Parasitoid and inquiline attack in the galls of four alien, cynipid gall wasps: host switches and the effect on parasitoid sex ratios

1. Four alien cynipid gall wasps of the genus Andricus are established and still spreading in the British Isles. The order, according to the northerli ness of their distribution boundary, is: A. corruptrix --> A. quercuscalici s --> A. lignicola --> A. kollari. All four aliens have a sexual generation in spring on Quercus cerris (introduced to Britain) and an agamic generati on in autumn on native oak species. 2. For 2 years 1994 and 1995, galls of both generations of the four alien s pecies were sampled at eight sites from the south of England to the north o f Scotland to determine the parasitoid and inquiline species that attack th e new galls. The spring generations of the invading species shared a parasi toid complex of four pteromalid species. Five species of inquilines and 11 species of parasitoids emerged from the autumn galls. 3. Two colonisation events were recorded for A. lignicola and A. corruptrix . On both occasions, the spring generations were found first at the new sit es, indicating that the agamic generation provides the colonisers for these invading species. After colonisation, the galls of both species were attac ked by parasitoids in their first season. 4. In spring, the invading species were among the most abundant cynipids at all eight sites. By sampling the whole local community of cynipid galls, i t was found that the parasitoid species attacking the spring galls of the i nvaders seemed to have shifted their attack to the new hosts. 5. The secondary sex ratios of the parasitoid species emerging from the sex ual galls of A. quercuscalicis (the smallest of the four) showed a strong a nd significant male bias at all sites and in both years. Parasitoid emergen ce from the galls of the sexual generations of the other three species (all about equal in size) was between 60 and 70% male, and variable among sites and between years.