This bud gall is monothalamous. The mature gall is green and covered with trichomes. The larval capsule has a yellowish translucent sclerenchymatous surface. On A. t. tridentata the prominent sclerenchymatous larval capsule of the mature gall (Fig. 30) is surrounded by a whorl of basally fused leaf vestiges around which are several whorls of linear or spatulate leaves that gradually increase in length outwardly. The gall is irregularly rosette in form. On both A. t. vaseyana and A. t. wyomingensis the mature gall (Fig. 31) is more normally rosette. The sclerenchymatous larval capsule is surrounded by several whorls of equally long, ovate leaves.

Thirty-one mature galls collected at Site 1 and 2 on A. t. tridentata had a diameter (measured at base including inner whorl of leaves) ranging from 3-6 mm (avg. 4). The sclerenchymatous capsules of these galls had a length range of of 3-4 mm (avg. 3.2) and the width or diameter of each was approximately 1 mm. The diameter (measured across the apices of the outer leaves) of 26 mature galls collected at Sites 15 and 21 from A. t. wyomingensis ranged from 8-21 mm (avg. 9.41).

Developing galls occur with the initiation of spring plant growth. On A. t. wyomingensis the developing gall was first found seasonally on March 23, 1970 at Site 12 and on A. t. vaseyana the following day at Site 27. The galls dry shortly after midge emergence the following spring and persist on the plant into at least late summer. Galls from which emergence has occured are readily recognized by the protruding larval capsule that is broken apically.