Abstract. 1. Biological invasions of exotic species pose a major threat to native biodiversity. Invaders are known to have direct impacts on native species; however, less well studied are the indirect impacts mediated through the integration of invaders into trophically linked communities.
2. A survey of the chalcid wasp parasitoid community attacking the chestnut gallwasp Dryocosmus kuriphilus was carried out over a 5-year period at 26 sites in north-western Italy. More than 415 000 galls were collected and more than 10 000 parasitoid specimens emerged. Twenty-seven parasitoid species belonging to six families (Eurytomidae, Pteromalidae, Torymidae, Eupelmidae, Ormyridae, Eulophidae) were identified using morphological and molecular methods; seventeen are new records for the parasitoid community associated with D. kuriphilus in Italy. The morphospecies complexes Megastigmus dorsalis, Eupelmus urozonus, E. annulatus, and Eurytoma pistaciae were the dominant species; another six morphospecies were encountered regularly but at low frequency; and 13 species were recorded only occasionally. The attack rate of any individual parasitoid species was low, although the more common species appeared to be increasing their use of this novel host.
3. Biases observed in the sex ratios of parasitoids emerging from D. kuriphilus galls suggest that parasitoid individuals are making life-history decisions to take advantage of the high abundance of this host. Overall, these patterns imply that there is an ongoing transfer of parasitoids between oak and chestnut galls, and hence a high potential for this invading species to have a major impact on native oak gall communities via indirect competition mediated through shared parasitoids.