Biological Control of Olive Scale and Its Relevance to Ecological Theory

Various authors have suggested that aggregation at local areas that have relatively many pests is an essential feature of parasitoids that are successful biological control agents. This behavior is thought to stabilize the pest-parasitoid system, and stability is considered essential for control. Such aggregation, however, can also be destabilizing. Strong aggregation independent of pest density is stabilizing, but leads to high pest density. We propose two models incorporating parasitoid distribution that is independent of pest distribution. In model A the parasitoids are gamma distributed and this yields May's (1978) negative binomial model. Stability requires strong aggregation (γ < 1). In model B the parasitoids are evenly distributed, which yields Nicholson and Bailey's (1935) unstable model. Using data from Huffaker and Kennett's study of the olive scale and its two introduced parasitoids in California, we show that the parasitoids do not aggregate to areas of high pest density. By comparing the observed distributions of parasitized hosts with those predicted by models A and B, we show that the system almost always fits the model A with γ > 1, and in some instances fit or was close to model B. In only one case out of eight is γ < 1. These results suggest that the system may be unstable, and this is supported by the observed fluctuations in scale numbers and parasitism through time. We conclude that spatial aggregation by parasites is not an essential feature of successful biological control. It may also be that stability is not essential for control, alternatively we may have failed to detect the salient density-dependent mechanism.