Goldenrod Ball Galls by Dave Erler

Have you ever noticed round ball-like growths on dried brown plant stems sticking through the snow pack in fields and meadows? What you were seeing was more than likely goldenrod ball galls. Goldenrod ball galls are a fascinating example of a plant and insect adaptation. The whole process begins in early spring as goldenrod plants begin a new year of growth. A small male fly (5mm) called a goldenrod gall fly (Eurosa solidaginis) walks up and down the goldenrod stem until he chooses a bud. There, he patiently waits until a female approaches. On the approach of the female he does a little dance, the couple mate, and the female walks away to find a spot to lay eggs. The eggs are actually injected into the goldenrod plant stem via the female’s ovipositor. Since adult gall flies are unable to eat, they perish within a few days of accomplishing their reproductive mission.

The eggs hatch in about 10 days and the fly larvae immediately begin to eat from within the rapidly growing goldenrod stem. As they eat, a chemical in their saliva causes the plant to grow abnormally, resulting in the ball-shaped gall. The larvae get everything they need: food, water, and shelter; thanks to the growing goldenrod plant the gall continues to grow larger, sometimes reaching the size of a golf ball. Late in the season, the larvae eat a tunnel that will serve as an escape hatch when it comes time to leave the gall. With winter approaching, the larvae retreat to the center of the gall and produce a chemical that keeps them from freezing and drying in the cold. In spring, the larvae pupate prior to metamorphosing into adults. The adults escape the gall by breaking through the thin layer of plant cells covering the exit tunnel previously created by the larvae. Adult goldenrod gall flies are weak fliers so they generally do not wander far to a new goldenrod plant to begin the cycle again.

Gall formation does not normally affect the overall health of the goldenrod plant. The goldenrod gall fly, however, depends entirely on just a couple of species of goldenrod. Although the goldenrod gall fly larvae are protected by the gall from most potential predators, they are still vulnerable. At least one species of beetle and two species of wasps seek out goldenrod galls and in turn eat or parasitize the fly larvae. Downy Woodpeckers and Black-capped Chickadees sometimes open wintering goldenrod ball galls to extract the larvae. Even humans get into the act. My own introduction to goldenrod ball galls came as a seven or eight year old when I learned that the larvae could be used to catch other food. Growing up in Minnesota, where digging worms in the winter was downright impossible, I was shown that the larvae could be put on a small hook and used as bait for ice fishing. Even an unopened ball gall could be rigged as an impromptu fishing bobber. To this day, whenever I see a goldenrod ball gall, it brings back memories of frigid mornings looking for round brown balls sticking up through the snow.