Edit: This post was recently included in the 182nd edition of the Friday Ark, a blog carnival focusing on animals of all sorts.
I’m sure we’ve all seen these in our local wetlands, cattail heads that have become all poofed out as winter progresses, like so much cotton-candy on sticks. I’d never given it much thought before, and if I had I suppose I’ve just assumed that the fluff has something to do with the cattail releasing its seeds to the wind, much like milkweed does.
A few weeks ago, my mom bought the book Insects: Their Natural History and Diversity (with a photographic guide to insects of eastern North America) by Stephen A. Marshall. Steve Marshall is a prof at the University of Guelph, where I did my undergrad, and was one of the instructors leading the Ecuador field course that I took. I didn’t spend much time with him (I knew the other prof better), but he was a really nice guy. So I thought it was cool that Mom had got this book, and I sat down to leaf through it. I was primarily interested in the 40-someodd pages of moth plates, and while flipping through those I spotted an image of fluffy cattail heads. Intrigued, I read the plate caption associated with the figure.
Turns out, those fluffy cattail heads you see in the middle of winter aren’t just the cattail doing its thing. Sure, it will naturally begin to loosen the fluff and lose some to the wind. But the cotton-candy formations? They’re the work of a tiny, drab little moth called the Shy Cosmet (Limnaecia phragmitella). Its caterpillars are appropriately known as Cattail Caterpillars. They feed on the seeds of the cattail in the fall and spring, overwintering inside the head as a larva. In late spring they pupate and emerge as an adult moth in early summer.
In order to ensure that they have a secure home for the entire duration of their stay, they spin tiny silken threads that act as a web holding all the fluff inside. The cattail will still loosen its seeds from the stalk to try to spread them on the wind, but the caterpillar’s netting holds them in place. The result is a rather lumpy cotton-candy appearance.
I thought this was all pretty cool, so I did a bit of searching on the web and discovered a post about the Shy Cosmet, made recently, by Gerry Wykes at Naturespeak (he calls Detroit home, so is within my rather broad neck of the woods). In it he brought in a cattail head and gently dissected it to expose the caterpillars hiding within. Now, I suppose I could have just taken him at his word, but I really wanted to go out and have a look for myself. Insatiable curiosity. In the name of science, of course. I brought in two heads for good measure, and opened them up in a tub to contain any wandering worms.
Gerry indicated that it would take a moment or two for the caterpillars to poke their heads out of the fluff, that they wouldn’t be immediately visible. So I sat there and patiently waited. And waited. Nothing happened. No heads, no tails even, not a hint of movement. I’ll admit that they should all be dormant at this time of year, but since he’d had such luck with his coming out as soon as they were warm, I was beginning to wonder if I’d picked a dud cattail head, one that really was simply just loosening up its fluff.
So I started gently teasing apart the clumps. I found lots of what looked to be frass in with all the seed heads. The seeds are tiny, flat and boxy at the end, and brownish in colour. The frass, on the other hand, was gray and round, spherical. There was lots of it, but no caterpillars associated with any of it.
Finally, after standing with my head bent over this dish for many minutes, I discovered one. Only one, just a single, lone, under-developed caterpillar. Perhaps the cattail head had already been picked over by foraging birds and this was the only guy to have survived. Perhaps there never were many to begin with, maybe I missed one or two. But I went through the entire two cattail heads and only found one. By the time I was done, they looked like this:
Here’s the caterpillar. I found him tucked in a clump of relatively undisturbed fluff, nearly comatose. He wiggled a bit when I first picked him out, but didn’t go anywhere. Gerry was describing his caterpillars crawling all over the place, making getting a good photo difficult. Mine was very photogenic.
This is the caterpillar posed with my mom’s finger for scale. He was tiny. Tiny tiny. This is why I wouldn’t be surprised if I missed a couple others, although I was paying close attention. Below is the caterpillar beside a measuring tape. You’re looking at the inches side of the tape. Each one of those black dashes is 1/16″. See? Tiny tiny, it’s not that my mom has big fingers…
I actually found the dried husk of an older caterpillar in the fluff as I was starting to clean up. This gives you an idea of what it should grow to before pupating. He’s got a ways to go.
