mature leaves than for young leaves. 7) For A. serpentaria plants, which grow sympatrically with A. reticulata, mature leaves and young leaves are equally acceptable to larvae. Mature A. serpentaria leaves are not markedly sclerophyllous. These results indicate that the causes of the observed seasonal increase in disappearance rates of larvae on A. reticulata can be traced ultimately to changes in the nutritional content of the foliage as it ages. They also suggest that a seasonal deterioration in the nutritional quality of A. reticulata foliage is a major selection pressure leading to the observed seasonal shift in the searching and oviposition preferences of females from A. reticulata in the early spring to A. serpentaria late in the spring. Finally, the results provide evidence supporting the contention that an important ecological function of sclerophylly is to defend plants from herbivores.