Intraplant movement of generalist caterpillars on alternative host plants

Background/Question/Methods Insect herbivores frequently move about on their host plants to obtain food, avoid enemies and competitors, and cope with changing environmental conditions. While numerous plant traits have been shown to influence patterns of movement of specialist herbivores, very few studies have examined how different host plant species can affect intraplant movement of polyphagous herbivores. The aim of this study was to characterize the movement patterns of two species of generalist herbivores (caterpillars in family Limacocidae:  Euclea delphinii Bdv. and Acharia stimulea Clem.) on six different host tree species that co-occur in an understory forest.  In a series of whole-plant censuses of small trees during the summer of 2008, the positions of individual larvae were recorded and the distance moved between censuses was measured.

Results/Conclusions For both E. delphinii and A. stimulea, multiple measures of movement varied significantly among host plants. Host plant effects on both movement rate and the cumulative distance traveled by larvae of both caterpillar species were strongest 1 and 3 days after deploying the caterpillars and attenuated somewhat in subsequent censuses. In the early censuses, median movement rates and average movement distances tended to be enhanced on red oak and black cherry and reduced on white oak, with other hosts intermediate. Because of the highly skewed nature of the movement data set, we also examined site fidelity (the tendency to remain on the same leaf or leaf cluster between censuses) on the suite of host plants; for both caterpillar species, larvae had the greatest site fidelity on white oak and the lowest fidelity on black cherry, with other species intermediate. While the mechanism(s) promoting or inhibiting movement on these different hosts are currently not known, potential mechanisms as well as the potential consequences of increased movement for caterpillar growth and development and mortality from natural enemies are explored and discussed.