Gall size determines the structure of the Rabdophaga strobiloides host-parasitoid community

1. The relationship between gall size and mortality of the willow pinecone gall midge Rabdophaga strobiloides ( Diptera: Cecidomyiidae) was examined by determining the fate of all galls in a 30-ha area in central Alberta, Canada over 4 years. It was found that gall size has a large effect on the type and intensity of mortality experienced by the gall midge, and consequently this factor has the potential to influence the dynamics of the host - parasitoid interaction through the creation of phenotypic refuges. 2. Total midge mortality ranged from 51% to 78% over the course of the study and was dominated by parasitism by Torymus cecidomyiae ( Hymenoptera: Torymidae) and Gastrancistrus sp. ( Hymenoptera: Pteromalidae) as well as predation by birds. Gall size had a strong, non-linear effect on the attack rates of each of these natural enemies. 3. Birds attacked the smallest size classes. Torymus cecidomyiae preferentially attacked medium diameter galls and thus avoided predation by birds in smaller galls. Gastrancistrus sp. preferentially attacked the largest galls and consequently suffered lower rates of predation by both T. cecidomyiae and birds. 4. This study emphasises the importance of understanding the interactions among mortality factors in order to describe adequately the susceptibility of R. strobiloides to parasitism and predation, and ultimately its population dynamics.