What’s Really Going On Inside Those Galls?? (It’s Not Just Fly Larvae)

Many of you already know about the big round galls on the stems of goldenrod plants (there are other kinds of galls on goldenrod too). For those of you who aren’t familiar with the story, the galls are created when a goldenrod gall fly (Eurosta solidaginis) lays an egg on the stem, the larva hatches, and it burrows inside the plant. The goldenrod plant tissue grows rapidly around the larva, creating the gall. The fly then feeds on the plant tissue, shielded from danger by the plant, until it eventually pupates and emerges as an adult fly.

Canada goldenrod (Solidago canadensis) with Maximilian sunflower in the background. The Nature Conservancy’s Platte River Prairies.
A big round gall on a Canada goldenrod plant.
A goldenrod gall fly larva inside a gall. Note the size and shape of it.

It’s a nice story, but that version is way oversimplified. As is almost always true in nature, the more you look into a story, the more complex and fascinating it becomes. For example, that fly larva is not necessarily safe inside that gall.

There are two small chalcid wasp species that attack the little gall fly larva. One wasp species (Eurytoma obtusiventris) lays an egg inside the fly larva before it burrows inside the plant, initiating a sequence of events that turns out poorly for the fly. After the fly larva enters the plant and the gall forms, the wasp larva inside the fly larva hatches (cue creepy music). It starts eating the fly larva from the inside, also stimulating it to create a pupa much earlier than it normally would. That pupa provides a nice shelter, within which the wasp larva completes its consumption of the fly and then creates its own pupa before eventually emerging as an adult wasp.

The second wasp, Eurytoma gigantea, lays its egg inside the gall itself. The female wasp has a long ovipositor that it can insert into the gall, though it can only do so on smaller galls because its ovipositor is only so long. When the wasp egg hatches, the fly larva gains an unexpected and very dangerous roommate. The wasp larva tears the fly larva apart with its mouthparts and eats it. Then it starts eating the same plant tissues the fly would have eaten.

The two wasp species have very different approaches to killing the hapless fly larva, but both are very effective. Rarely, individuals of both wasp species end up sharing the same gall. After dispatching the fly, the two wasps can sometimes both survive and exit the gall after they pupate and become adults. Honor among parasitoids, I guess, but I’m not sure how they both come out of that situation alive. According to researchers, it happens.

I’ve known the basics of the gall fly and wasp stories for years now, though I usually don’t remember the specifics of the wasp tactics. As a result, when I spontaneously decided to collect and cut open a few goldenrod galls I found on a recent hike, I wasn’t surprised that two of the galls contained a creature that definitely wasn’t a fly larva.

This definitely isn’t a goldenrod gall fly larva. The shape is all wrong. If you look to the left of the larva, you can see what might be the remains of the fly larva.

However, as I studied and photographed the non-fly larvae, they didn’t look much like larval wasps. They were too skinny and their bodies were the wrong shape. To the internet I went…

That’s when I learned about Mordellistena unicolor, a tiny beetle that also feeds on the tissue inside galls created by goldenrod and the goldenrod gall fly. A female beetle lays an egg on the outside of the gall in mid-summer and it subsequently hatches and burrows inside. It then tunnels around inside the gall, feeding on whatever it finds – mostly the plant tissue of the gall itself.

As I read about the beetle, I learned a new vocabulary word: inquiline. An inquiline lives in the same home as another creature and (generally) doesn’t cause the host any harm. Some of the papers I read categorized M. unicolor as an inquiline. Others called it a predator. In both cases, the authors acknowledged that the beetle larva was likely to eat the larva of the goldenrod fly if it happened upon it. Does that make it a predator or just a really bad houseguest? Either way, the fly larva loses again, unless the beetle never comes across it.

I’m pretty sure the skinny larvae I found were baby beetles. I also found what looked like the desiccated remains of the gall fly in at least one of the galls inhabited by a beetle larva. I can’t confirm that the fly was killed by the beetle, but it was dead nevertheless.

Between the two wasps and a beetle, there’s a lot of traffic inside those goldenrod galls that our original story that sold galls as a safe haven for the larvae of goldenrod gall flies. “If the wasps don’t getcha, the beetle probably will”, would make a good cross stitch wall-hanging for fatalistic flies. And yet, enough fly larvae survive each year to continue the whole process. That’s lucky for the flies, but also for the wasps and beetle, whose entire life strategy hinges on the flies making galls.

Now, you might be wondering, “What happens if a wasp is already there when the beetle arrives?” It’s a good question. As far as I could find, the beetle is just as happy to munch its way through a wasp larva as it is a fly larva. And, of course, in the case of E. obtusiventris, the wasp larva is INSIDE the fly larva, so it’s a kind of turducken situation for the beetle. (I couldn’t come up with a good portmanteau of wasp, fly, and larva that works as well as ‘turducken’ but you know I tried.)

So, if the first wasp finds the fly larva before it gets in the gall, the fly dies. If the second wasp finds a gall that’s small enough, it’ll lay an egg inside the gall and the fly dies. A beetle might enter the picture and, if so, there’s a good chance that the fly will die then too – or whichever wasp had already dealt with the fly. But not always.

That’s definitely a more complex story than “Fly larva burrows into a stem and a gall forms to protect and feed it.” How about just a little more complexity?

Ok. During the winter, there are at least two bird species that see goldenrod galls as a great place to find a snack. Downy woodpeckers and black-capped chickadees are both known to drill holes into galls and extract the larva from within. In fact, the woodpecker can apparently get the larva out within 30 seconds, which is pretty spectacular. Downy woodpeckers tend to make small neat holes (often slightly conical), where chickadees make bigger, messier holes. Now you have something to look for on your next winter prairie hike!

