Invasive species disrupt ecosystems through multiple mechanisms. This study examines the impact of a social wasp invader, Vespula pensylvanica, on two radiations of Hawaiian Hymenoptera: Hylaeus bees and Nesodynerus wasps. Because Hawaii has no native social insects, endemic solitary bees and wasps may be particularly sensitive to predation and competition from yellowjackets. Although endemic solitary Hymenoptera serve as primary pollinators of native flora and important predators, little is known about interactions between endemic Hymenoptera and social wasps, such as V. pensylvanica. Observational surveys in Hawaii Volcanoes and Haleakala National Parks demonstrated that Hylaeus and Nesodynerus exhibit mutually exclusive distributions with respect to V. pensylvanica. To identify the cause of this pattern, we examined how densities of solitary Hymenoptera respond to the removal of V. pensylvanica colonies. Moreover, we used molecular and physiological analyses to determine the extent to which yellowjackets prey on Hylaeus and Nesodynerus. Lastly, we quantified competition for prey between V. pensylvanica and Nesodynerus using trap nest and diet overlap analyses.
Experimental removal of V. pensylvanica colonies resulted in increased densities of both Hylaeus and Nesodynerus at floral resources within two months of removal. These responses are best explained by native Hymenoptera moving into foraging areas after yellowjacket removal. Hylaeus bees commonly occurred in the yellowjacket diet and multiple lines of evidence reveal that yellowjackets prey on these bees. In contrast, we found no native solitary wasps in the diet of V. pensylvanica. Analyses of Nesodynerus trap nest data show that these solitary wasps overlap in food resource use and colonize areas where yellowjacket colonies have been removed. Through predation on native bees and competition with native wasps, yellowjackets play an important and underappreciated role in structuring Hawaiian plant and arthropod communities.