A Cecropia Tragedy

2015 August 19

Samantha found this cocoon stuck to a tree branch in fall of 2014. It was clearly a giant silkmoth cocoon (probably a Cecropia Moth). At this point we’d had good success in overwintering silkmoth cocoons for a couple of years running, so we put it in an insect cage on the cellar steps like we’d done in previous years.

Then, in the spring, we put the cage and cocoon on the front porch like we’d always done before, and waited. And waited. And waited some more.

And then, on June 12, 2015, I noticed that there were a couple of what looked like house flies inside the cage. And the next day, there were a whole bunch of flies. Uh oh.

It was looking like we had a parasitic fly attack here. So I dissected the coccoon to see what was going on in there. The cocoon consisted of an outer envelope of waterproof silk, and inside was a smaller, more oval-shaped coccoon.

And the inner coccoon was, of course, stuffed with the remains of fly pupae.

Dumping out the contents, we can see that there were thirty or so fly pupae in there, all of which had emerged and left their husks behind.

The actual moth pupa was just this hard, shrivelled mummy.

There was also the skin that the caterpillar has shed when it pupated.

Well, these were pretty clearly some sort of tachinid fly, of which there are hundreds of species. Here’s another one, that I refrigerated enough to get it to hold still for pictures while still having it alive to pose.

They have the same sort of mouthparts as house flies do. Basically a sponge on a stick, that they use to lick up fluids to eat.

I think I know what these are, and it is kind of a shame that we are finding them: they are quite likely to be Compsilura concinnata, a generalist tachinid fly that attacks a wide range of different host caterpillars. And the problem is, that these are a non-native species that the Department of Agriculture brought to the US around 1906, in an attempt to control Gypsy Moths.

And, as is so often the case when one imports predatory species without thinking it through, it didn’t work as planned. The gypsy moths had evolved along with these flies back in the Old Country, and so they have good enough natural defenses against them that the flies only attack about 5% of gypsy moth caterpillars. Which is to say, the flies were a failure for their intended purpose.

However, if our purpose was to wipe out the native giant silk moths, then the flies are a rousing success. The giant silk moths have adequate defenses against the native tachinid flies for them to coexist with them. But these imported flies are different. Instead of laying eggs on the caterpillars that the caterpillar can then attempt to deal with before they hatch, their eggs hatch inside the mother fly’s body, and she injects live maggots into the host. As you can imagine, this speeds things up immensely, and the maggots can mature and kill the caterpillar in as little as 10 days.

And, of course, since they have over 200 known hosts, the tachinid flies can pretty much attack whatever caterpillar they see, and aren’t necessarily caught in the boom/bust cycles of their hosts. So if there is a gypsy moth outbreak, they can breed up to huge number on them, and then when the gypsy moth population collapses the (now very numerous) flies can just move on to the silkmoths, tent caterpillars, monarch caterpillars, sawflies, you name it[1]. And if their new hosts’ defenses prove not to be up to the task, well, it’s too bad for them.

So anyway, it isn’t really a surprise that these flies are in the area, seeing as how the gypsy moths moved in five or six years ago. So now we are stuck with them.

Incidentally, I did not release the flies. We killed them all. More of a symbolic gesture than anything else, I guess, but it beats letting them go to kill more silkmoths.

[1] For the last few weeks, Rosie has been collecting the caterpillars of Cabbage White butterflies that have been eating the broccoli in our garden, and putting them into a cage to pupate. She has probably 20 chrysalises, several of which emerged as butterflies. But, lately I’ve also been seeing what look like these same flies in the cage, too. So even little caterpillars, that are only big enough to support one fly larva, are being attacked here.

3 Responses
  1. August 19, 2015

    What a sad fate for the silk moth. Maybe this can partly explain the collapse of the monarch butterflies?
    Thanks for the investigation and thorough analysis.

  2. August 19, 2015

    They might be a contributing factor to reducing the monarch population, yes. Although we haven’t seen any parasitized monarchs locally. The monarch caterpillars we rear that die before maturing mostly seem to have some sort of disease that makes them turn black.

  3. August 19, 2015

    Ouch. The Law of Unintended Consequences strikes again.

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