Strength of enemy release from parasitoids is context-dependent in the invasive African Fig Fly,Zaprionus indianus

Abstract

Understanding the mechanisms underlying the success of biological invasions is essential to employ effective prediction and management strategies. The escape from natural enemies in invaded regions (enemy release hypothesis) and increased competitive ability are hallmarks of invasive species; however, these two processes are rarely studied within the same context. Here, we examined the effect of enemy release on the competition outcomes of a successful invasive insect pest in North America, the African fig fly (Zaprionus indianus). Parasitoid wasps such asLeptopilina heterotomathat parasitize drosophilid larvae may seek out established species with known host suitability over a novel species, so we hypothesizedZ. indianusmay have low susceptibility to parasitoids, giving them a competitive advantage over co-occurring drosophilids. We tested this hypothesis by comparing the adult emergence rates fromZ. indianuslarvae reared alone or in competition withDrosophila hydeiorD. simulanslarvae in the presence and absence of parasitoid wasps. These interactions might be influenced by larval density, so we tested competitive interactions under low and high larval densities. At low larval densities,Z. indianusemerged at equal rates toD. hydeibut outcompetedD. simulans, and these outcomes were not affected by parasitoids. However, at high densities, the addition of parasitoids shifted competition outcomes in favor ofZ. indianus, suggesting enemy release provides a competitive advantage under some circumstances. These results indicate that the strength of enemy release inZ. indianusis widely dependent on contextual factors such as density and competitor species. Further investigation of how these results apply to field environments could offer insight into howZ. indianusalters ecosystems and how productive biological control may limit the spread ofZ. indianus.Short AbstractInvasive species may succeed in new environments in part because they are less susceptible to diseases and parasites that have co-evolved with local hosts, giving invaders a competitive advantage. We tested this hypothesis by competing an invasive fruit fly against established species in the presence of parasitoid wasps that lay their eggs in fruit fly larvae. We found that the invasive species generally outcompeted other species in the presence of parasitoids, but the extent of its advantage depended on the species it was competing against and the number of larvae present.