The secret of the Goldenrod Gall Fly

When we think of Goldenrod, at least two things might come to mind—its colorful bloom and those enormous round balls that we often see on the stem of these plants in fall. They are so prominent, these balls, and have such interesting features that they have been studied widely. On our walk through the Biocore Prairie last Saturday with Seth McGee as our leader of the Biocore Program, we observed one in its beginning stages and had a 101 lecture on “what happens here.”
 
We noticed a gall in the making on a rosinweed, which also attracts the goldenrod gall fly (Eurosta solidaginis) or ball maker. Earlier in spring a goldenrod gall fly inserted a fertilized egg into the bud of the plant.  After the larva emerged, in a week or so, it ate its way to the base of the bud and into the spongy middle of the stem and induced a gall. The hormones in the saliva of the larva interacted chemically with the plant tissue, resulting in the growth. Seth  cut the growing gall open to show us the little larva inside. The larva had grown, feeding off the plant. The plant tissue around also expanded in response to the chemicals exuded by the larva. Before the gall ball would harden later in the year, the larva would dig an escape tunnel with its pair of mouth hooks all the way to the plant’s epidermal layer that surrounds the gall. It would survive the winter without freezing to death by emitting further chemicals that act as anti-freeze. In spring after pupation,  the new fly would turn its head in direction of the dermis, pump all its blood into its head to strengthen it, and ram with all its might through the dermis into the open, free to fly. Next time you see a gall on a goldenrod with a hole in it, you will know how the fly escaped. Photos and text, Gisela Kutzbach