Harvester Caterpillar

2014 November 1

Sandy found a couple of these in a cluster of wooly alder aphids on September 10, 2013. This one had dropped to the ground under the aphid cluster. It was probably all scrunched up like that because it was getting ready to pupate.

Looking at the underside, we can see that it is a caterpillar, as it has the caterpillar-type head, six true legs, and the prolegs running down the abdomen.

It’s a little bit hairy, but not too much. It wasn’t very big, as we can see from the fact that it fit neatly between the lines on a standard sheet of notebook paper (a quarter inch)

I’m pretty sure that these were Harvester Caterpillars[1], Feniseca tarquinius. Before they started getting ready to pupate, they would have looked a lot more caterpillar-like. They were evidently eating the aphids. And this is where you say, “What? Did you say it was a butterfly caterpillar eating aphids?
Yes I did.

It seems that the caterpillars of this species are, in fact, the only known exclusively-carnivorous caterpillars in North America. Oh, sure, some caterpillars will turn cannibalistic if they are overcrowded, and maybe some others will eat other small creatures if opportunity arises and it isn’t much trouble, but for the most part your typical caterpillars are pretty dedicated to a herbivorous lifestyle.

These, on the other hand, have discovered that (a) the flesh of other insects is way more nutrient-dense than the leaves of plants, and (b) eating aphids is not much more difficult than eating leaves. Especially the woolly aphids, which are practically immobile because of the wax “wool” they secrete. By eating aphids, the harvester caterpillars can mature very fast, getting in multiple generations per year even this far north.

They are supposed to pupate as a chrysalis attached to a leaf, that looks kind of vaguely like a skull, or maybe a monkey head. They may overwinter either as pupae, or as older larvae.

It is kind of unusual to see them, both because they aren’t particularly common, and because they hide well. The caterpillars tend to burrow in under the remains of the aphids they’ve been eating, mainly to protect themselves from the ants and wasps that come to tend the aphids (There is always something tending the woolly aphids on our alder trees. Some years it is carpenter ants, some years it is the Formica ants, and this year it was swarms of yellowjackets and bald-faced hornets. They’re all after the same thing, though: the sweet “honeydew” that the aphids secrete, specifically to lure in defenders like that).

The adult harvester butterflies (which are small, orange butterflies) are also not commonly seen because they don’t go to flowers much, preferring things like animal droppings, carrion, and masses of aphids secreting honeydew. Plus, they look a lot like a number of other small, orange butterflies, and so I might not recognize them if I did see them.

[1] Not to be confused with a Caterpillar Harvester, which is something completely different:

(There’s at least one machine of this general type working out in the pine plantation behind our house right now. They start before sunrise, and continue until well after sunset. I don’t know exactly how many loggers with chainsaws one of these machines is equivalent to, but it’s probably on the order of 50 or so. And the probability of somebody getting killed or maimed is much, much lower.)

2 Responses
  1. November 2, 2014

    Jeezey Moe, that CAT looks like something out of a Terminator movie!

  2. November 3, 2014

    KT: Yep. The logging business has changed almost beyond recognition since I was a kid. But it all goes on out of sight in the forests, and I don’t think that most people really realize what sorts of monsters are stalking the woods these days.

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