The white or gray polythalamous gall (Fig. 34) appears to be a concentration of individual, ovoid galls (Fig. 35) that form a globuliferous mass around a stem. The gall is a firm mass of plant tissue with centrally located larval chambers. The surface is densely clothed with long trichomes giving it a fuzzy appearance.

Sixty-three mature galls collected from A. t. tridentata and A. t. wyomingensis at Sites 1, 21, and 24 ranged in width (diameter perpendicular to stem) from 30-15 mm (avg. 9) and in height (diameter parallel to stem) from 3-10 mm (avg. 6). These galls had 1-11 (avg. 5) larval chambers.

Galls develop from summer to the following spring with most noticeable growth occurring before winter. The first sign of the developing gall was in early June when immature larvae were found. At this time ephemeral leaves were near full growth, the persistent leaves were nearly dry, and the gall appeared as a small white globose structure at a node. Ten of these developing galls collected on June 18, 1969 at Site 1 ranged in width from 1.5 to 2.5 mm (avg. 1.9) Galls begin to dry and decompose after midge emergence. However, old gray colored galls were observed persisting on plants for at least a year.

- 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^©