It helps so much when you finally get a name for it

We’ve been seeing these things sticking to walls every fall for some years now. We mostly find them on masonry walls and windows, although that doesn’t necessarily mean they prefer those surfaces – it may just be that they are easier to spot there.

They look like either little animal droppings, or some sort of sand-covered snail. Poking at them shows that they are firmly attached to whatever surface they are on. When they are pulled off, they feel a bit “crunchy” because of the sand on the surface, but also tough and flexible enough that they can be stretched out a bit. They are a tube of silk, with sand stuck all over them for camouflage and protection.

Inside this one, there was a nondescript chunk of organic material that looked like it might have been alive at some point, but didn’t look alive anymore, along with a bunch of shed skins.

As common as these are, you’d think that everybody would know about them, but I’ve never heard them discussed by anybody. I submitted the pictures to BugGuide, but this is a busy time of year for them, and the pictures evidently got buried in the large numbers of admittedly more interesting insects that are beings submitted this fall. At any rate, the few people who might have known what it was apparently didn’t see it. So, I kept searching around on the internet. I figured that something this common would have to crop up somewhere, but I was hampered by the lack of a name, and a shortage of unambiguous search terms. I was getting nowhere. Anything related to “sand covered cocoons” gave links to ant lions and such, and mentions of “spiral” and “helix” tended to get items relating to DNA.

Finally, I hit on the search string “coiled cocoons sand covered”, and Bingo! It turned up as the third picture down on this page. At first, I was worried, because the author of that page didn’t seem to know what it was either. But then, I saw the edit added at the bottom saying what it was. Aha! It’s a Snailcase bagworm, Apterona helix.

Once I had a name, it all fell into place. Even though the BugGuide page doesn’t give much information, several others do.

It turns out that snailcase bagworms are, of all things, a type of moth. Like so many other insects around here, they are native to Europe, and got accidentally introduced to North America in the 1940s – probably as cocoons stuck to objects being imported. They have some pretty unique features: first off, they are parthenogenic females. There are no males of this species in North America, the females just essentially clone themselves when they reproduce[1]. This is very unusual for moths. Another unusual feature is that they have no wings, and they never really come out of their cases. Their lifecycle apparently goes like this:

-Eggs overwinter in the protection of the sand-covered case of the parent, and hatch out in the spring. The young then lower themselves to the ground on silk threads, and pick up some debris to start making their own cases.
-The young then go and eat pretty much any vegetable matter they can find. They aren’t very fussy, and will eat most anything from grass, to broadleaf plants, to algae that they find growing in crevices on rocks.
-As they grow, they keep adding silk and sand to their cases. The cases are extended in a spiral, kind of like a typical snail. This spiral shape keeps the whole thing compact, so they aren’t dragging this long, unwieldy shell behind them.
-In the late summer, they find a surface to stick themselves to, and pupate. They emerge from the pupa in the early autumn, but they don’t actually come out of the case. Instead, they lay their eggs inside of the case, and then die, still stuck to the surface. The eggs stay in the case until spring, and the whole thing repeats.

They are considered pests, but not for the normal reason. While they do eat a lot of kinds of plants pretty indiscriminantly, they generally don’t do much damage aside from some cosmetic holes in the leaves. No, the thing they do that annoys people is they stick themselves all over house siding, cars, windows, etc. in a highly unsightly manner. They don’t actually hurt anything if you just leave them there, but it ends up looking like some small creature left droppings all over the building. They stick so hard that fairly extreme cleaning methods, like high pressure water jets, are needed to remove them. They can be picked off by hand, but when you pick them off of house siding they frequently pull a little flake of paint along with them, which obviously doesn’t do the paint job any good.

Something I find interesting is that they not only look like snails, in a lot of ways they act like snails. They drag a shell around with them for protection while they slowly cruise around for food, and when they reproduce they do it without benefit of males[2]. Since they aren’t finicky about food, are well armored against most predators their size, and don’t need to find mates, they don’t have to move fast or fly around. Crawling slowly is good enough. They have lost wings, prolegs, and in fact pretty much all of the features that would identify them as moths (other than the ability to make silk). All in all, it looks like another case of convergent evolution. Form follows function. If you live like a snail, eventually your descendants end up looking like snails, because the shape of a snail is very well suited for that lifestyle. That’s why the snails look like that themselves, after all.

Of course, that’s not to say that they don’t get around, but they mainly travel complements of us. We haul all sorts of things around the world willy-nilly, and when we do, there they are stuck to the sides. Why do the work when somebody else will do it for you, after all?

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[1] I got to wondering whether it’s just here in North America that there are no males. Maybe the ones that hitch-hiked over happened to be just females, and so they went from partially-parthenogenic to completely parthenogenic? Do males of this species still exist in Europe? So, I did a search on “do male apterona helix exist”. I didn’t get an answer, though. Instead, I turned up a lot of stuff that was unhelpful, and some that was just plain weird. Like this site claiming that Jesus was a parthenogenically-born natural transsexual. Um. Ok.

[2] Snails aren’t quite the same, they are hermaphrodites (combined male and female), not parthenogenic. So, if you have two snails they can mate with each other, but if you have two snailcase bagworms all they can do is keep cloning themselves. Although, I think most species of snail are able to fertilize their own eggs if they don’t have a mate handy, which gives pretty much the same effect.