One of the responsibilities I have at the Garden is looking after the library. After trying (and failing) to identify this plant, I ordered a copy of Ron Russo's Field Guide to Plant Galls of California and Other Western States for the library as a reference. Though it didn't help in that identification, the book has quickly become a favourite among the staff here at UBC Botanical Garden for its crisp photography and intriguing subject matter -- I think there will even be a book review in our next issue of Davidsonia.
The midge-induced gall on this Artemisia tridentata (big sagebrush) is not accompanied by a photograph in the book. However, Rhopalomyia medusa is mentioned in the image-accompanied entry on Rhopalomyia medusirrasa, a closely-related species. On Rhopalomyia medusirrasa, from the book:
"This midge induces globular, leafy-pubescent, polythalamous [many-chambered] galls on the bud of Great Basin sagebrush (Artemisia tridentata). These large galls are actually composed of numerous leaflife structures that are covered with long, forked hairs...Galls begin development in October, rest during the winter, and reach full size the following spring. These spring galls measure 20 to 25 mm in diameter and contain up to four larvae. Adults emerge in April or May. The larvae, pupae, and adults of this species [Rhopalomyia medusirrasa] are basically indistinguishable from those of Rhopalomyia medusa. The major differences exist with the galls. The galls of the woolly bud gall midge [Rhopalomyia medusirrasa] have the long white hairs, while the galls of Rhopalomyia medusa are hairless."
Gall midges are relatives of pests known to many indoor gardeners, fungus gnats (search the UBC BG Forums for "+fungus +gnat" (without quotes) to see the many discussions). For photographs of members of the genus Rhopalomyia, head on over to bugguide.net: Rhopalomyia. There are no images of either of Rhopalomyia medusa or Rhopalomyia medusirrasa, but you'll get a good sense of the various life stages of these small insect species from the others photographed.
I also note from this entry that I've never done an entry on Artemisia tridentata for BPotD. That's something I'll have to correct in the near future!