Rhopalomyia hirtipomum appears to be anatomically identical to R. pomum but is responsible for a slightly different gall. That of hirtipomum is densely covered with long trichomes, whereas that of R. pomum has short, prostrate, sparse trichomes on the surface. In addition, galls of the former are always monothalamous.

This monothalamous gall (Fig. 9) is globose and quite similar to the monothalamous gall of R. pomum. However, the gall surface is densely clothed with long trichomes, giving it a fuzzy appearance. The gall is generally solitary on either the upper or lower leaf surface, but can occasionally be found in confluent clumps (Fig. 10). Fifty-nine mature galls were measured: the diameter extending from the gall base ranged from 8-15 mm (avg. 12) and the diameter perpendicular to this ranged from 8-13 mm (avg. 11).

Developing galls were first found on October 23, 1969 at Site 1 on A. t. wyomingensis. They appeared as fuzzy swellings with diameters of 1-3 mm on a persistent leaf surface. The long, dense trichomes were white, while the surface of the gall was reddish. Galls had the same appearance on A. t. vaseyana f. xericensis when found at Site 27 on December 29, 1969. At both sites this appearance persisted until March, when growth resumed. As the gall grew, their surface coloration gradually changed to grey. Maximum size of the gall was found to coincide with the presence of pupae and with new stem growth of approximately 2.5 cm in length. Shedding of the galls occured in July and August, at the same time as those of R. pomum.

- Robert G. Jones, Raymond J. Gagné, William F. Barr: (1983) Biology and taxonomy of the Rhopalomyia Gall Midges (Diptera: Cecidomyiidae) of Artemisia tridentata Nuttall (Compositae) in Idaho^©