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Caterpillar season!

 On recent forays, I have been noticing caterpillars everywhere. Most are small early instars - just little tubes that are easily missed. Those that survive will grow dozens of times bigger and more meatier than they are in their earliest instars, or growth stages. The first big seasonal flush of caterpillars coincides with spring leaf out. Many of the early caterpillar species are ones who overwintered in the egg stage; their hatching coincides with the appearance of leaves, which will be their food. In a perfectly orchestrated symphony, our migratory songbirds arrive just as this caterpillar bloom is starting. Inestimable scores of caterpillars become the fuel which propels the warblers, vireos, tanagers, orioles and most of the other songbirds (and some nonpasserines) that we enjoy observing.

The eastern deciduous forest biome, which cloaks much of the eastern half of the United States, stretching from the Gulf Coast to southern Canada, grows the lion's share of caterpillar biomass in northern North America, along with the great boreal forest of the far north. This is why the migratory pathways of so many of our Neotropical migrant songbirds goes through eastern North America, even if the species ultimately heads far to the northwest to breed. Hungry migrants are assured of finding lush food sources when they travel through a region rich in broad-leaved trees and their attendant caterpillar crops.

As for the photo above, it's a bit of a fooler. While the animal is quite caterpillar-like, it is actually a sawfly larva. Sawflies are in the order Hymenoptera (bees, wasps, ants), not the Lepidoptera (moths and butterflies). It's the larvae of the latter that we call caterpillars, and which are arguably (or not) the most important foodstuff for a great many of our bird species. Sawflies do not seem to be particularly well known, and I'm unsure of this species. It is eating black oak, Quercus velutina. Many sawfly species seem to be "chemically protected"; they sequester toxic compounds that render them unpalatable to birds. However, I have seen birds eat sawflies on several occasions.

Now this was a cool find! I was with Ann Bowe, a friend from Kentucky, yesterday. I was down there to seek out the Kentucky lady's-slipper (success!), and along the way Ann spotted this beauty. It is the caterpillar of the Canadian owlet moth, Calyptra canadensis. I had never seen it before, at least that I recall. Adding to its intrigue, this species eats only meadow-rues in the genus Thalictrum. They are big cats, and meadow-rues are typically rather spindly herbs. The three caterpillars on this meadow-rue were making mincemeat of the plant, and the lower leaves had been stripped down to bare petioles. Caterpillars that are rather large and conspicuous, and feed during the day when birds are active, often possess chemical properties that make them distasteful to birds (as mentioned under the sawfly above).

I was, it should go without saying, quite excited to find this little oddity. I had a few short hours to photograph this morning, and ran over to a nearby local patch, Kiwanis Park. It is an oasis of high quality habitat along the Scioto River, and is loaded with a diverse native flora. After getting some keepers of various birds doing interesting things, I was making the short drive home when I felt a tickle on my arm. Glancing down, I saw this caterpillar inching along.

It is the filament bearer, Nematocampa resistaria, another species that I knew of but had never seen. I quickly detoured into a parking lot, got a vial from the trunk, and caged the animal. Upon arrival home, I placed it on my serviceberry, Amelanchier arborea, and made some images. The odd protuberances on the caterpillar's dorsal surface probably act as disruptive camouflage, perhaps helping it to blend in when at rest. When disturbed, it elongates the filaments significantly, as here. I suppose it is possible that the caterpillar may use them as flails - whips by which it can repel would-be parasitoid flies or wasps.

In any event, I hope this filament bearer flourishes on my native serviceberry. They are known to eat a wide variety of woody plants, including species in the rose family, such as serviceberry. If all goes well for it, the caterpillar will morph into a rather undistinguished brown moth. If one of the local birds gets it before that, well, that's life on the food chain.


Julia McGuire said…
so cool. I never thought to have a walk or hike specific to caterpillar hunting. thanks for sharing!
cosmichorror said…
Fantastic, and timely. I came in from outside earlier today and I had a Filament Bearer stuck to my pants. I had never seen anything like it. I got some shots of it, and then spent nearly an hour looking at caterpillar identification sites trying to identify it, with no luck. Then stumbled across this post. Now I know.
Jim McCormac said…
Thank you, Janet, and I'm glad this post was helpful, "cosmichorror"

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