A spatial model for the successful biological control of Sitona discoideus by Microctonus aethiopoides.
A new model for the biological control of the pest weevil Sitona discoideus by the parasitoid Microctonus aethiopoides is presented. Based on an earlier empirical model, the new model was expanded to include explicit dispersal in a coupled map lattice metapopulation. Model results were consistent with observed data from New Zealand, mimicking successful biological control with a reduction in host density from 1984 to 1991 and a sustained suppression of around 75% thereafter. The metapopulation model was locally stable because of strong host density-dependence and a non-random parasitoid attack function. Metapopulation structure had little effect on the local dynamics, except in transient behaviour such as initial rates of parasitoid spread or the response to a local perturbation. The survival rate of dispersing weevils was nevertheless important in determining overall weevil abundance. Assuming that the observed low survival rate of weevils during dispersal (around 0.3%) was related to the relative scarcity of its host-plant (Medicago sativa) in the landscape, the model suggested that local weevil density could substantially increase with an increase in area of crop planted. Although the extent of biological control would be sustained in relative terms, increasing the crop abundance could allow a substantial increase in absolute pest density. With appropriate adjustment, the model also simulated the unsuccessful biological control observed in Australia. Advancing weevil oviposition in autumn (reflecting warmer autumn temperatures in Australia) and reducing parasitism rates among aestivating weevils (reflecting a lack of summer development of parasitoids in Australia, compared with the atypical development of a proportion in New Zealand), led to an inability of the parasitoid to significantly reduce weevil populations in the model. This host-parasitoid system is unique for the quality and duration of monitoring conducted before, during and after parasitoid introduction. It also represents one of the few biological control case studies to be thoroughly evaluated through use of empirical models such as the spatial and non-spatial versions presented here.