Out in the Woods

Gall Fly galls on Goldenrod stems

Photo: thekeepershouse

Goldenrod Gall Fly

By Kevin McKeon, Maine Master Naturalist

With summer’s greenery mostly gone, winter walks can reveal hidden life. Quiet your gait a bit and look in meadows, forest openings, and along trail sides and road ditches for last summer’s goldenrod stems; the easily noticed bulges on the stems are created by the larvae of the Goldenrod Gall Fly, Eurosta solidagnis, and the Goldenrod Gall Moth, Gnorimoschema gallaesolidaginis.

The male fly emerged in the spring, patiently awaiting the female’s emergence so he can perform his little dances to attract her. If she likes what she sees, mating ensues, after which she’ll take very short flights to eventually lay up to 100 eggs on young goldenrods — first “tasting” the plant’s buds with chemical sensors on her feet and antennae to ensure a good site for her eggs to mature.

After about 10 days, a larva emerges from the egg and burrows into the plant. Chemicals in its saliva cause the plant to begin forming the growth that we know as the gall. As the larva feeds on the tissues, the plant sends more nutrients and starches to the area to both replace the eaten ones, and to produce a protective scar tissue at the wound. So, the goldenrod is constantly healing itself from the larva’s eating, effectively enlarging the larva’s protective round gall. The larva will molt twice during its growth stage, preparing for its winter hibernation; with insects, this dormant state is called diapause.

Gall Moth gall on goldenrod stem. Notice the tiny exit hole where the moth emerged.

Photo: K. McKeon

At full growth, the larva molts a final, third time and enters diapause. It begins the production of sorbitol and glycerol, the insect’s form of anti-freeze. This process protects it from cell-damaging freezing. (Several other creatures of our forest, fields, and waters have this ability, creating head-scratching among research scientists trying to duplicate this process for human uses — like keeping donated organs viable for longer time periods to enhance the transplantation segment of our health care system.) In early spring, the larva pupates, undergoing complete metamorphosis into an adult fly, emerging from its gall to inflate its wings and start the whole cycle again.

A smaller, oblong-shaped gall on goldenrods is created by the moth, using a slightly different tactic. The egg is laid on a dying leaf in the fall where it overwinters in the soil litter. After hatching in spring, the larva seeks a goldenrod sprout where it eats through to the center of the stem; The goldenrod responds by creating the gall. After feeding inside the gall all summer, the larva begins to chew an exit hole, leaving a protective thin outer skin covering. It then creates a silk lining and pupates. After four to six weeks, the adult moth emerges, pushes through the exit hole, finds a mate, and life continues.

These galls don’t harm the plant significantly and the adult flies don’t bother people; the flies have no mouthparts to bite or even eat! In fact, these galls are quite beneficial to the ecosystem. The larvae inside serve as a food source for a variety of wildlife, particularly birds, which can feed on both the larvae and the adult insects. This creates a valuable addition to the landscape’s food chain, helping to sustain birds and other creatures in the area.

As your winter walks slow and you espy a goldenrod bulge, a closer look may reveal a hole where it’s been pecked open by a visiting chickadee or downy woodpecker, or even the smaller exit hole from the insect’s emergence. Galls tend to occur in patches, as the insects’ flights are short from where they emerged. So, next time you spot a goldenrod gall, remember this weird growth is actually a little home.

Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our Out in the Woods columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net