Have you ever noticed a swelling on the stem of a goldenrod and wondered what it was? This swelling is called a gall and is a deformation of the plant’s growth caused by an insect. A specific gall insect creates each type of gall on each plant species. “Gall insect” refers only to the fact that the insect has the ability to create these deformities. It does not belong to an all-encompassing family of gall insects; in fact, gall insects can be species of flies (including midges), wasps, aphids, or moths.
Galls take many forms and can be found on myriad plant species, such as grape, raspberry, cherry, oak, willow, witch-hazel, and goldenrod. Some galls form on plant stems, while others appear on leaves. Buds and twigs can bear yet other types of galls. If the gall is round and grows on a goldenrod stem, it is caused by a goldenrod gall fly (Eurosta solidaginis).
The goldenrod gall fly is tiny, only about a quarter inch long. During the adult stage of its life, which is only about 2 weeks long, the fly does not eat. This time is spent mating and laying eggs. The female fly will lay her eggs in the stems of emerging goldenrods in the spring. A hatching larva (white and grub-like) will burrow further into the plant stem to create itself a safe home. Chemicals in the larva’s saliva that mimic plant hormones cause the stem to form a ball-shaped growth, or gall. While the outside surface of the gall is hard and protective, the inside is softer and very nutritious. There the larva lives, feeding on the plant’s inner tissues while the gall continues to grow to about the size of a golf ball. It is humbling to think that the chemical secretions of a tiny insect can cause a plant to deform in such a way as to create the perfect house and pantry for the insect’s needs.
The goldenrod gall fly larva will live within the gall from one spring to the next. Fall temperatures will cause the fly larva to accumulate glycerol and sorbitol in its body fluids, allowing it to survive freezing and thawing many times over the course of the winter. In the spring it will chew a tunnel through the gall to the outside. But it still needs to pupate, to transform from this larval stage into an adult fly. So it crawls back into the center of the gall, not emerging from its escape tunnel until the transformation is complete.
Downy Woodpeckers and chickadees have discovered the nutritious meal that lies hidden within the goldenrod gall. You may find one of these industrious birds perched upon a withered goldenrod stem this winter as it pecks a hole in a gall to reach the tasty larva within.