Island invasions by the non-native vinegar fly Drosophila suzukii and its parasitoid wasps.

Understanding how island characteristics influence the establishment and impact of invasive species and their natural enemies could inform both island biogeography and biological control theory. We studied the occurrence and relative abundances of the globally invasive vinegar fly, Drosophila suzukii, and its recently introduced larval parasitoids, Leptopilina japonica and Ganaspis kimorum, across islands with varying sizes and levels of human-mediated transport in the Gulf and San Juan Islands of British Columbia (Canada) and Washington State (USA). Across 2 years and 58 sites, we collected D. suzukii and its parasitoids from wild blackberry, Rubus armeniacus, fruit. We predicted that parasitoids were more likely to be present on larger islands with higher levels of human activity and higher D. suzukii densities, and that the less specialized parasitoid species (L. japonica) would be more likely to establish on islands. We detected D. suzukii across all islands, indicating widespread establishment of this invasive pest. In contrast, we observed parasitoids on fewer than half of the islands. Leptopilina japonica was the only parasitoid of D. suzukii detected on islands. Parasitoid presence was significantly positively associated with host density and marginally positively associated with island area, but not average annual vehicle ferry traffic (an indicator of human-mediated propagule pressure). Parasitism levels were low throughout the study region and we did not observe lower relative abundances of D. suzukii on islands where parasitoids were present-in fact, the relative abundance of D. suzukii tended to be higher on islands where the parasitoid L. japonica was detected. These findings suggest that island characteristics, host density, and a consumer's host specificity may be associated with the spread of natural enemies from mainland areas to islands, but that the natural enemy's presence may not inevitably result in pest population suppression on islands, at least over the short term.