Stability in the host-parasitoid relationship of Dialeurodes citri and Encarsia lahorensis in the citrus orchard

The citrus whitefly, Dialeurodes citri (Ashmead), a citrus pest, has been prevalent in Israel since 1975. The parasitic wasp Encarsia lahorensis (Howard) has been successfully used for its biological control since 1980 and the host-parasitoid system is stable. This study uses field data from four and a half years in order to examine whether heterogeneity of parasitism and risk aggregation can explain the stability. After establishing that the probability of parasitism is not constant over patches, we examined the question of parasitoid aggregation, dependent or independent of the host, at different patch levels. At the tree level we found an inverse relationship between the proportion of parasitism and host density. At the leaf level, taking the tree effect into consideration, the host density dependence was weak and nonsignificant. At the leaf level, a combined examination of both types of heterogeneity in parasite distribution - host density dependent heterogeneity (direct or indirect) and host density independent heterogeneity, was performed using the (CV)(2) > 1 criterion. The mean (CV)(2) value over different sampling dates was greater than one. Host density independent heterogeneity had the greater contribution to stability. The (CV)(2) findings at leaf level in the plot, combining leaf and tree effects, represent both aggregation at the tree level ( inverse density dependence) and aggregation at the leaf level ( host density independence). The heterogeneity findings for parasitoid distribution, together with the stability, which was directly observed, support our hypothesis that aggregation of risk is an important mechanism in the stabilising interaction between the citrus whitefly and its specific parasitoid, E. lahorensis.