Are galling insects better protected against parasitoids than exposed feeders?: a test using tenthredinid sawflies

Data mostly from the published literature were used to assess the effect of galling on the number of parasitoid species per host species in the phylogeny of nematine sawflies from free external feeders (colonial and solitary) to leaf gallers and shoot gallers. The strongest effects of galling were the total elimination of the species‐rich cocoon‐attacking guild of parasitoids, and eonymphal parasitoids, from the parasitoid community on shoot gallers, all of which are in the genus Euura. All tachinid larval parasitoids were also eliminated by the galling habit. The cumulative effects of these exclusions resulted in a decline in mean number of parasitoid species per host species from almost sixteen species on external colonial feeders to 4.0 species on shoot gallers. General patterns in per cent parasitism by non‐tachinid and tachinid larval parasitoids, eonymphal and cocoon parasitoids, on exposed feeders to shoot gallers, showed declines in non‐tachinid attack and elimination of tachinid, eonymphal and cocoon parasitoids. But leaf gallers tended to be attacked more than exposed feeders by non‐tachinid larval parasitoids. The galling habit had a long‐term impact by reducing the number of parasitoid species attacking nematine sawfly gallers and per cent mortality inflicted, so that natural enemies may have been important as a selective factor in the evolution of galling nematine sawflies.