Stability in the host-parasitoid relationship of and in the citrus orchard

The citrus whitefly, (Ashmead), a citrus pest, has been prevalent in Israel since 1975. The parasitic wasp (Howard) has been successfully used for its biological control since 1980 and thehost-parasitoid system is stable. This studyuses field data from four and a half years inorder to examine whether heterogeneity ofparasitism and risk aggregation can explain thestability. After establishing that theprobability of parasitism is not constant overpatches, we examined the question of parasitoidaggregation, dependent or independent of thehost, at different patch levels. At the treelevel we found an inverse relationship betweenthe proportion of parasitism and host density.At the leaf level, taking the tree effect intoconsideration, the host density dependence wasweak and non-significant. At the leaf level, acombined examination of both types ofheterogeneity in parasite distribution – hostdensity dependent heterogeneity (direct orindirect) and host density independentheterogeneity, was performed using the () > 1 criterion. The mean () value over different sampling dateswas greater than one. Host density independentheterogeneity had the greater contribution tostability. The () findings atleaf level in the plot, combining leaf and treeeffects, represent both aggregation at the treelevel (inverse density dependence) andaggregation at the leaf level (host densityindependence). The heterogeneity findings forparasitoid distribution, together with thestability, which was directly observed, supportour hypothesis that aggregation of risk is animportant mechanism in the stabilisinginteraction between the citrus whitefly and itsspecific parasitoid, .