Interference and the ideal free distribution: oviposition in a parasitoid wasp

The interference ideal free distribution (IFD) model of Sutherland makes a number of predictions that have yet to be tested and that have implications for the validity of subsequent extensions to the theory. We tested these predictions in a study using different densities of the parasitoid wasp, Venturia canescens, foraging on patches containing different densities of its host, Plodia interpunctella. Our results support a number of the interference IFD model's general predictions. Gain rate decreased because of increased interference at higher density. Although gain rates on the two patches differed slighdy, this would be expected allowing for some sampling behavior and perceptual constraints. Early in each experiment when patch assessment is likely to occur, wasp movement was higher and gain rates lower. However, die more specific prediction of Sudierland's model, that proportional patch use should be constant and independent of density, was not upheld. Contemporary IFD models use only one of several equally valid potential relationships between gain rate, interference, and competitor density. The results of this study provide support for the additive model developed by Tregenza et al. (companion article).