This gall is a composite growths of the branch tips and buds (Fig. 19). At maturity it consists of 2-4 short, malformed, developing stems that are grouped in a globose mass of white trichomes from which green leaves protrude. The surface of these stems is densely clothed with white trichomes which are longer than the linear leaves arising from these stems. Hidden by the dense trichome mass, sclerenchymatous larval capsules are on the axis of the stems. These capsules are elongate cylinders with a rounded apex and a thin hard wall. The galls are generally found concentrated on one branch.

Fifty mature galls ranged in height from 8-15 mm (avg. 11.68). Their width ranged from 8-16 mm (avg. 12.88). Five of the larval capsules were approximately 2 mm in height and 1 mm in width.

Early development of this gall was noted on June 5, 1969 at Site 24. The immature gall is recognized as a mass of white trichomes subtended by a whorl of developing ephemeral leaves at the tip of a developing stem. The galls increase in size during the summer and reach what appears to be maximum size in late August when the ephemeral leaves begin to dry. The galls remained unchanged in appearance after adult emergence and throughout the winter. The following spring the trichomes on the gall surface become drab. The old galls persist up to two years and perhaps longer in sheltered parts of the plant.

- 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^©