Jeff Mitton Natural Selections
Jeff Mitton Natural Selections

I had seen the curious growths on big sagebrush before, but they had not captured my interest. But this last summer, in successive trips to the area around Comb Ridge and Natural Bridges National Monument in Utah, I saw many big sagebrushes, Artemesia tridentata, with one to five of these odd growths, and their ubiquity tweaked my curiosity.

Initially, without thinking critically, I dismissed them as flowers, or perhaps fruits. But the flowers of sagebrushes are small, yellow and have five petals. Furthermore, these growths were approximately the size of golf balls, much too large to be fruits or seed pods from such small flowers.

By process of elimination, I formed the hypothesis that these growths were galls produced by some sort of animal. This was not a brilliant epiphany but the most logical and likely possibility, for galls are common on plants.

Animals induce plants to create galls by chewing or injecting compounds similar to plant hormones. The chewing causes callus tissue to develop, and eggs can be inserted into that tissue. Analogues of plant hormones cause unusual development in meristematic or growing tissues. A common example of this sort of gall is made by the spruce adelgid, which sucks juices from growing spruce twigs and introduces a chemical that initiates development of a female cone. But development goes awry, producing something that looks like a brown female cone adorned with dried needles, but is a gall containing larvae.


A gall often has a hard, durable outer case to discourage predators, so it serves as a safe haven for defenseless developing larvae. The inside is typically moist and soft, rich in protein and sugars, supplying the larvae with everything needed for growth and development. Juvenile gall insects have the luxury of living in an edible fortress.

Many groups of animals induce plants to produce galls, including viruses, bacteria, mites, fungi and nematodes. But from my perspective, insects create the most common and conspicuous galls. Galling insects include beetles, moths, butterflies, aphids and thrips. If species diversity is the metric of interest, the groups with the most galling insects are cynipid wasps and tiny flies called midges.

I was traveling when I became interested in sagebrush galls, so I was not prepared to collect live material. If I had collected galls, I could have brought them back to the laboratory of my entomologist colleague Deane Bowers, where they could have been reared and identified. But I was on a camping and photography trip, so I took photos of galls in several places. When I returned to Boulder I ran a digital search for galls on big sagebrush, and I quickly found photos that looked like those in the photo illustrating this column.

Big sagebrush galls are produced by midges, a very diverse group of tiny flies in the family Cecidomyiidae, which contains over 6,000 species. Most midges are only 2 to 3 millimeters long and have hairy wings and long antennae. This particular species is the wooly bud gall midge, Rhopalomyia medusirrasa.

Midges are in the galls for perhaps half of one year. Adults lay eggs on big sagebrush buds in summer and the eggs hatch in fall. Larval feeding induces galls to form in October and larvae spend the winter in the galls. In spring, a gall may be home to up to four larvae, which emerge as adults in late spring or early summer.

The midges have bewildering diversity. A closely related midge forms galls of the same size on sagebrush, except that the galls lack the white hairs; a total of seven species of flies create galls on big sagebrush. Fifteen closely related species of midges form galls on creosote bush, Larrea tridentata. Midges are hardly visible, but their galls are everywhere.
