Purplish-Brown Looper

2010 December 18

We were walking in the woods out back on July 23, and as usual we were casually checking over any milkweed we passed to see if there were any Monarch caterpillars on them. By this time, the Monarch season was tapering off, so we weren’t finding too many of them. But, we did find this caterpillar clinging to a milkweed stalk:

Now, this is an odd thing to find on a milkweed plant. It is an inchworm, and an excellent stick mimic. It looks almost exactly like a short branch on, say, an apple tree. In its correct location, it would probably have been practically impossible to see. But, on the bright green milkweed stalk, it stuck out like a sore thumb. It obviously didn’t belong there.

To figure out where it did belong, it would be nice to figure out what it was. There are a number of twig-mimic inchworms, but this one has some identifying features: it was quite large (well over an inch long), and has a two-lobed lump partway down its back:

It also tended to hold its head tucked under, kind of like a fist,

and it had kind of a flat, receding forehead.

We were able to find a pretty close match in “Caterpillars of Eastern North America”. It looks to be the Purplish-Brown Looper, Eutrapela clemataria, which in the adult form is more often known as the Curve-Toothed Geometer moth. These are large moths, and the adults are excellent leaf mimics. As expected, their normal caterpillar foodplants are things like ash, basswood, birch, cherry, elm, maple, oak, poplar, willow, and a bunch of other woody plants. In general, anything that has a branch that resembles its camouflage is something that it is likely to eat. I thought that it looked a lot like an apple twig, so when we let it go we put it on one of our apple trees. Where it promptly pretended to be a stick and vanished from sight.

Notably absent from this foodplant list is milkweed. So, what was it doing where we found it? Well, I can think of two explanations, one an innocent mistake, and the other a result of something being terribly wrong. First, the innocent mistake: It could easily have fallen off of a nearby tree, crawled around looking for a new one, and accidentally went up a milkweed stalk instead. In that case, we probably saved its life by putting it onto that apple tree. But, the other possibility is that it was intentionally seeking out a somewhat toxic plant. It could have been suffering from a parasitoid infection of the kind we have seen before, and attempting to self-medicate[1]. Of course, it looked OK, but then again, they almost always do, right up until the parasitoid grubs erupt through their skins. The only reliable ways I know of to detect a parasitoid infection are to either wait for them to emerge, or to dissect the suspected host. I did neither of those things (because, at the time, I didn’t realize that appearing on an inappropriate and somewhat toxic foodplant might be a symptom of infection), and so we will never know for sure. In the future, I think that whenever we find one of these suspiciously misplaced caterpillars, we will keep them just to make sure.

[1] It occurs to me that there could be two goals to the self-medication. On the one hand, it is possible that the caterpillar could knock out the parasitoids enough to survive. On the other hand, even if (or maybe even especially if) the caterpillar is doomed, there would be an advantage if it went off and committed suicide by poisoning itself rather than just letting the infection run its course. As we have seen before, some of these parasitoids can produce hundreds or thousands of offspring from a single host. If unchecked, these would then spread out and infect the future offspring of this caterpillar’s siblings. So, if the caterpillar can kill itself in such a way that the parasitoids also die, then that would still aid the survival of its relatives by reducing the number of wasps around to lay eggs on them. And by the kin-selection principle, this behavior would be selected for if the siblings carry the same genetics. If suicide is the only aim, then it becomes much easier for the caterpillar to accomplish than a cure. Any toxic plant will do, not just those with medicinal properties. And, if it is also a plant that is completely incompatible with its camouflage, then the caterpillar is simultaneously proclaiming to all predators, “I do not want to live! Eat me!” Which would kill off any parasitoids even more effectively than poisoning.

15 Responses
  1. December 29, 2010

    Great post. As for the suicide deal, it’s almost like the caterpillars have a collective intelligence like some kind of alien race from a science fiction novel.

  2. July 25, 2011

    I just found one of these today at work. I work at Apple Works Orchard in Trafalgar, Indiana. My fellow apple pickers nor I have ever seen one of these Loopers before. He was found on an apple tree in the orchard and he was eating the apple leaves. At the time we found him he was stretched out and suspended from a silk thread, but his back legs were still on a leaf. I think I shall keep him, feed him, and let him turn into a moth.

