Abstract
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 Trissolcus japonicus (Ashmead) and T. cultratus (Mayr) (Hymenoptera: Scelionidae) associated with Halyomorpha halys (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. Trissolcus japonicus showed high acceptance of parasitized hosts up to 72 h following oviposition by T. cultratus, despite a very poor developmental outcome. In contrast, T. cultratus generally avoided ovipositing in H. halys eggs containing T. japonicus early-instar larvae but did not avoid parasitizing H. halys that contained eggs and third instar larvae. The adaptive value of this behaviour was supported by developmental outcome: T. cultratus outcompeted T. japonicus eggs but not early-instar larvae, and a trophic shift occurred wherein T. cultratus developed as a facultative hyperparasitoid on third instar T. japonicus larvae. Trissolcus japonicus guarded egg masses 8-12× longer and displayed more aggressive interactions than T. cultratus, suggesting T. japonicus is the superior extrinsic competitor. Development as a facultative hyperparasitoid provided a competitive niche for Asian T. cultratus and confirms its instrinsic competitive superiority. This also occurs in a biologically distinct European population of T. cultratus, suggesting that facultative hyperparasitism as a competitive strategy is retained in geographically separated populations that have not coevolved with H. halys or T. japonicus.