These little caterpillars form one of the primary food sources for Red-winged Blackbirds upon their arrival here in the spring. It seems like they’d need to eat a lot of them to get much nutrition, but evidently it works for them. A couple springs ago I got a photo of a female Red-wing poking at cattail heads. I thought at the time she was looking for nesting material, but having learned this, it seems apparent that she was actually searching for caterpillars. Who knew? Cotton candy indeed.
As for the book, I highly recommend it. It’s got great photos for an identification reference, and excellent information to complement them. The notable entomologist E.O. Wilson is quoted on the cover: “I wish I’d had Stephen Marshall’s book when I started out in entomology. Its countless photographs and notes bring alive the vast diversity of the insect world.” That’s like Roger Tory Peterson endorsing a bird reference book. There’s so much cool stuff in the book, lots to look at. The only downside: it weighs a tonne. Probably almost literally. At 730 pages and nearly two inches thick, a field guide it’s not. Still, it has the best collection of printed moth photos I’ve seen, plus so much other stuff, I ended up getting a copy myself (should be in the mail). It retails in store for $95, though generally cheaper online, but I found a copy on eBay for $33 plus shipping, so if you look around you should be able to get a good deal on it. Amazon has it on sale for $60. It’s also got a couple images of inside the book so you can take a peek before you buy.
15 thoughts on “Cotton candy for blackbirds”
Great, fascinating post! Kinder Surprise seed pods! I’ve never heard of these.
If I get a chance I’ll check out the nearest marsh to look for lumpy seed heads, snag a few and bring them home. My very quick web-search suggests that this is a European species, also widespread in eastern N Am. I wonder if it or something close occurs or has been introduced out here (south coastal BC).
And great pics, too.
I learn so much from you and am heading to the nearest marsh at the first opportunity. Beautiful pictures to go along with the fascinating information.
Wow! Next time, I’m bringing my cattail fluff home, rather than looking it over and leaving it behind.
I wonder what I’ll find on this side of the continent. The Shy Cosmet doesn’t occur here, but we have Red-wing blackbirds in all the marshes, on the cattails. So we may have our own local variety of cattail fluff bugs.
Hugh and Susannah, from what I understand based on my reading, there are actually a few species of critter that have close associations with cattail, so one of these guys might be out your way. Another moth is Henry’s Marsh Moth (Simyra henrici/Simyra insularis), which ranges coast to coast. Its young are also called Cattail Caterpillars, but I’m not sure if they might eat leaves instead.
Hugh, I actually don’t know if the species is introduced or not, but I know that there’s many moth species that are found naturally on both continents. A British mothing friend of mine regularly refers to British guides and is familiar with many species also from there. Of course, there’s probably also a lot of species that came to North America with shipments of various stuff!
And thanks for the compliments, Beth! Good luck to everyone, hope your cattail heads contain more than mine did!
I’m impressed with your investigative technique. Good Job!
Insects: Their Natural History and Diversity – I could use this reference. I’ll check it out.
Thanks Mary – hope you find the book as interesting as I did!
This is fascinating. I live in a very swampy area of Ontario, and see blackbirds on the cattails all the time…I never realized what they were looking for.
Thank you for the educaitonal segment of my day!
Glad you enjoyed it, Shelley! I was fascinated to discover this as well. It’s amazing that you can live someplace your whole life (mine’s admittedly not that long yet, but still..) and still find new things to learn about it.
It’s been a long time. But I’ve confirmed the identity of the cattail caterpillars I found. The post on it is here.
That’s awesome, thank you very much. I love researching on the internet. I find all sorts of very convenient details as if by accident. I’m writing a young adult fantasy book in a naturalistic setting, and it always seems that when I find a new element on the internet, it points me to another, usually one I’m already working on. I knew there were redwing blackbirds in my environment, and now I know why. Good ole’ cattails.
My husband will not be thrilled to discover the cattails I brought into the house a couple of years ago are why we have grubby larvae in our basement, however.