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Complex Life Cycles in Andricus kollari (Hymenoptera, Cynipidae) and Their Impact on Associated Parasitoid and Inquiline Species
K. Schönrogge, P. Walker and M. J. Crawley
Vol. 84, No. 2 (Feb., 1999), pp. 293-301
Stable URL: http://www.jstor.org/stable/3546724
Page Count: 9
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The cynipid gall wasp Andricus kollari was introduced to southwestern Britain in the middle of the nineteenth century and has since spread throughout the British Isles. A. kollari produces a sexual generation in spring on Turkey oak (Quercus cerris; an introduced tree in Britain) and a parthenogenetic 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 diameter in the spring galls and up to 30 mm for the autumn galls). In 1994 and 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 emerged 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 parthenogenetic females, however, emerges when the spring galls, containing the next sexual generation, are already fully developed. Eggs laid by the females of the second cohort thus cannot contribute to that spring generation, but stay dormant in the buds of Turkey oak and hatch as the spring generation a year later. The proportion of females of this second cohort increased from south to north, and in the north of Scotland the populations densities of A. kollari showed substantial differences between years, consistent with a two-year life cycle. Here we study the effects the different gall properties and the geographical variation of a complex phenology have on the communities 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 north. Similarly, parasitoid abundance in the autumn galls decreased from an average of 6 individuals per gall in the south to 1 per gall in the north (in contrast to the spring galls, the autumn galls are sufficiently large to contain inquilines and can yield more than one individual per gall). Parasitism 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-density year in the north, which suggests a satiation effect whereby the parasitoid species cannot follow the sudden increase in the host population.
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