Costs of counterdefenses to host resistance in a parasitoid of Drosophila

The ability of a parasitoid to evolve enhanced counterdefenses against host resistance and its possible costs were studied in a Drosophila-parasitoid system. We reared Asobara tabida (Braconidae, Hymenoptera) exclusively on D . melanogaster to impose artificial selection for improved counterdefenses against cellular encapsulation, the main host defense against parasitism. C ontrols were reared on D. subobscura, the main host of the population of wa sps from which the laboratory culture was derived and a species that never encapsulates parasitoids. We observed improved survival and avoidance of en capsulation in all five selection lines compared to their paired control li nes, although there was unexpected variation among pairs. Improved survival was associated with parasitoid eggs becoming embedded in host tissue, wher e they were protected from circulating haemocytes. There were no difference s among lines in average adult size, fat content, egg load, or performance on D. subobscura. However, the duration of the egg stage in selection lines was longer than that of control lines, probably because of reduced nutrien t and/or oxygen supply when eggs are embedded in host tissue. We suggest th at this delay in hatching reduces the probability of parasitoid survival if another parasitoid egg is laid in the same host (superparasitism or multip arasitism) and hence is a cost of enhanced counterdefenses against host res istance.