Document: JAM-3-99-157

The impact of parasitoids and plant spacing on the spread of insect outbreaks.

UMBANHOWAR, J.* and A.HASTINGS

University of Californa,Davis, CA 95616 USA ¹

Abstract:
Many factors control the spatial spread and temporal persistence of outbreaks of insect herbivores. Especially important are the distribution of host plants and natural enemies. Populations of Orgyia vetusta exhibit multi-generational, spatially limited outbreaks whose spread is limited by parasitoids. We describe and analyze a model based on the biology of O. vetusta. There are four major results of this analysis. First, we find that the spread of an outbreak is greatly slowed by the presence of wide ranging parasitoids. Second, in contrast to similar models where herbivore and parasitoid have overlapping generations, the spread of an outbreak through a continuously distributed host plant cannot be prevented, regardless of the movement abilities of herbivore or parasitoid. Third, if plants are considered to be discrete patches, outbreak spread can be prevented by sufficient inter-plant distance. Fourth, the model predicts the outbreak will collapse, due to overexploitation by parasitoids after reaching a critical size. Observational data are consistent with the third and fourth prediction.

Keywords: outbreak,parasitoids,herbivore

This abstract is being presented at: 8:00 AM in session:
Oral Session #4: Herbivore Responses to Plants.