Large Moth that Resembles a Cicada – Carpenter Moth

2015 November 7
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I was walking on campus with Sandy and Rosie on June 25, 2015 when Rosie spotted this on the outside wall of the library. I thought it was an early cicada at first, but on closer examination saw that it was something else entirely.

It’s actually an old, tattered, somewhat balding Carpenter Moth, Prionoxystus Robiniae. They’re also known as Locust Borers, although they go after lots of tree species other than locust. They are pretty huge as moths go, with their bodies being about an inch and a quarter long:

Males have orange hindwings, but this one does not and so it is most likely a female. Her abdomen was huge, with plenty of room for eggs.

I don’t see any sign of mouthparts, so there are pretty good odds that this is another of those huge moths that just live for a week or so on stored body fat, and don’t actually eat as adults.

These moths are in the superfamily Cossoideae, which are unusual moths because instead of eating leaves like most other lepidoptera do, their caterpillars burrow under bark and bore into wood. This species in particular gets deep enough into the wood that it actually damages the lumber, and so it is regarded as a pretty serious pest.

Like a lot of other large, wood-boring insects, the caterpillars of these moths spend several years maturing inside the tree trunk. This is partly because wood is hard to digest (and therefore they grow slowly), and partly because it really is a pretty safe place to hang out, and so there is no hurry to mature. Oh sure, there are woodpeckers and such that will drill into logs looking for these big, fat caterpillars[1], but this threat is nothing compared to the leaf-eating caterpillars risking being eaten by pretty much every passing bird.

[1] The related Witchetty grubs, in Australia, are commonly eaten by people in the outback. They’re supposed to taste like almonds when raw, and like eggs wrapped in crispy chicken skin when fried. There are several species collectively known as Witchetty Grubs, and one of them turns into a monster moth like the one in this video that Eric Eaton talks about. By “monster”, I mean about the size of a medium mouse. I linked to Eric’s blog instead of the original video, because the original video was evidently misidentifying the moth.

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