The effect of dispersal on the population dynamics of a gall-forming midgeand its parasitoids

1. Theoretical studies predict that limited amounts of dispersal of individ uals in host-parasitoid systems can both enhance the stability of the subpo pulations and promote the co-existence of competing parasitoid species. We investigated the effects of dispersal on the population dynamics and parasi toid community structure of a natural host-multi-parasitoid system consisti ng of the midge Rhopalomyia californica that forms galls on the shrub Bacch aris pilularis and the parasitoids that attack the midge. 2. An experiment involving the release of midges into a field with a low ba ckground density of galls demonstrated that the midges, on average, travell ed approximately 1.7m in their lifetime. This suggests that the appropriate spatial scale at which to look at the effects of dispersal is relatively s mall. 3. Dispersal of midges and parasitoids between individual bushes was experi mentally eliminated in a caging experiment. The midge populations in all of the uncaged replicates displayed dynamics that were similar to each other, while the dynamics of the midge populations in the caged replicates diverg ed. The midge dynamics on the uncaged bushes were not significantly more st able than those on the caged bushes. 4. Dispersal among bushes was found to play a major role in co-existence of the competing parasitoid species. There was a dramatic drop in the parasit oid species diversity on the caged bushes, with only a single parasitoid sp ecies, Platygaster californica, persisting at high numbers in the caged pop ulations. In accordance with theoretical models, P. californica is the para sitoid species in the community that has the highest attack rate and is mos t effective at searching for hosts in a restricted area. Alternative explan ations for this pattern are discussed.