Question: What are the life-history costs for a predatory insect of surviving parasitoid attack., and can parasitoid attack alter predator-prey interactions? Hypotheses: Survivorship is influenced by host age. Hosts that suffer parasitoid attack grow. p more slowly and consume fewer prey. Those that survive attack are smaller as adults and show reduced survivorship. Organisms: The aphidophagous hoverfly Episyrphus balteatus, its endoparasitoid wasp Diplazon laetatorius and its prey, the pea aphid, Acyrthosiphon pisum. Site of experiments: All experiments were conducted in controlled temperature rooms and chambers in the laboratory. Methods: Episyrphus balteatus larvae of each instar were exposed to attack by Diplazon laetatorius, then dissected to measure the encapsulation response (a measure of immunity). Second instar larvae were either attacked or not attacked by D. laetatorius. Their development rates and numbers of prey consumed were noted. The size and survivorship of surviving (immune) and control hoverflies were compared following eclosion. Conclusions: Successful immune response increased with larval age (first instar 0%, second instar 40%, third instar 100% survival). Second instar larvae that successfully resisted parasitoid attack were larger as pupae (but not as adults) and showed reduced adult survivorship. Female adult survivors were more likely than male survivors to have died within 16 days of eclosion, but there was no difference between unattacked male and female control hoverflies. Attacked larvae, irrespective of immune status, consumed fewer aphids than unattacked individuals. Episyrphus balteatus suffers significant costs of resisting parasitoid attack, and parasitoid attack can reduce the top-down effects of an insect predator, irrespective of whether the host mounts an immune response or not. Episyrphus balteatus (Syrphidae) : Predator, Host Diplazon laetatorius (Ichneumonidae) : Parasite Acyrthosiphon pisum (Aphidoidea) : Prey Biometrics; Nutrition; Diet; Prey; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Immunology and repair mechanisms; Tissue reactions; Ecology; Population dynamics; Predators Acyrthosiphon pisum; Insect predators; Predator feeding rate; encapsulation response to hymenopteran parasites relations; Episyrphus balteatus (Diptera) Diplazon laetatorius; Dipteran hosts; Host encapsulation response; costs & consequences; Episyrphus balteatus Episyrphus balteatus; Age; Size; Weight; Insect prey; encapsulation response to hymenopteran parasites relations; Acyrthosiphon pisum (Hemiptera); Feeding rate; Hymenopteran parasites; Encapsulation response to parasites; costs & consequences; Diplazon laetatorius; Encapsulation; Survival none