PUPAL PARASITOID ATTACK INFLUENCES THE RELATIVE FITNESS OF DROSOPHILATHAT HAVE ENCAPSULATED LARVAL PARASITOIDS

1. The evolution of host resistance to parasitoid attack will be const rained by two factors: the costs of the ability to defend against atta ck, and the costs of surviving actual attack. These factors have been investigated using Drosophila melanogaster and its parasitoids as a mo del system. The costs of defensive ability are expressed as a trade-of f with larval competitive ability, whereas the costs of actual defence are exhibited in terms of reduced adult fecundity and size. 2. The co sts of actual defence may be ameliorated by the host-choice decisions made by Pachycrepoideus vindemiae, a pupal parasitoid. If larvae that have successfully encapsulated a parasitoid develop into poorer qualit y hosts, then these may be rejected by ovipositing pupal parasitoids. 3. Pupae developing from larvae that have encapsulated the parasitoid Asobara tabida are smaller and have relatively thinner puparia. Thinne r puparia are likely to be associated with a reduction in mechanical s trength and possibly with a decrease in desiccation tolerance. 4. Pach ycrepoideus vindemiae that develop in capsule-bearing pupae are smalle r than those that emerge from previously unattacked hosts. This suppor ts the prediction that ovipositing female P. vindemiae should avoid at tacking capsule-bearing hosts. However, in choice experiments with 1-d ay-old pupae, P. vindemiae females oviposited preferentially in hosts containing a capsule, whereas there was no preference found with 4-day -old hosts. This appears to be a maladaptive host choice decision, as the female pupal parasitoids are preferentially attacking hosts that w ill result in a reduction of their own fitness. 5. The increased likel ihood of attack by a pupal parasitoid is another cost of actual defenc e against larval parasitoid attack.