Open-Mesh Cocoon – Diamond-Back Moth, or Spongillafly?

2014 November 12

On September 22, 2013, Sandy found this peculiar mesh structure on a branch in the stand of poplars northwest of the house.

Here’s another shot for scale. The grid on the board underneath it is half-inch, so it was only about a quarter of an inch long.

One end was torn open, showing where whatever was inside had left again.

From the intact end, it looks kind of like the rib pattern of a zeppelin:

So, it appears to be a cocoon, but of what?

At first, I thought it was the Diamond-Back Moth, Plutella xylostella. This is a small moth that probably came over from Eurasia in the 1850s, and eats plants in the brassica family (cabbages, mustards, and the whole complex of agricultural crops that have been derived from them). Around here, they’d probably be eating Yellow Rocket, a wild plant in the mustard family that we have in some abundance[1]. Diamond-back moths are mainly notable for their very fast breeding rate (they can have up to seven generations per year in warm climates), and for their tendency to make migrating swarms. They make a cocoon that looks very like the one we’ve been looking at.

However, down in the comments, Charle Eisman (author of the excellent guide, “Tracks and Signs of Insects and Other Invertebrates), says that its association with poplar trees pretty much cinches it as being Wockia asperipunctella. This doesn’t have a common name, although Eiseman says some people want to call it a “Wookie moth”, both because that’s pretty close to the pronunciation of the genus name, and because the moths have kind of shaggy wings.

Another, longer-shot possibility (which I nevertheless can’t help mentioning because up until now I didn’t realize such things exist) are the Spongillaflies, family Sisyridae. These are small insects that are related to lacewings and antlions, that make a very similar mesh cocoon (although, from the pictures on BugGuide their cocoons more closely resemble geodesic domes than zeppelins).

[photo by Charles Schurch Lewallen]

Their noteworthy feature is that they have aquatic larvae, that eat, of all things, freshwater sponges, like the ones in the genus Spongilla. Yes, there are freshwater sponges. They evidently grow as more of a film on rocks than most marine sponges do, are green because they have a cooperative arrangement with algae that grow on them, and don’t have much of a skeleton (probably because of the low mineral content of fresh water), and so they are usually mistaken for masses of algae. But they are sponges, all right. And these particular insects have specifically adapted to eat them. Go figure.

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[1] The “yellow” part of the name of “yellow rocket” is obvious enough, they have a profusion of bright yellow flowers starting quite early in the spring. I’m not positive about the “rocket” part, but it likely has to do with the way that they shoot up ahead of most other plants starting immediately after the snow melts. Apparently, the greens are edible, particularly in the very early spring (the plants overwinter as a rosette of leaves, and they are supposed to be best while they are still slightly covered with snow). They aren’t exactly an invasive plant, as they have been growing all around the Northern Hemisphere for longer than anyone has been keeping records. They are a pretty aggressive weed, though.

7 Responses
  1. November 12, 2014

    I just looked up ‘rocket’ in my Unabridged. It comes from rochetta, an Italian word (derived from a Germanic word) meaning small distaff, a distaff being the small rod that holds the wool when you’re working a spinning wheel. Not that I ever have, I’m just sayin’.

    The fireworks/missile rocket has the same root. I suspect the English word for the plant predates the missile so they both refer to the length, not the shooting into space [aside: how about that Philae, huh?] I’d look it up in my OED but I’ve been on the computer all day and even with the magnifying glass I don’t think I could read the print that small!

  2. November 13, 2014

    Anne: You’re probably right. The Wikipedia page for the “yellow rocket” plant suggests that people have been eating it and using it for a burn treatment for probably most of recorded history, so the common name should be pretty old.

    Still, the image of a whole field of little flower-missles rocketing in the air kind of amuses me. Even if that isn’t the original derivation, I’m more than happy to use their high growth rate to justify the common name now.

  3. November 15, 2014

    Amazing.

  4. Cory Lawrence permalink
    March 16, 2016

    Hello,

    I’m producing a story for a science news program called Daily Planet on Discovery Channel in Canada and I’m looking for some footage or images of the spongilla fly cocoon.

    I was wondering if I could include your images in my story?

    Thanks,

    Cory

  5. March 16, 2016

    Cory: the spongilla fly cocoon picture isn’t mine. As I noted in the posting, it is from this bugguide page: http://bugguide.net/node/view/124515/bgimage

    You will need to check the BugGuide contributor’s page, http://bugguide.net/user/view/815 , to see what the terms of use are for this image. It is under Creative Commons.

    If you want to use any of the Diamondback Moth cocoon pictures, those are mine, and you are welcome to use them.

  6. January 9, 2017

    This cocoon was not made by a diamondback moth but another moth without a common name: Wockia asperipunctella (Urodidae). The fact that you found it among poplars clinches it. Compare with this one.

  7. February 21, 2017

    Thanks, Charley. I added this information to the posting.

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