Among the numerous kinds of arthropods that are known to induce galls are several hundred species of sawflies in the subfamily Nematinae (Family Tenthredinidae) that pick on species of willow (Salix) and poplar (Populus). Full-blown gallers seem to have evolved from species that cause leaves to fold or curl around their grubs. These grubs feed externally in the sense that they eat from the surface down, but are well protected inside the deformed leaves. The more derived types of galls are consistently formed by particular lineages of sawflies, and the most derived type of gallers seem to be the sawflies that induce deformities in stems, buds, or leaf petioles where the grubs feed internally.
The larvae of the Willow Red-Bean Gall Sawfly are grub-like and feed in a gallery inside the bean gall. The gall can be anything from pale to green to red (usually most intense on the upper side of the leaf). The adults are small black sawflies – we have pictures of lots of these that are not identifiable, but we offer one that may look more or less like the adult (remember, take any identification based solely on a picture with a grain of salt).
*Tommi Nyman, Alex Widmer, & Heikki Roininen. Evolution of Gall Morphology and Host-Plant Relationships in Willow-Feeding Sawflies (Hymenoptera: Tenthredinidae). Evolution 54(2), 2000, pp. 526–533