Complex life cycles in Andricus kollari (Hymenoptera, Cynipidae) and theirimpact on associated parasitoid and inquiline species

The cynipid gall wasp Andricus kollari was introduced to southwestern Brita in in the middle of the nineteenth century and has since spread throughout the British Isles. A. kollari produces a sexual generation in spring on Tur key oak (Quercus cei ris; an introduced tree in Britain) and a parthenogene tic generation in autumn on the native oak species Q. robur and Q. petraea. The galls of the two generations differ considerably in size (< 5 mm in di ameter in the spring galls and up to 30 mm for the autumn galls). In 1994 a nd 1995 we sampled eight populations of A. kollari at sites from the south of England to the north of Scotland. The females of the autumn generation e merged in two cohorts: one in the autumn of the year of gall induction and the second in the spring of the following year. The second cohort of parthe nogenetic females, however, emerges when the spring galls, containing the n ext sexual generation, are already fully developed. Eggs laid by the female s of the second cohort thus cannot contribute to that spring generation, bu t slay dormant in the buds of Turkey oak and hatch as the spring generation a pear later. The proportion of females of this second cohort increased fr om south to north, and in the north of Scotland the populations densities o f A. kollari showed substantial differences between years, consistent with a two-year life cycle. Here we study the effects the different gall propert ies and the geographical variation of a complex phenology have on the commu nities of parasitoids, inquilines and predators associated with A. kollari. We measured predation rates by birds and parasitoid attack in the galls of both generations. While there was an increasing tendency towards a two-year life cycle from the south to the north, bird predation rates on the spring galls decreased in 1994 from > 50% in the south to less than 5% in the nor th. Similarly, parasitoid abundance in the autumn galls decreased from an a verage of 6 individuals per gall in the south to 1 per gall in the north (i n contrast to the spring galls, the autumn galls are sufficiently large to contain inquilines and can yield more than one individual per gall). Parasi tism rates in the spring galls showed no geographical trend in 1994, but a clear decrease from south to north in 1995. This year, 1995, was a high-den sity year in the north, which suggests a satiation effect whereby the paras itoid species cannot follow the sudden increase in the host population.