I’m pretty sure this gall was opened up by a downy woodpecker, given the size and shape of the hole.

When I say the birds extract and eat the larvae, they do, but not if it’s the larva of Eurytoma obtusiventris (the wasp that eats the fly larva from the inside out). At least according to one study from 1974-75, if a woodpecker opened a hole to a gall and found that wasp larva, it tended to just leave it there. There was no report on what happened if a woodpecker found the other wasp or the beetle, and no statement on the preferences of chickadees regarding larval species.

What have we learned, then? The goldenrod gall fly lays an egg on the stem of goldenrod and if its larva successfully hatches, it burrows inside the stem, causing the plant to create a big round gall around it. Inside, the fly larva starts to eat, unaware (we assume) that it might soon die a horrible death from one of many sources. It might get eaten by a wasp larva already inside its body, it might get torn apart and consumed by a different larval wasp, it might be fatally burrowed-through by a beetle larva, or it might be excavated and swallowed by a bird. If, by some miracle, none of those things happen, an adult goldenrod gall fly will emerge after the winter ends and try to keep the whole cycle going. I bet those successful flies have no idea how fortunate they are.

If you want to learn more, there are lots of research on goldenrod galls and the insects you can find inside them. Here are two examples. I wasn’t able to link you directly to the free PDF versions, but if you copy and paste the titles of these articles, you should be able to access them.

Mortality factors affecting Eurosta solidaginis (Diptera: Tephritidae). James H. Cane and Frank Kerczewski. Journal of the New York Entomological Society (1976) 84: 275-282.

Variation in selection pressure on the goldenrod gall fly and the competitive interactions of its natural enemies. Warren G. Abrahamson, Joan F. Sattler, Kenneth D. McCrea, and Aruther E. Weis. Oecologia (1989) 79:15-22

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About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

23 thoughts on “What’s Really Going On Inside Those Galls?? (It’s Not Just Fly Larvae)

  1. Chris; as expected, great job. For Chris, ( the occasional human), and Karen J, who recognized him for that title, there was a time ( mid last century), when it wasn’t so “occasional “. At least not in states farther north than Nebraska. Such as Wisconsin. As kids, ( in the late 40’s and fifty’s), by early December, we headed to the good plots of goldenrod. We gathered the small round thingys ( never heard of galls). We ignored the ones with holes. We put the thingys into an onion bag and hung them in the shed…..hoping somebody would say, “ hey, wanna go ice fishing for panfish?”😇. Well we were ready with our bait.
    What I did learn today, was the reason some thingys, had larger grubs. Thanks Chris.
    Harold E

  2. What shape holes do the flies and wasps make when they emerge (if they’re so lucky)? I’ve always assumed the little round holes were the exit door, but it sounds like they’re at least as likely to be woodpecker feeding entraces.

    • It’s a great question. I didn’t find any solid descriptions of that – and I looked because I was wondering the same thing. I think those holes are smaller than those made by woodpeckers and are very clean around the edges – as opposed to more of a conical shaped hole (usually) when made by a woodpecker. The photo I included in my post of what I think was a woodpecker hole was a guess and I based it on the fact that the hole seemed bigger than it would need to be for the tiny fly or wasp and I thought I saw some evidence (?) of it being created from the outside. The insects create their exit tunnel in the fall but leave the very edge sealed up so they can easily break out in the spring.

      • “The insects create their exit tunnel in the fall but leave the very edge sealed up so they can easily break out in the spring.” Ah-ha! So the big round holes probably are woodpeckers. Yep, I always thought they were insect exit holes, too. This explains everthing!

  3. You know we prairie managers use fire-fall, winter, spring and summer. Any thoughts about what seasonal fire does to the gall?

    • Excellent question. I have seen galls lying on the ground after a fire goes through and they look relatively intact. That doesn’t mean the insect is alive inside. And it doesn’t mean other galls didn’t burn. I have no idea what happens to the insects! Lots of guesses, but no good answers.

      • Of course, that’s one of the reasons we don’t burn everything–ensures some will survive to continue the species. Thanks.

  4. So much juicy prairie trivia here to feast on! (pardon the pun) Thanks Chris, this adds greatly to what I know of the story of goldenrod galls!

  5. This is an absolutely wonderful story of interactions, that intricate balance between survival and death in the tiny realms within the prairie. It will be shared with kids of all ages going forward!

  6. A really fun close look into this hapless larva’s tribulations. I’m on its side, it did all the work. I had my daily laugh, turducken. Thank you.

  7. Love the turducken metaphor! An additional subtlety is that only the fly larva has the mouth parts to eat an escape route thru the gall. It does so and turns back round into the gall before it pupates, giving the resultant adult fly, which lacks the boring mouth parts, a path to the outside world.

  8. I don’t know which one of those larva it is but one of them has a small “tail”.
    They are know as “mousies”.
    They’re used as bait for Ice Fishing.

  9. Hello Chris, I am the editor for the The Prairie Garden, a non-profit volunteer organization dedicated to the advancement of horticulture in the Prairies. Members of the Prairie Garden are volunteers, who annually publish a book on gardening for our customers in western Canada and the northern plains of the United States. A copy of this blog post was forwarded to me by Linda Dietrick, a fotmer editor of ours. I am wondering if you would consider writing an article on any horticulcurtural topic that is of interest to you for our upcoming issue. If your are, I would be pleased to hear from you and I will send you the particulars.

  10. Yay! Some answers to a mystery we found on our Colorado Foothills property, but among the willows in the wetlands. We thought it had to be something with bug eggs but couldn’t seem to find the right words to research it correctly. Thanks for the insights, and humor! Now, we’ll do some more digging once they show up again in the summer. Looking forward to cutting a few open now!

  11. Thank you for this educational moment. This is fascinating and I hope to use this info with older students and adults.


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