Bruce Spanworm Moths

2013 October 19

On November 4, 2012, I decided that since it wasn’t quite freezing out (it was 34 degrees F, or about 1 degree C), it was worthwhile to leave the porch light on overnight to see what we got. The biggest things were these two moths, one with slightly blurred markings:

And one with pretty distinct markings:

Aside from the slight difference in markings, they looked pretty much identical, so I’m pretty comfortable assuming that they are both the same species.

In this case, the time of year that they were flying, and the temperature at the time, was a better ID feature than the markings anyway. There are very few moths that fly in November when it is barely above freezing. So I used the BugGuide Advanced Search to look for Lepidoptera, caught in November, in Michigan. There were only three species that came up, and one of them looked like a pretty good match: the Bruce Spanworm, Opherophtera bruceata. The other two were the Winter Moth and the Autumnal Moth, but the Winter Moth is much darker gray, and the Autumnal Moth has a row of double-dots along the wing edges where the Bruce Spanworm only has single dots.

These are both males, because the females have only tiny vestigial wings and can’t fly. They hang around on the bark of the trees that their caterpillars feed on, mainly Maple, Beech, and Aspen (although they will sometimes eat other trees, like willows, if they have to). So the males find the females, they mate, and then the females lay little green eggs (that later turn orange) in the tree bark to overwinter.

The caterpillars in the spring are a variety of inchworm (green, with pale yellow stripes running the length of the body).

Um. You know, the BugGuide picture of the caterpillar looks pretty familiar.

Actually, I think this might be one of the ones I posted in March 2012 in the “unidentified green caterpillars” posting – a little green caterpillar photographed in early June 2009, using my old point-and-shoot camera:

It’s an inchworm with the same diffuse dark stripe running down its back, with pale yellow-white stripes on either side. Just like the Bruce Spanworms on BugGuide.

So, maybe it’s not unidentifiable after all!

Comments are closed.