A temporal trophic shift from primary parasitism to facultative hyperparasitism during interspecific competition between two coevolved scelionid egg parasitoids.

Understanding competition between scelionid parasitoids that exploit the same host may provide insight into strategies that allow coexistence on a shared resource. Competition studies typically focus on interactions between native and exotic parasitoids that do not share an evolutionary history; however, coevolved parasitoids may be more likely to demonstrate strategies to avoid or exploit a shared resource. We examined intrinsic and extrinsic competition between Asian (Ashmead) and (Mayr) (Hymenoptera: Scelionidae) associated with (Stål) (Hemiptera: Pentatomidae) that share an evolutionary history. Interspecific interactions were assessed by providing parasitized egg masses to each species at various intervals post-parasitism, and measuring host acceptance, developmental suitability, and guarding behaviour. showed high acceptance of parasitized hosts up to 72 h following oviposition by , despite a very poor developmental outcome. In contrast, generally avoided ovipositing in eggs containing early-instar larvae but did not avoid parasitizing that contained eggs and third instar larvae. The adaptive value of this behaviour was supported by developmental outcome: outcompeted eggs but not early-instar larvae, and a trophic shift occurred wherein developed as a facultative hyperparasitoid on third instar larvae. guarded egg masses 8-12× longer and displayed more aggressive interactions than , suggesting is the superior extrinsic competitor. Development as a facultative hyperparasitoid provided a competitive niche for Asian and confirms its instrinsic competitive superiority. This also occurs in a biologically distinct European population of , suggesting that facultative hyperparasitism as a competitive strategy is retained in geographically separated populations that have not coevolved with or .