Goldenrod Gall Fly: A Solitary Life Inside

This week I have been focusing on Goldenrod Ball Galls, the home of the insect known as the Goldenrod Gall Fly.

When I walk among the goldenrods in Eliza Howell Park in December, it is not to admire the flowers or to observe pollinators and preying insects, as it was in September. Now it is primarily to look for signs of insects overwintering in the egg or larval stage.

The absence of leaves makes it easier to locate the galls on the stems of some goldenrod species (Canada Goldenrod is one species they favor). They are usually found halfway or more up the stalk.

Goldenrod Gall Flies have a very interesting/unusual life cycle. They live about a year (if they are not eaten by predators) and almost all of that year is spent inside the gall, in the larval and pupal stages. Winter months are spent in diapause, dormant time.

In Spring, the female fly lays eggs on young goldenrod stems. After eggs hatch in about 10 days, the larva burrows down into the plant stem and begins eating. This stimulates the plant to grow around it, providing the ball-shaped shelter with plant fibers inside for food. The fly larva does not harm the overall health of the plant

Each gall has only one developing fly inside. It stays here, alone, for about 11 months, before finally emerging as an adult in the spring, just in time to mate and for the female to lay eggs before they die.

The adult is a little smaller than a house fly. It has no mouth parts and is incapable of eating. It’s adult role is limited to reproduction and usually lasts less than 2 weeks.

Since the adult cannot chew its way out of the gall, in the fall, before going into diapause, the larva excavates a tunnel to the outside, leaving only the surface layer. This tunnel is for the adult to use when it emerges from the pupal stage in the spring.

Some of the galls that I have located recently have had a hole drilled into them.

The fly larva is nutritious food for birds that can get to it, like woodpeckers and chickadees. I suspect that these holes were made by a Downy Woodpecker, which often forages quite close to the ground.

Photo by Margaret Weber

I could take a look at the inside of a gall, without interfering with the life processes inside, by cuting into a bird-opened one.

The center is a cozy-looking “nest” for the solitary resident.

Cutting open one half through the predator’s opening shows the direct line to the nesting center. It looks like the bird found and used the escape tunnel the larva had made.

The Goldenrod Gall Fly has been studied and written about quite extensively (perhaps because it makes for a good study project in the winter?), so I was able to expand upon my field observations easily by doing a little online research.

A lifetime that is 95% prior to adulthood is a great contrast to what I once considered “normal.”

An adult animal unable to sustain its life is also a great contrast to what I once considered “normal. “

One of the reasons I am fascinated by this insect, which I do not see, is that it leads me to think again, perhaps more deeply, about the diversity of animal life.

Winter is a time for seeing some of what we miss on nature walks in the busier seasons of the year. It is also, for me, a time of reflection on the (sometimes unexpected) wonders of nature.

2 responses to “Goldenrod Gall Fly: A Solitary Life Inside”

  1. Redefining “normal”—–interesting.


    1. Yes, expanding my understanding of the range of animal life cycles.


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