These familiar bulbs that form on goldenrod stems are caused by a developing fly larva. CWC photo

Have you ever noticed the bulbs that form on the stems of goldenrod? Believe it or not, those bulbs house the larvae of an insect called the goldenrod gall fly. This fly is completely dependent on the goldenrod and cannot complete its lifecycle without the plant.

In the spring, the adult flies lay their eggs in the stem of the goldenrod where the larvae hatch. When the larva starts to feed on the plant, it secretes a chemical in its saliva that causes the goldenrod to grow irregularly, forming the large bulb, called a gall, which will be the larva’s home for the next year. The more the larva eats, the bigger the gall gets. In the fall, the larva will chew a tunnel leading out of the gall, but they don’t leave just yet. They return to the gall to spend the winter, where they begin to produce antifreeze-like glycerol inside their bodies. This chemical, coupled with the gall’s protection, help the larva survive the winter. In the spring, the larva develop into pupa and then into an adult. The adults emerge from the gall using the tunnel they made as a larva, and then start the process all over again.

During the winter, when food becomes scarce, goldenrod gall fly larvae are a food source for birds like woodpeckers and chickadees. There are also two species of wasp that prey on the gall fly larva. The wasps only deposit their eggs inside a goldenrod gall, and the wasp larva feeds exclusively on the gall fly larva.

Speaking of wasps, they can also cause galls. You may have seen a golf ball-sized fruit or seed hanging from an oak tree. These “oak apples” are actually a wasp gall! The oak apple gall wasp has a strange life cycle consisting of alternating generations. Wingless fertile females develop in the roots of the oak, then emerge and crawl up the tree, where they lay eggs in newly-forming leaf buds. As the eggs develop, they cause a gall, which is spongy and fibrous on the inside and papery on the outside. The larvae develop in the galls, with this generation consisting of male and female wasps. After mating, these females burrow into the soil and lay their eggs in the roots of the oak, where the next generation of wingless fertile females will develop. How bizarre!

If you’d like to see some galls, check out one of our preserves and keep your eyes open for strange bumps on plant stems and leaves. Goldenrod galls will be very easy to find at our Loomis Goose Creek Preserve, and our Dobbins Woods Preserve has numerous oak trees where you can look for oak apple galls. Be sure to share your gall photos with us on Facebook, Twitter, and Instagram!

The Chautauqua Watershed Conservancy is a local not-for-profit organization that is dedicated to preserving and enhancing the water quality, scenic beauty and ecological health of the lakes, streams, wetlands and watersheds of the Chautauqua region. For more information, call 664-2166 or visit chautauquawatershed.org or facebook.com/chautauquawatershed.