Old Sagebrush Stem Gall

The stem gall tephritid fruit fly (Eutreta diana) causes galls to form on tall sagebrush (Artemesia tridentata) following oviposition of a single egg in its axillary or terminal buds. In my last post (“Sagebrush Stem Gall”) I noted that the gall begins to develop in the fall, remains small throughout the winter and finishes developing in the spring. An old sagebrush stem gall is brownish.

In addition to the younger stem galls I found on tall sagebrush near our home (Lookout CA), there were also old galls on the plants. Adult stem gall tephritid fruit flies emerge in May and only produces one generation per year. Thus the old galls should not contain larvae in the fall.

Out of curiosity I opened some old galls. I was fascinated to find the pupae cases left by the adults when they left the gall. Looking closely at the pupa case in the gall it can be seen that one end is broken open and a “trail” goes from the broken pupa case to the outside surface of the gall, the exit path of the adult. The open end is also visible in the pupa case removed from the gall. No fecal material is in the gall, typical of E. diana galls.

In one photograph the difference in size can be seen between young (green) galls and the brown “mature” galls.

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