1. Data from parasitoids attacking galls of Giraudiella inclusa Fr. (Diptera: Cecidomyiidae) from Phragmites australis Trin. were used to assess the factors dictating species richness and coexistence in a community. 2. Species richness of the 14 parasitoids could be related to the high frequency and apparency of both the galls and the Phragmites monocultures. Four non-galling species of Cecidomyiidae attacking P. australis had only 2-3 parasitoid species, presumably due to the less visible and less heterogeneous microhabitat offered to the parasitoids. 3. Distribution of midge galls was due to several plant characteristics with percentage parasitism and parasitoid coexistence determined by the site of gall induction. The stem-boring moth Archanara geminipuncta Haw. (Lepidoptera: Noctuidae) was a key factor for the heterogeneous gall distribution and thereby favoured coexistence of the galls' parasitoid species: Giraudiella-galls induced on side shoots, grown from Archanara-damaged shoots, increased percentage parasitism of two parasitoid species and reduced parasitism of one species. 4. Parasitoid foraging was concentrated on temporally and spatially separated subpopulations with the microscale of host density (gall cluster size per reed internode) influencing percentage parasitism only in two out of eight species, and with the macroscale (gall abundance per habitat) having no effect at all. These findings do not meet expectations for density-dependent host regulation, but support a resource heterogeneity hypothesis. Both midge and parasitoid populations appeared to be 'limited from below' rather than 'controlled from above'. 5. The prediction that host density is negatively correlated with parasitoid diversity was not supported.