    Thanks for the post and the great photos,

  3. July 25, 2011

    Also, we sprayed for Japanese Beetles about 10 days ago. So I wonder if he survived the spray or if he is less than 10 days old.


  4. July 26, 2011

    Matthew: It could have survived the spray, especially if they used something specific to Japanese Beetles rather than something broadly toxic. I suspect that insecticide sprays normally have pretty poor coverage when sprayed on dense foliage, with leaves shielding other leaves, so there are probably a fair number of insects left behind even after application of a broad-spectrum insecticide.

    I wouldn’t think that it would have gone from an egg to something large enough to catch your eye in just ten days (although the egg stage of insects does tend to be a lot more insecticide resistant than the larval or adult stages). But, there are some caterpillars that grow that fast (monarch caterpillars go from egg to chrysalis in just two weeks, for example). So it isn’t completely impossible.

  5. Barb permalink
    June 4, 2015

    i live in southwestern pa and I’m pretty sure I just saw one of these caterpillars eating my basil plant. Could it be?

  6. June 4, 2015


    I think it could be, yes. You are far enough south of us that caterpillar eggs laid in early spring could be fully grown by now. And, if it is eating something toxic to deal with a parasitoid infection, basil is exactly the sort of thing it would pick.

  7. Barb permalink
    June 4, 2015

    Thanks Tim. I’m going to let him continue to munch on the basil then. Maybe he’ll be able to cure himself. I couldn’t figure out where all of these charcoal like pellets were coming from around the basil. Can one inchworm make that much poop? It’s a lot!

  8. June 4, 2015

    Yep. Caterpillars don’t actually do a very good job of digesting the leaves that they eat, so they pass a lot through. A single caterpillar can make an astounding amount of poop. Although, there could certainly be more than one of them around.

  9. Barb permalink
    June 4, 2015

    So far all I can see is the one. He seems to be hanging with his mouth in the plant and front legs but the rest of him is just hanging downward. How would I know if he’s making a cocoon, and would he do it on the basil plant? Thanks for all your input. I find these creatures amazing!

  10. June 5, 2015

    Usually caterpillars will crawl off somewhere to make a cocoon, they usually don’t seem to like to do it on their food plants. They normally don’t let their bodies hang down like that, either, which suggests that it is not healthy. There is an excellent chance that this one has an infestation of parasitic wasp larvae, or possibly some disease.

    If you put it in a jar with some basil leaves, you will likely see it pupate, and if it has parasitoids you’ll eventually see tiny wasps (or maybe flies) emerge from it within a couple of weeks. And if it doesn’t have parasitoids, you should get a moth within about the same amount of time (maybe up to a month).

  11. Barb permalink
    June 5, 2015

    Thank you very much for all the help. I really appreciate it and I’m enjoying learning about these creatures! He seems to have reattached to the basil again and isn’t hanging from one end. He’s really munching away and is getting around the plant.

  12. Alice Pratt permalink
    July 4, 2015

    I just found one on a yellow Millionbells (hanging basket) plant, eating the flowers.

  13. Theresa Herbst permalink
    May 10, 2018

    HI, thank you for this page that helped me identify the caterpillar i found munching on a patio plant. The potted plant that I found the caterpillar on is called “mona lavender” Plectranthus plepalila. I noticed the caterpillar feces on the ground and it took me atleast a week to finally find the caterpillar! they look like sticks when they are resting! I put it in a glass vase with some of the flowers he likes and I changed the leaves everyday for 5 days. Today he has curled himself up in a leaf. I cant wait to see him emerge. I wonder how many days the metamorphosis takes??

  14. Theresa Herbst permalink
    May 10, 2018

    oh yeah, I’m in Guatemala City, Guatemala. Most information i found online so far was only mentioning its range in North America.

  15. Anne Marie Devereaux permalink
    July 27, 2019

    We thought we had a mouse around the summer savory, so we bought a mouse trap. I had moved the plant up to a table and when we got home noticed more poop on the table. It was then I saw the caterpillar on the plant. Didn’t know it was a moth until I started looking for a worm, grub etc that looks like a twig / stick. Came across this post.

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