The Effect of Native Vegetation on the Susceptibility of Aristolochia Reticulata (Aristolochiaceae) to Herbivore Attack

Plants growing in dense stands of native vegetation are often less susceptible to herbivore attack than isolated plants or plants growing in monoculture. Three mechanisms have been suggested that may account for this phenomenon; (1) maintenance of a complex predator fauna in dense vegetation; (2) inhibition by complex vegetation of the ability of an herbivore to discover its host plants; and (3) decreased residence time of herbivores on plants growing in diverse vegetation. This study demonstrates that for both adults and larvae of the pipevine swallowtail butterfly, Battus philenor, inhibition of discovery is the primary cause of decreased susceptibility of its host plants, Aristolochia reticulata, to attack when grown amid native vegetation. In addition, this study demonstrates that a seasonal decrease in the susceptibility of host plants to discovery by adults is correlated with a seasonal increase in the height and density of the native vegetation. A similar inverse correlation between vegetation structure and susceptibility to discovery by larvae was detected. However, seasonal changes in vegetation structure do not influence susceptibility to larval discovery. This differential effect of changing vegetation structure on searching efficiency is probably due to differences in the way larvae and adults search.