CHANGES IN NUMBERS IN THE PARASITOID COMPLEX ASSOCIATED WITH THE DIAMOND-BACK MOTH, PLUTELLA-XYLOSTELLA (L) (LEPIDOPTERA), IN VICTORIA

Between 1972 and 1974 a study was made of parasitoids of P. xylostella [Diadegma cerophaga, Thyraeella collaris, D. rapi, Apanteles spp., Spinolia sp., Stictopisthus sp., Antrocephalus sp., Eupteromalus sp. and Brachymeria sp.] naturally infesting 4 successive crops of cabbages which received no pesticide treatment. Parasitism fluctuated in each crop, averaging 49% during the study. The major parasitoid spp. were D. cerophaga, T. collaris and D. rapi, which averaged 93% of parasitism over the study period. Six minor primary parasitoid and 1 hyperparasite species were recorded. Parasitoid activity was high during spring and summer, although extremely high temperatures during summer reduced numbers of both the host and its parasitoids. Drought conditions accentuated this. Parasitism in autumn depended upon the severity of this depletion and on the ensuing rate of recovery. The more important larval parasitoids were closely synchronized with host numbers, with a distinct lag during the winter period.