The mature gall is tube-shaped (Fig. 11). The surface is dark reddish and covered with the same white trichomes as on the normal leaf surface. The gall has a single larval chamber at the base, is hollow in the center and has a thin septum near the apex. The hollow portion is utilized by the pupa to facilitate escape, with the pupal exuvium remaining at the ruptured apical septum.
In appearance the developing gall differs only in size from that of the mature gall.

Gall elongate-cylindrical, not tapered at apex...Rhopalomyia tubulus Gagné

The 3–6 mm long gall has a single larval chamber at its base and a thin septum near the apex through which the adult escapes...Host: A. tridentata