Adult Goldenrod Gall Flies do not feed and live only long enough to mate, after which the female uses her ovipositor to lay her egg in the stem of the goldenrod. When the egg hatches, the larva begins to eat and grow within. The saliva of the larva stimulates plant tissues to multiply abnormally, forming a grocery store and a protective home for the young insect.
At the end of the fall when the goldenrod begins to turn brown the nearly grown larva will chew its way to the outer edge of the gall, but not emerge. It will spend the winter dormant inside this ball after producing a natural “antifreeze” in its cells. In early spring of the following year, it will pupate and emerge as an adult, breaking through the last layer of plant tissue. The adults will search each other out and mate without a meal, and the cycle will start again.
Despite the appearance of a safe place to grow up, there are predators that will feed on the fly larva. Woodpeckers and chickadees often break into the gall, and there are a variety of insect species that will also attack the fly larva. One species of wasp lays its egg in the gall, where the larval wasp will have its own food supply while it grows in the protective ball.
Bass falls and Trout Brook preserves have abundant goldenrod communities along trial edges and are good places to search out this insect gall. Once the stems become brown you can tell which are last years and which might have a current resident by looking for the existence of an exit hole.
Oak Apple Galls, Pineapple Galls (on Willows), and Blueberry Ball Galls are other insect galls you might look for as you are exploring SVCA’s preserves this fall.