The cynipid gall wasp Diplolepis rosae is more successful in North America than in Europe because of enemy release

Abstract

The Enemy Release Hypothesis predicts that introduced species in their new range are freed from natural enemies (e.g., pathogens, parasitoids and predators) that control their populations. Diplolepis rosae (Hymenoptera, Diplolepididae), native to the Western Palearctic, induces readily apparent galls on wild roses (Rosa spp.) that support a robust component community of inquilines, parasitoids and hyperparasitoids in its native range. D. rosae was introduced to North America in the mid‐1800s, and has since become widespread and common across the continent. We compared the insect communities associated with D. rosae galls from Canada and the US Pacific Northwest with those of Eastern Europe. Throughout its introduced range, parasitism rates were lower compared with galls in their natural range. Component communities were also less diverse and species‐rich. The relationship between gall size and parasitism rates showed no significant difference between the two continents. These results show that the component community in its introduced range is depauperate and provide support for the Enemy Release Hypothesis.