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Goldenrod Gall Fly

You've seen 'em, if you've spent anytime at all walking through old fields. I'm talking about those round growths that form on the stems of goldenrod; kind of looks like the plant tried to swallow an apple and it lodged halfway down the stem. This growth, called a gall, is formed by a highly specialized insect called the Goldenrod Gall Fly, Eurosta solidaginis. Solidago is the genus of goldenrods, as recognized in the specific epithet of the fly's scientific name.

But it's just not any of the 25 species of goldenrods recorded from Ohio that will do; the fly is quite specific in its demands. The host plant must be Tall Goldenrod, Solidago altissima, a very abundant and widespread plant, and along with Canada Goldenrod, S. canadensis, the goldenrod of old fields. These two species of goldenrods have variously been lumped and split, so similar are they, and I've always leaned towards lumping them. But, recently recognized evidence suggests that they truly are good species, and this obscure fly factors into this. The fly only chooses Tall Goldenrod, apparently, shunning the very similar Canada Goldenrod even when both species occur together. This suggests a chemical difference between the two goldenrods that is significant.

Good ole Eurosta solidaginis, female. A tiny critter and nowhere near as obvious in the fields as are the galls that they create.

Here's the male. Rather studly, these boys. They lurk on goldenrod buds, watching for prospective mates. When a lovely lady is spied, the male launches into a performance designed to win her over, by rocking rapidly to and fro. If his magic casts a spell over her, mating commences. This coupling can last from 15 minutes to an hour+, with an average time of 40 minutes. People actually are out there that watch this sort of thing, you know. In the case of this fly, I will bet that it was some long-suffering grad student who was made to observe the amorous antics of these flies so that we could all learn about the copulatory habits of Eurosta solidaginis. Whatever the case, these insectivorous studs have tremendous staying power.

OK, after the baby-making is over, the female injects an egg into the stem of a suitable Tall Goldenrod. Interestingly, there are different groups of this fly known as "host races", and another uses Giant Goldenrod, Solidago gigantea, as its host. This goldenrod is not weedy and far more habitat-specific than Tall Goldenrod.

On into the wacky world of bugs we go. The female is quite adept at detecting appropriate host plants, using chemical sensors located in its antennae and feet. If things look OK, in goes the egg. In a fascinating experiment, researchers attempted to trick a fly into ovipositing in a non-host plant. They did this by enwrapping the imposter plant with a leaf from Tall Goldenrod. Sure enough, the fly starting to drill in, but when its ovipositor connected with the alien plant, it quickly pulled out and split the scene, showing that sensors are also present in the ovipositor.

Once the egg is in the plant, this foreign body stimulates the goldenrod to produce specialized gall tissue, probably directed by secretions from the larva once it hatches. If you've ever felt one of these galls, they are hard as rock and good protection. The photo above shows a gall that has been split open with a knife; you can see the larva in the small chamber that it excavates by eating away some plant tissue. Adult Goldenrod Gall Flies don't eat, only the larvae do.

As the worm matures and the gall becomes fully formed, it drills a small escape hole to the outside. Look closely at one of these galls in late fall/winter and you'll see it. Come spring, the larva uses this to escape to the outside world, mature, and start the process all over.

So, what good is this bug? Well, I'm sure they have functions that we don't even know about, but if you are a Downy Woodpecker they're mighty tasty. Downys know what they are and often work the galls in old fields, drilling in and pulling out the larva. If you see our smallest woodpecker out in goldenrod fields, that's probably what they are up to. Look closely at enough of these galls in the winter, and you'll sooner or later see the larger ragged holes that the Downy Woodpeckers excavate in their pursuit of these morsels.


Heather said…
Absolutely fascinating, Jim! I have seen these before, and I'm so excited to now know what the cause is. Thanks for the detailed info about how the egg gets laid - isn't it amazing how the experiment failed - that fly was not going to be duped! One thing you don't mention is the life span of the adult fly. I can only assume it is only long enough to reproduce, since you mention the adults don't eat. Thanks for the science fix!
MJ said…
You say "Come spring, the larva uses this to escape to the outside world, mature, and start the process all over." I think they must emerge as adults. After mine emerged I cut open a gall and the spent puparium was inside. What do you think? Thanks.

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