Factors affecting parasitism in the oak-galler Neuroterus quercusbaccarum (Hymenoptera: Cynipidae)

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We investigate the influence of seven explanatory variables based on individual characteristics of galls on parasitism in the oak-galler, Neuroterus quercusbaccarum(sexual generation). The community consists of three species of parasitoid and one species of inquiline (which is lethal to the galler). Our analysis shows that there is considerable spatial heterogeneity in parasitism from site to site and from tree to tree within sites. With regard to the placement of galls on tree organs, galls on catkins are less parasitised than those on leaves. Gall size does not explain this difference because the external diameter of catkin galls is not significantly different from those on leaves. We hypothesise that the precocious abscission of catkin galls prevents their exploitation by parasitoid species with long developmental times. Moreover, there is a distinct sequence of parasitism, reflected by a partitioning in the sizes of galls attacked by each parasite species. However, the growth dynamics of the galls themselves show that just external diameter is not the only size parameter affecting the niches of the different parasites. Even though Synergus spp. is one of the earliest acting parasites, it attacks galls with thicker walls relative to external diameter than occurring in unparasitised galls from which the galler emerges. A delay between gall growth and feeding activity of the gallmaker would induce non-linear growth of gallwall thickness, with major consequences for the accessibility of the host larva to the parasitoid.

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