The other day I wandered through some tall sagebrush (Artemisia tridentata) on our property (Lookout CA) looking for sagebrush galls. Artemisia species are galled by many species of midges, tephritid fruit flies and a eriophyid mite. Tall sagebrush (also called big sagebrush and Great Basin sagebrush) seems to host a particularly large number of galling insects. I was not disappointed.
Nearly every tall sagebrush had sagebrush stem galls. These galls are formed in the terminal stem tissue of new shoots by a stem gall tephritid fruit fly – Eutreta diana. These fruit fly galls are stimulated when the female fruit fly deposits her egg directly into axillary and terminal buds. Development begins in the fall and the gall with its larva remains small throughout the winter. In the spring the gall resumes growth and reaches maturity. The adult fruit fly emerges in May.
The gall itself is spindle-shaped, integral (not detatchable or part of the stem) and monothalamous (one-chambered with usually only one larvae). Grey-green in color, the gall is covered in soft hairs (pubescent) and usually has leaves emerging from its sides. A rosy purple flush often colors the gall. This flush is usually caused by sunlight, however there are suggestions that gall parasites or inquilines (insects that eat the gall tissue and/or the insect in the gall) may cause the rosy color. Inside the gall there is some white frasslike material but no solid feces. As the gall ages it turns brownish.
These pictures are of younger fall sagebrush stem galls and the tephritid fruit fly larva. More information on Artemesia tridentata, including photographs of the three-lobed leaves, can be found in my earlier post “After the Storm”. The wooly bud gall midge is another insect that galls A. tridentata.