Willow leaf traits affecting host use by the leaf‐gall‐forming sawfly

Gall‐forming insects usually have very restricted host ranges, but plant traits affecting patterns of host use have rarely been examined. The sawfly Phyllocolpa sp. (Hymenoptera: Tenthredinidae) forms leaf‐roll galls on three of seven Salix (Salicaceae) species that occur together on riverbanks in central Japan. We have attempted to explain this host‐use pattern by invoking three plant traits: synchrony of leaf flush with the oviposition period of the sawfly, intrinsic leaf quality as a potential larval food, and leaf morphology. Two Salix species frequently used by the sawfly, Salix eriocarpa and Salix pierotii , had similar leaf traits suitable for larval survival. The third species, Salix serissaefolia , was used relatively less often and the sawfly frequently stopped laying eggs on the plant during oviposition, suggesting ovipositional selection. S. serissaefolia had the smallest leaves, and survival of sawfly larvae was lower on S. serissaefolia than on S. eriocarpa and S. pierotii , because of gall destruction, by other herbivorous insects, and leaf‐size restrictions. Among the four unused species, Salix chaenomeloides had a late leaf‐flush phenology, Salix gracilistyla had inferior leaf quality, and Salix gilgiana had linear leaves; these seemed to be critical factors for non‐use. Salix subfragilis was also unused, but the reason for this could not be explained by the three leaf traits studied.