One of the most common galls found on tall sagebrush (Artemisia tridentata) is a spongy gall induced by the sponge gall midge (Rhopalomyia pomum). Tall sagebrush, also commonly called Great Basin or big sagebrush, is found between 4,000 and 10,000 feet in cold arid or semi-arid habitats throughout the West.
The galls induced by R pomum are green, reddish brown to purple in color. These galls can be round and have a singular larval chamber (monothalamous) or irregularly shaped, deeply fissured or multilobed with more than one larval chamber (polythalamous). Ruth A Hufbauer (2004) writing in the “Western North American Naturalist” suggests that the form is determined by the number of parasitoids (parasites that attack the midge) present, not the number of midge larvae present. Early in the season parasitoids attacking before gall development may contribute to lobe formation by disrupting cues from the midge eggs or larvae to the host plant.
Gall development begins in October and after overwintering, the gall attains full growth in the spring. The larval chamber is located near the base of the gall. Adults emerge in May or June with the females depositing their eggs in tall sagebrush buds. Even when a plant is infected with a large number of galls, the general health and vigor of the host is not affected.
Many tall sagebrush along Elkins Loop #2 and the Quarry Trail in Ash Creek Wildlife Refuge (Modoc County CA), where these photographs were taken, are infected with R pomum sponge galls.