Range expansion and enemy recruitment by eight alien gall wasp species in Britain

1. Biological invasions involving continuous range expansion differ from discontinuous introductions in that invaded and native ranges remain connected, potentially allowing pursuit of range expanding species by their natural enemies. The establishment in Britain of eight alien herbivorous gall wasps (Hymenoptera: Cynipinae) provides a rare opportunity to study continuous range expansion and naturalenemy recruitment in a guild of related and ecologically similar species. 2. Four aliens (Andricus kollari, A. quercuscalicis, A. lignicolus,and A. corruptrix)reached the UK before 1990, while four more recent invaders (A. aries, A. grossulariae, A. lucidus,and Aphelonyx cerricola) reached Britain by 2000.We provide the first parasitoid records for the recent invaders, update community development for the earlier set, and use dates of first record to estimate rates of spread for all eight species. 3. While the recent invaders are restricted to southern and eastern England, three of the early invaders have reached northern Scotland. From their origins in southern England, invading gall wasps have expanded their distributions across the UK at mean rates ranging from 1.4 km⁄year to >20 km⁄ year. Variation in range expansion rate was not related to life history differences, including voltinism or host oak association. 4. All species have recruited native parasitoid enemies since their arrival, and we found no evidence of pursuit by non-native natural enemies from continental Europe. Our results suggest that over timescales predicted for rapid climate change, herbivore⁄parasitoid communities are unlikely to expand their range as sets of interacting species. Rather, we expect host range expansions to trigger local reassembly of communities.