The goldenrods are blooming, and I’m taking note of the big stands. They are spectacular now. But I want to check on them again this winter, when the dead, dry stems are harder to spot.
I want to learn more about the galls that grow on goldenrod stems.
You can see the galls now — most are the size of a taw or a golf ball. The gall is caused by the goldenrod gall fly, Eurosta solidagninis. The adult fly, smaller than a housefly and sporting brown and khaki camo, doesn’t have a functional mouth, so it doesn’t live long. The flies mate and lay eggs on goldenrods in spring.
Georgia has about 35 species of goldenrod, genus Solidago. The fly prefers some, meaning it can discriminate species better than I can. It’s a humbling thought if you were raised, as I was, on the notion that humans are exceptional.
The tiny larvae hatch quickly and dig into the stem of the plant, eating as they go. The larvae release a chemical that the plant takes to be a growth hormone. The goldenrod produces a gall of soft, edible tissue around the larvae. Through the winter, the larvae eat and grow and change forms as insects do. Since the adult fly can’t chew its way out of the gall, the grub eats a passage almost to the surface of the gall, leaving a membrane-thin hatch.
The gall seems like a safe place for a grub. But it’s not.
Two kinds of wasp prey on fly larvae.
Eurytoma obstusiventris catches the grub before it can burrow into the goldenrod and lays an egg inside it. The wasp egg hatches inside the fly larva and eats its way out. If you’ve seen the movie Alien, you’ve got the gist of it.
The other wasp, Eurytoma gigantea, has a long egg-laying tube called an oviposter and deposits her egg directly into the gall. The wasp larva grows up in the gall, eating plant tissue, until it blunders into the fly larva and eats it too — a bit of extra protein.
A beetle, Mordellestina unicolor, lays eggs on goldenrod. The larvae hatch and eat their way into the galls. Opinion seems to be divided on whether the beetles eat only plant tissue or eat the fly and wasp larvae too.
One other thing — the detail that interests me: Two birds that are common at our feeders have discovered that goldenrod galls are good sources of protein-rich food in winter.
Downey woodpeckers and Black-capped chickadees loot the galls.
If you see a gall that has a hole in it that looks as if it were made by a neat shop teacher with a drill, you’re looking at the work of a woodpecker.
If you see a gall that looks as if it’s been mauled by a shop student with a pickax and crowbar, you’re looking at the work of a chickadee.
I’ll check in on the goldenrods this winter. I want to see which is primary pillager in our neighborhood: the woodpecker or the chickadee.
• Source: I was put on to this story by Chris Helzer’s post “What’s Really Going On Inside Those Galls?? (It’s Not Just Fly Larvae” at The Prairie Ecologist, March 8, 2023. It’s here:
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