As you walk around Fermilab in the fall keep your eyes open for a round lump on the stems of many of the goldenrod plants that grow throughout the site. This gall is produced by the activity of the Goldenrod Gall Fly, Eurosta solidaginis. This fly's larva (a chunky white maggot) spends the winter well-hidden and surrounded by food within the goldenrod stem.
During the summer a female adult goldenrod gall fly lays an egg on the stem of the goldenrod plant. The egg hatches and the larva immediately bores into the stem and begins to eat. The activities of the growing larva stimulate or "irritate" the plant and it responds by making extra thick layers of plant tissue around the larva. These extra layers form the gall. In the fall, the larva forms an exit tunnel that ends close to the outer surface of the stem, then returns toward the center of the gall for the winter. In the spring it pupates and by late May or early June emerges from the gall, breaking through the thin surface of its exit hole, as a fly with banded wings. Adults usually live about 2 weeks.
A similar gall that is eliptical, not round is produced by the Goldenrod Gall Moth, Gnorimoschema sp.