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

1. Theoretical studies predict that limited amounts of dispersal of individuals in host-parasitoid systems can both enhance the stability of the subpopulations and promote the co-existence of competing parasitoid species. We investigated the effects of dispersal on the population dynamics and parasitoid community structure of a natural host-multi-parasitoid system consisting of the midge Rhopalomyia californica that forms galls on the shrub Baccharis pilularis and the parasitoids that attack the midge. 2. An experiment involving the release of midges into a field with a low background density of galls demonstrated that the midges, on average, travelled approximately 1.7m in their lifetime. This suggests that the appropriate spatial scale at which to look at the effects of dispersal is relatively small. 3. Dispersal of midges and parasitoids between individual bushes was experimentally 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 diverged. The midge dynamics on the uncaged bushes were not significantly more stable 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 parasitoid species diversity on the caged bushes, with only a single parasitoid species, Platygaster californica, persisting at high numbers in the caged populations. In accordance with theoretical models, P. californica is the parasitoid species in the community that has the highest attack rate and is most effective at searching for hosts in a restricted area. Alternative explanations for this pattern are discussed.