Quadrate (or Square-Eyed) Dagger Moth Caterpillars

2012 February 11

Sandy found several of these caterpillars eating the leaves of one of our young cherry trees on July 25, 2011.

These are pretty distinctive. The only caterpillar I could find on-line that has that combination of double-orange dots running down the back and the white “racing stripes” is the Interrupted Dagger Moth, Acronicta interrupta. But, that didn’t look quite right, and although BugGuide mentions that the caterpillars of the Quadrate Dagger Moth, Acronicta quadrata, look similar, there weren’t any online pictures of its caterpillar to compare. But, as it turns out, in January I just acquired a copy of the new book Owlet Caterpillars of Eastern North America[1]. And right there on page 298 is a picture of the Square-Eyed (or Quadrate) Dagger. Which is a dead ringer for my specimens.

In any case, Dagger Moths are medium-sized gray moths, usually with long, straight “dagger” markings on their wings. The adults are pretty well camouflaged, but their caterpillars tend to be brightly-colored and often furry.

The long hairs on this caterpillar, which can barely be seen in the side view above, but are pretty evident in the face shot below, look to me more like sensory hairs than defensive hairs. They are too spindly to really ward off predators effectively, but are long enough to give quite a bit of advance warning when the caterpillar is approached by, say, a parasitic wasp.

The bold colors make me think that these caterpillars are noxious to eat, so probably their primary predators are things like parasitic wasps rather than birds. And, like all caterpillars, their main business is eating.

I suppose they could be a pest species, since they obviously eat cherry leaves, and are reported to also eat leaves from apples, plums, and all sorts of other fruit trees. But, to really be a pest, leaf-eating caterpillars need to get to high enough numbers to defoliate the tree. They could have been a problem on our little cherry tree (which didn’t actually have that many leaves to spare), but probably wouldn’t have affected a bigger tree very much.

In fact, plants sometimes benefit from having some of their leaves eaten. It is well-known that judicious pruning of fruit trees and grape vines improves the quantity and quality of fruit, and moderate browsing by deer or insects is a lot like pruning. I particularly remember an article that I read in a farming magazine when I was a kid, where corn plants were cut off just above ground level when they were at the 2-leaf stage. The stalks that grew back were actually taller and stronger, with better root systems, than the adjacent untouched cornstalks. I particularly remember that article because, just a few days after I read it, we got confirmation of its results when the cows got out one night[2]. One of them (I think it was Emily, although we had about 30 cows and it could have been any of them) got into our garden and cropped off half of the just-sprouted sweet-corn plants before we herded her back out. And the plants that she cropped were between one and two feet taller at the end of the season, and with bigger ears, than the ones that she didn’t touch.

[1] An amusing point: I have a bunch of insect guides at this point, and I’ve noticed that, on average, the more specific they get, the bigger they are. The general-purpose guides are about the size of a short paperback book, but Caterpillars of Eastern North America is the size of a big book like a dictionary, and Owlet Caterpillars of Eastern North America is the size of an epic fantasy trilogy like The Lord of the Rings. This is because the general-purpose guides only have representatives of each insect family, but the more specific ones try to give IDs down to the genus or species.

[2] One of the cows (Dot) was a snitch. On the occasions when the cows got out during the night, she would immediately go and stand under my parent’s bedroom window and scream. At this point you are probably thinking, “wait a minute, cows don’t scream, they moo!” To which I reply, OK, try telling her that. To top it off, her face was almost completely black except for the tiny white dot in the middle of her forehead (hence her name). So you can imagine what it was like for my parents to be abruptly woken up around midnight by this un-cowlike screeching noise coming from a huge, formless black thing outside their window.

5 Responses
  1. February 11, 2012

    Your bit on racing stripes made me wonder how fast caterpillars crawl and how widely their top speeds vary. A very cursory search revealed they crawl at 0.7 mph.

  2. February 11, 2012

    Gorgeous photos, by the way.

  3. Carole permalink
    February 11, 2012

    According to Dr Douglas Tallamy, members of the cherry and plum tree family are the third most valuable plant for wildlife because they support so many insects. They are the larval host for 438 moths and butterflies and many other insects feed on them. He points out that 96% of land birds eat or feed their young insects. Even if the caterpillars taste nasty, it could be that the eggs or moths make a good meal so I would encourage you to smile when you see them feasting on your trees. Dr Tallamy wrote Bringing Nature Home, a book I really enjoyed.

  4. February 12, 2012

    KT: That’s actually a lot faster than I would have thought. I guess that explains why they are sometimes running so fast that I have a hard time getting pictures.

    Carole: I can believe it. It seems like everything is munching on the cherry trees. One would think that this would really set back the trees’ growth, but they certainly seem successful enough. Maybe the trees are able to optimize their growth better when they aren’t producing defensive chemicals, and so they end up keeping up with the competition in spite of being eaten heavily.

  5. February 18, 2012

    Hmm. They seem too small for proper NASCAR sponsor decals …

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