Can vertebrate predation alter aggregation of risk in an insect host–parasitoid system?

1. Insect host–parasite systems allow investigations of the trophodynamics of ecological communities within a well-formed theoretical context. A little explored feature of such systems involves the interplay between generalized consumers and host–parasitoid dynamics. I report a study investigating how the impacts of generalized consumers, viewed here as interaction modifications, may influence the stability of a particular interspecific interaction. 2. In a study involving overwintering oothecae of the praying mantis Stagmomantis limbata (Hahn), birds damaged 36% of oothecae, 85% of which had also been parasitized by winter-active, multivoltine torymid wasps of the genus Podagrion. Birds preferentially preyed upon oothecae oviposited high on trees, but such predation was often incomplete, leaving both viable and parasitized mantid eggs inside damaged oothecae. 3. Two factors allowed bird damage to influence the distribution of risk of parasitism among oothecae and among hosts. These were (1) that some parasites − but no mantids − emerged successfully prior to bird predation and (2) that extensive post-damage foraging by parasitoids occurred, but that the nature of this foraging was altered little by oothecal damage. 4. In all, bird damage engendered a fourfold increase (from 1·5 to 6·5) in the CV of parasitism risk among hosts (a stability criterion that has been proposed for host– parasitoid interactions) and increased the relative importance of host-density-dependent parasitism. The role of timing of the two natural enemy impacts for stability of the host–parasitoid interaction is discussed.