By Susan Thurn,
Cable Natural History Museum
As I was walking down the road, the goldenrod stalks were waving in the wind. In late summer, the yellow colors of the goldenrod lights up the fields and roadsides. Now, its dead stems sometimes show life, with a round ball evident on some of the stems. This round ball is an insect gall, the home of a small fly spending the winter protected inside this plant. My body shivers in the cold, just thinking of what it might be like to be an insect inside that small ball, tucked inside a plant. What an interesting place to spend the winter!
A goldenrod gall is formed by a small fly, a fruit fly. The insect’s relationship with goldenrod began last spring when a female fly laid one egg in May to early June on the leaves of a goldenrod stem. After four days, the egg hatched and immediately the young larva chewed into the stem. The larva injected chemicals into the plants, causing cell division to increase and form the round gall. Its hard corky exterior protected the inner nutritious starches that provided food for the larva. As the gall grew, the fly larva created a central chamber that it lived in until the next spring. During the summer, the white larva fed on the inside of the gall, growing until early October. Since the adult flies have no mouthparts with which to chew, the larva then chewed a tunnel out to the surface of the gall, leaving a thin protective exterior over the outer opening.
During the winter, the mature larva went into a diapause, or dormancy. As winter approached, the water content in their bodies was converted from glycogen into glycerol, acting as antifreeze to keep ice crystals from forming in their bodies. Additionally, they had the plant gall to provide physical protection from predators.
In the spring, the fly larva will leave its dormant state and pupate inside the gall. Once it emerges as an adult fruit fly, it will crawl along the previously created tunnel, inflate a balloon-like body part between its eyes to create an opening in the thin covering, and escape its winter home. As an adult, the fly is smaller than a house fly, and lives about ten days. The cycle then begins again.
The goldenrod gall is not harmed by the insect’s presence. It seems like an ideal, and amazing, way for a small insect to survive and adapt to the harsher elements of our winter season. However, the small fly is still susceptible to dangers. Downy woodpeckers and chickadees are known to feed upon the insect larvae. Two species of parasitic wasps deposit their eggs through the growing gall tissue into the fly larvae, so that a small black wasp may emerge in spring instead of a fruit fly.
Wander the snowy fields in search of some goldenrod ball galls. Take a closer look at one of nature’s amazing delights. Enjoy nature in our own back yards.
For over 42 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility and exhibit, On Lake Owen: The Art of Walter Bohl, in Cable at 13470 County Highway M. Also find us on the web at www.cablemuseum.org to learn more about our exhibits and programs.