Immature larvae were found June 13, 1969 at Site 1 on both A. t. tridentata and A. t. wyomingensis, coinciding with the first sign of gall development. By late fall the larva reaches maturity. They are a darker orange color than larvae found earlier and they completely fill the sclerenchymatous capsule in the center of the gall. Overwintering occurs at this state of development. Adults were emerging by March 24, 1969 at Site 1, whereas in 1970 they emerged before March 20 at this site and before March 19 at Site 24. Emergence was in progress as late as May 18, 1968 at Site 8.

This ovoid bud gall has vestiges of spatulate leaves (Fig. 33). Its smooth, green surface is densely covered with trichomes. Maturity is reached in the fall after which degeneration begins. The gall surface dries and hardens, becoming shiny yellow-brown and losing its trichomes and the leaf vestiges. By late fall the tissue beneath the surface layer dries to a hard black mass which surrounds the 1-4 sclerenchymatous larval capsules containing the overwintering larvae. These dry galls persist on either vegetative or flower stems into the following spring until shortly after the midges emerge.

Seventy-eight mature galls from the three subspecies of A. tridentata collected from Sites 1, 15, 21 and 27 ranged in diameter from 5-10 mm (avg. 7) and in height from 6-11 mm (avg. 8). No size differences of the galls could be attributed to host plant or location. Ten immature galls collected from A. t. tridentata at Site 1 on June 13, 1969 had a diameter ranging from 204 mm [presumably a typo, most likely meant "2-4"] (avg. 3).