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Summary. 1. The length of time that a gall‐forming midge, Rhopalomyia califomica, was vulnerable to attack by four parasitoid species was measured in the field at two locations.2. The midge had a restricted window of vulnerability to each of the parasitoid species, but similar windows of vulnerability were found at the two sites.3. A stage‐structured model was used to illustrate that the length of the vulnerable window should have no effect on the fraction parasitized by a single parasitoid species if that species is the only parasitoid attacking the host in a coupled host‐parasitoid interaction. However, the length of the window of vulnerability can have a positive effect on the fraction parasitized by a species in competition with other parasitoid species.4. The length of the window of vulnerability can help explain the relative field abundance of four of the common parasitoid species of R.californica.

Summary1. 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 eects 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 ®eld 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 eects 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 signi®cantly 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 eective at searching for hosts in a restricted area. Alternative explanations for this pattern are discussed.