The gall ... constitutes a modified capitulum. It is cylindrical, 5–10 mm long and 2–3.5 mm wide, usually slightly wider at base than at the tip. The gall is green but densely covered by short, white hairs that give it a whitish, fuzzy appearance. Galls are almost always situated among normal capitula, and are accompanied by tiny leaves and/or ray florets at their base. Each gall contains a single smooth, thick-walled chamber. Inside this chamber is an inner, conical chamber with very thin walls, containing a single larva, facing downwards. Larvae turn to face upwards inside the gall before pupation. The gall’s wall is much thinner apically than laterally, and galls that contain pupae sometimes have slits at their tips ... Galls become apparent in mid August and pupation takes place in late August to mid September.

From vase shaped flower gall on Solidago canadensis [note: Plakidas does not appear to distinguish S. canadensis from its close relatives]