It was just supposed to be a quick trip across town for a brief photography session before breakfast this morning. The light was nice, the breeze was nearly calm, and it was still far from the near 100 degree temperature forecast for later in the day. I certainly didn’t expect to find something that consumed much of the rest of my day.
On the other hand, what a fun way to have my schedule derailed!
Take a look at the photo below. That surprisingly colorful and beautiful circular pattern on a goldenrod leaf is something many of us see regularly but pay little attention to. For those in the know, it is a gall – a swelling found in plant leaves, stems, or other parts that acts as a hiding place for the larva of an insect. In this case, it’s a gall produced by an ambrosia gall midge (Asteromyia carbonifera). The midge lays an egg on the leaf, the larva hatches and burrows into the leaf and a gall forms around it, giving it a relatively safe and hidden place to feed and mature.
That part of the story was familiar to me, though I definitely didn’t have the name “Asteromyia carbonifera” at the top of my mind. What set me off on my journey-of-the-day and made it difficult to focus on other work I needed to do was what I saw when I looked closely at some other nearby galls through my camera’s macro lens.
What I saw on close inspection of lots of nearby galls appeared to be small flowers emerging from the gall. I’m no expert on galls, and only a semi-pro botanist, but I do know that galls don’t produce flowers. The only explanation I could come up with in the moment was that it might be some kind of fungus, but galls aren’t created by fungus, right? They’re the swelling of plant tissue around the larva of an insect. Or at least they are in the world I’ve been living in.
As it turns out, the world I was living in was very limited. That’s not surprising, given the number of new discoveries (to me) I make each year about prairies.
Once I started reading up on this topic, I had a hard time stopping. I did anyway, because I had multiple meetings and other tasks to attend to. But during every other moment I could scrounge, I kept investigating and learning from those who have studied the galls of midges on goldenrod leaves.
To start with, larvae that develop inside galls are safe from some dangers, but often prone to attacks from parasitoid wasps. I’d read about that in other examples of galls – most prominently those that form on the stems of goldenrod plants (I wonder what makes goldenrod so attractive to gall-making insects??). Parasitoid wasps have long ovipositors (tubes they lay their eggs through) on their rear ends and can insert those through plant tissue to reach their intended targets inside wasps.
Gall midge larvae inside goldenrod leaf galls like those pictured above are attacked by several wasp species. One, studied by Arthur Weis in a fascinating 1982 short article in the journal Ecology, is Torymus capite. As he describes in his note, the wasp inserts its ovipositor into the gall and then probes around for a larva, flexing the long appendage to and fro. When the ovipositor connects with a larva, the wasp lays an egg on it that will later hatch and consume the larva. Very cool.
The same Weis article, however, introduced me to a much more fascinating story that linked to the ‘flowers’ I’d found. It turns out that there is a fungus that is symbiotic with the gall-making midge. That fungus grows in the gall and eventually forms a hardened ‘shell’ called a stroma over the gall that makes it much more difficult for the wasps to reach the midge larvae inside. Scientists think the formation of that stroma may be triggered chemically by the saliva of the feeding larvae.
The wasps can still penetrate the shell with their ovipositors, but because of the thickness of the fungal layers, the wasps can’t probe around – they can only go straight in and out, which makes it nearly impossible for them to find larvae. Often, they try a few times and then give up and move on.
I kept looking and found a second, more recent journal article by Jeremy Heath and John Stireman III that investigated further and clarified a lot more about what is happening. First of all, the fungus (Botryosphaeria dothidae) is introduced by the gall midge during oviposition. Without going into obsessive detail, when the egg is laid on the leaf, spores of the fungus are attached to it. As the egg hatches and begins burrowing into the leaf, the fungus also enters the leaf. In fact, in this case, the gall is not actually made of plant tissue. It’s made of fungus.
The fungus grows within the leaf and creates a chamber, within which one to several midge larvae crawl around and feed. Here’s the next thing. The larvae aren’t feeding on the leaf. They’re feeding on the fungus! The fungus acts as both protector and food for the larvae.
But wait, it gets better. As the gall matures, it develops the thick protective layer, but also fills the chamber with rapidly-growing mycelium that envelope each larvae in individual pupal cells. Basically, the fungus makes cocoon-like structures for the larvae to pupate in!
No one seems to know how the midge picks up the conidia (spores) of the fungus, but it’s assumed that they are somehow actively hunting and collecting those. Then they rely on the fungus to provide food and shelter for their larvae. In return, the fungus gets transportation to appropriate places to grow and then seems to depend upon the larvae for what the authors call ‘hyphal proliferation.’ In short, both species seem to benefit greatly from the relationship.
At our latitude here in Nebraska, gall midges can have multiple generations (maybe three?) per year, which means this process repeats itself a couple times during each growing season. Wasps do still take out some of the midge larvae, but mostly during the early days of the gall formation, before the fungus creates its full protective armor.
I’m an ecologist, which means I study (and revel in) interactions between species. It’s what makes me love prairies and prairie management. I’ve been working in prairies for more than 30 years and hope to be doing it at least 30 more. By the time I’m done, I’m supremely confident that I’ll still be learning about the complex interplay between grassland species. Some interactions I come upon are merely interesting. Others, like this one, blow me away. What a fantastic world we live in!
(If you’re interested in another equally-fascinating story – this one involving beetles and bees – here’s a post from four years ago that won’t disappoint.)