ALIEN HERBIVORES AND NATIVE PARASITOIDS - RAPID DEVELOPMENTS AND STRUCTURE OF THE PARASITOID AND INQUILINE COMPLEX IN AN INVADING GALL WASPANDRICUS-QUERCUSCALICIS (HYMENOPTERA, CYNIPIDAE)

1. Rapid and substantial changes have occurred in the parasitoid and i nquiline community associated with the agamic galls of Andricus quercu scalicis since it invaded Britain in the late 1950s. The number of par asitoid and inquiline species has risen from one to thirteen over a 15 -year period. Although the number of species has been relatively consi stent over the last 8 years, the species composition has changed consi derably and in a highly characteristic way during this period. 2. The parasitoid complex can be divided into two broadly distinct sets of pa rasitoid species; one set attacks only the gall former whereas the oth er set concentrates on the inquilines living in the wall of the gall. 3. The most dramatic change, however, is in the abundance of inquiline s which were reported to be virtually absent in earlier studies on thi s community in Britain. Over a period of only 5 years, between 1988 an d 1993, inquiline attack rose from less than 0.01 to an average of 0.2 6 inquilines per gall. The intensity of inquiline attack is geographic ally heterogenous, with high inquiline numbers restricted to south-eas t England. Because of the relatively high specificity of the parasitoi ds, high inquiline abundance is positively correlated with parasitoid species richness in knopper galls. 4. Parasitism rates, particularly o n the gall former, were generally low (<10%). Over the last 5 years, h owever, seven parasitoid species have been consistently recorded and t he mortality caused by these species has increased continuously. The s pecies composition of the community associated with this alien gall wa sp in Britain has quickly converged to the community known from its na tive range in continental Europe. Parasitoid species known to attack t he galls of A. quercuscalisis on the continent have been recorded from it in Britain for the first time mainly in areas where inquilines hav e recently become abundant. 5. Since rates of parasitism of the gall f ormer are still low, parasitoids are unlikely to play a major role in the population dynamics of this invading gall wasp at present, but the rapidly increasing inquiline and parasitoid attack could be a source of increased mortality for native cynipid species which are the altern ative hosts of those parasitoid species.