Intraplant movement of generalist slug caterpillars (Limacodidae: Lepidoptera): effects of host plant and light environment.

Abstract

Insect herbivores frequently move about on their host plants to obtain food, avoid enemies and competitors, and cope with changing environmental conditions. Although numerous plant traits influence the movement of specialist herbivores, few studies have examined movement responses of generalist herbivores to the variable ecological conditions associated with feeding and living on an array of host plants. We tested whether the movement patterns of two generalist caterpillars (Euclea delphinii Boisduval and Acharia stimulea Clemens, Limacodidae) differed on six different host tree species over 10 d. Because these tree species vary in the range of light environments in which they commonly grow, we also compared the movement responses of E. delphinii caterpillars to two contrasting light environments, sun and shade. For both caterpillar species, multiple measures of movement varied significantly among host tree species. In early censuses, movement rates and distances were highest on red oak and black cherry and lowest on white oak. Site fidelity was greatest on white oak and lowest on black cherry. Movement of both caterpillar species varied inversely with mean predator density on five of the six host trees. Other ecological predictors (e.g., leaf size and the density of other herbivores) were unrelated to movement. Light environment altered behavior such that caterpillars in the shade moved and fed more often, and moved greater distances, than caterpillars in the sun. Although the mechanism(s) promoting or inhibiting movement under these different conditions requires further study, the consequences of increased movement for caterpillar development and mortality from natural enemies are discussed.