Sagebrush is a predominant plant of the arid Utah and Nevada landscape. Plants that grow in arid regions have less competition for sunlight from other plants, but competition for water is fierce. The foliage of desert plants is a potential source of water and nutrients for numerous herbivores. The foliage often grows slowly and is heavily defended. Desert plants often produce a variety of chemicals to deter herbivores from eating their leaves and taking their precious water and nutrients.
Insect herbivores need water and plants to survive. Since the leaves are protected by toxic chemicals, other ways of getting their food are essential. The sponge gall midge, Rhopalomyia pomum, is a common sight on sagebrush in Utah and the American Southwest. The sponge gall midge is in the same family of flies as the Hessian fly, an important pest of wheat.
The gall flies lay their eggs in the stems of their host plant, in this case, sagebrush, Artemesia tridentata. The egg hatches and the larva secretes saliva into the plant. The saliva contains factors that alter the growth of the plant. Instead of its typical stem, the plant produces a swelling or “gall” around the developing insect. The gall is a feeding site for the insect and the plant is induced to deliver nutrients to its insect parasite. The gall can provide the insect some protection from predators.
However, in any area where food is scarce, other insects will find a food source. The sponge gall midge has several parasitoids, tiny wasps that probe the galls with their ovipositor, and lay their eggs inside the developing midge. The eggs of the wasp parasitoids hatch inside the midge. Instead of a midge emerging from the gall, the parasitoid wasps will emerge.
The sponge gall midge is a subject for ecological studies. In 2009, Beckenbach and Joy published a study comparing the genes of the sponge gall midge to the genes of Hessian fly and other insects. The Hessian flies used for the study came from Purdue.
The sponge gall midge is aptly named. The gall is feels spongy, like a sponge or a cotton ball when squeezed.
Sponge Gall Midge
Living With Insects Blog 
Great info. Is is also true that the marble size “cotton balls’ found as inflations of Artemisia stems, many found here in central Oregon, is caused by a Rhopalomyia species of midge fly? I would like to know the species as I have observed the small red ants here attack them if they are found on the ground, and am looking to see if they climb the Artemisia to do the same.
Rhopalomyia species can produce a “woolly” or cottony gall in some species of Artemisia. It helps to know the species of Artemisia and positive ID requires collecting the midge from the gall.
In some cases, multiple species can be present in a single gall. The galls can also contain parasitoid wasps that feed on the midges and caterpillars that harbor in the gall. You might ask someone in your state extension office.
As I suspected. Thank you for your reply. The species is Artemisia tridentata, at about 5000ft. and on the edge of the Crooked River canyon about three miles south of Prinville, Oregon.
Finally an answer to my years-old question! I thought these were some kind of berry or seed on the Sagebrush native to my eastern Washington home. Thanks!
Glad to know what this is finally! Been wondering for years, hiking around the Moab desert. Thanks for a great article!