The Goldenrod Gall Fly: An Insect with a Fascinating Life History and Valuable Role in the Ecosystem

Overview

Sometimes when we are walking through our yards or communities, we don’t see the critters themselves as much as we see the signs they leave behind. Such is the case with the goldenrod gall fly (Eurosta solidaginis). Goldenrod gall flies can be found throughout most of North America. There are two subspecies – one in the eastern half of the U.S. and southern Canada, and one that is typically found further west.

What do Goldenrod Gall Flies Look Like?

Adults:

The adults are a little smaller than a housefly with a brown body, brown eyes, and wings that are mottled with clear patches and brown patches that are a little darker than the body. The adults never really leave the goldenrods and are active at a time when no one is paying the goldenrods much attention, so most people never see the adults.

Galls and larva:

The galls produced by the goldenrod gall fly are mostly round, ping-pong to golf ball-sized, swollen places found on goldenrod stems. They are usually on the upper half to a third of the stem. These galls are often most visible in the winter after all of the surrounding vegetation has fallen off the goldenrod stem. (The galls that form the leafy rosettes on goldenrods and sometimes catch our eyes in the middle of summer because they “look weird” are from a different type of insect.)

The larva are white to creamy colored, fat, and sometimes described as barrel-shaped or oval. You’ll only see it if you open up a goldenrod gall. However, not everything you find in a goldenrod gall is a goldenrod gall fly larva. Sometimes the goldenrod gall fly larva will be preyed upon and replaced by either a beetle larva or a parasitoid wasp larva.

Life History of the Goldenrod Gall Fly

Goldenrod gall flies emerge in the mid- to late-spring. The males emerge first, then the females. They typically don’t move far from the patch of goldenrods where they emerged. The males will stand on a goldenrod and “dance” to attract a female. If the female finds the male acceptable, then they will mate and she will fly off to lay her eggs. Adult goldenrod gall flies typically only live a couple of weeks.

A female goldenrod gall fly may lay as many as 100 eggs over the short course of her adult life. She lays each egg just barely inside a goldenrod stem, near a growing terminal bud. When the egg hatches, the larva burrows into and down the stem until it finds the “right place” to stop. I haven’t found anything that tells us why the larva stops where it does, but it tends to be in the top half to a third of the plant.

Chemicals in the larva’s saliva causes the plant to begin forming the growth that we know as the gall. The plant tissues in the gall are high in starches and other nutrients that the larva feeds on as it develops. As the larva feeds on the tissues, the plant sends more nutrients and starches to the area to replace the ones that the larva ate. That goldenrod stem may not grow quite as tall as nearby stems that don’t have galls, and that stem’s seed production may be slightly lower that year, but the gall and its host don’t appear to have any long-term effects on the goldenrod.

The larva will continue to feed and grow until late fall. In mid- to late-fall, the larva will chew a tunnel through most of the gall towards the outside edge, but it will leave a thin layer of tissue between the tunnel and the outside world. The larva will then move back into the main part of the gall and go into essentially an insect form of hibernation. A type of natural antifreeze produced by its body will keep it from freezing during the winter.

In the spring, the larva will pupate and metamorphosize into the adult goldenrod gall fly. Adult goldenrod gall flies don’t have mouthparts which is why the larva has to chew the exit tunnel in the fall. The adult goldenrod gall fly crawls down the exit tunnel and then use fluids to inflate a balloon-type structure on its forehead to push open the last remaining section of the tunnel. Once the tunnel is completely open, the goldenrod gall fly can crawl on top of the gall and begin pumping fluids into its wings before the mating process starts and the cycle begins again.

Are All Goldenrods the Same?

There are over 100 different species of goldenrods in North America. Kentucky, alone, has a little over 30 species. But the goldenrod gall fly only uses a handful of those species. Exactly how many of those species it uses appears to still be a matter of debate depending on which resource you look at, but the two primary species of goldenrod used by the goldenrod gall fly are tall goldenrod (Solidago altissima) and giant goldenrod (S. gigantea).

Apparently, not all goldenrods respond to the chemicals in the goldenrod gall fly larva’s saliva in the same way. Only a few species will start producing the extra tissue growth and continue to produce that extra tissue growth throughout the growing season. The larva relies on the tissue growth for food, so if the plant won’t produce the tissue growth or stops producing the tissue growth, then the larva won’t survive.

Female goldenrod gall fly larva can actually tell whether or not a given goldenrod plant is one of the species that will produce the galls or not. They may land on other species, but they’ll quickly fly away without laying an egg. Researchers in one study even wrapped tall goldenrod leaves around the stems of other goldenrod species to try and trick the goldenrod gall fly larva. The females landed on the wrapped stems and started to oviposit, or lay an egg, but quickly pulled out and flew off without actually laying the egg. As soon as the ovipositor entered the stem of the goldenrod, they could tell that it wasn’t the right type despite being wrapped in the tall goldenrod leaves.

Goldenrod gall flies are so picky about what species of goldenrods they use that in the eastern U.S., there appears to be two distinct races developing. One race prefers tall goldenrod, while the other one prefers giant goldenrod. The two races rarely interbreed. Scientists think this may be the early stages of the development for a third subspecies.

I find all of this extremely interesting on so many levels. One of the reasons is that most resources indicated that Canada goldenrod (S. canadensis) is not one of the species used by goldenrod gall flies. However, Canada goldenrod is one of the species that I generically refer to as “tall field goldenrods.” It spreads by vegetative runners and can be very aggressive. It is also the one that is commonly assumed to be the predominant field goldenrod in most areas. Yet, I have lots of goldenrod galls in my fields. So, if goldenrod gall flies don’t use Canada goldenrod, then I don’t have as much Canada goldenrod in my fields as I thought I did. I may have to dig into the keys next year to see which goldenrod species I actually have. (Identifying field goldenrods to species isn’t easy, which is why they are so often lumped together.)

The Goldenrod Gall Fly’s Role in the Ecosystem

The goldenrod gall fly is an important food source for several other insects and multiple species of birds. There are two species of parasitoid wasps that specialize on goldenrod gall flies. One species of parasitoid wasp will lay its eggs in the goldenrod gall fly eggs. The wasp larva won’t hatch until the goldenrod gall fly larva has grown large enough to provide a good meal. The other species of parasitoid wasp is active later in the growing season. The females of that species will stick her ovipositor through the gall to lay an egg. Once the egg hatches, the wasp larva will eat the fly larva and then continue feeding on the tissues within the gall.

A beetle species will also lay its eggs on the goldenrod gall. After hatching, the beetle larva will chew into the gall and begin feeding on the tissues inside the gall. It will often kill the fly larva although it’s unclear whether it actually eats the fly larva. Both species of parasitoid wasps and the beetle species will overwinter in the gall as either a pupa or a larva, depending on the species. That’s why you can never be sure what you will find if you open up a goldenrod gall.

Goldenrod gall fly larva are also valuable winter food sources for downy woodpeckers, chickadees, and several other species of birds. When they find a gall, they will peck it open and eat the larva. Sometimes the larva will be that of either a parasitoid wasp or the beetle, instead of the gall fly larva, but either way it is still a nutritious meal for the bird.

Summary

The goldenrod gall fly is an often-over-looked insect. Most of us never even see the adults. All we ever notice is the galls, and even then, we have to cut the gall open to see the larva. Yet, the goldenrod gall fly has a fascinating life history and plays a valuable role in the ecosystem.