Cecidomyiidae

Rhopalomyia

Artemisia cana

Alaska

Arizona

California

Colorado

Idaho

Minnesota

Montana

Nebraska

Nevada

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North Dakota

Oregon

South Dakota

Utah

Wyoming

Alberta

British Columbia

Manitoba

Saskatchewan

Artemisia tridentata

Washington

Rhopalomyia medusirrasa

Adults, larvae, and pupae of this species are anatomically indistinguishable from R. medusa. Both form the same type of gall but that of R. medusirrasa is densely covered with long trichomes, whereas that of R. medusa is naked. 

This gall (Fig. 40) resembles that of R. medusa but differs in that all radiating filaments are densely clothed with long trichomes. These trichomes generally mask the reddish and green colors of the filaments and give the gall a white or gray appearance.

Twenty-four mature galls from Sites 6 and 25 had a diameter (measured perpendicular to stem) ranging from 12-25 mm (avg. 17). Thirty-one galls that were collected April 14, 1969 at Site 6 prior to ephemeral leaf development had a diameter ranging from 6-16 mm (avg. 10) and contained 1-4 larvae (avg. 2). The first sign of gall development was in early October with the appearance of small clumps of leaflike structures covered with trichomes at the nodes. The galls grow during the fall and again in the spring until about the time pupae are present. After adult emergence the galls dried but retained their gray appearance and persisted on the plants as for those of R. medusa. 

Biology and taxonomy of the Rhopalomyia Gall Midges (Diptera: Cecidomyiidae) of Artemisia tridentata Nuttall (Compositae) in Idaho

Rhopalomyia medusirrasa
Woolly-bud Gall Midge

These large galls are composed of numerous leaflike structures covered with long forked hairs, which give the galls an overall white-green color. 

Plant Galls of the Western United States