Birch Sawfly (Arge pectoralis)

When is a caterpillar not a caterpillar?

Birch Sawfly (Arge pectoralis)
A Birch Sawfly (Arge pectoralis) larva, Gregory Canyon, Boulder, Colorado, USA. July 21, 2012.

When it’s a sawfly larva, of course! Yesterday morning at the beginning of a photography class, we stopped for breakfast at a picnic table in Gregory Canyon. There were some little caterpillars on the table…and then I noticed there were caterpillars on the tree next to the table. Lots of caterpillars. The tree was nearly stripped of leaves, and caterpillars were falling out of the tree onto the table, which I could have done without (I suspect the one that fell into my iced chai could have done without that fatal experience as well).

When I got home, I determined that these were not, in fact, caterpillars, but sawfly larvae, most likely the Birch Sawfly (Arge pectoralis). The tree was probably a Water Birch (Betula occidentalis). Sawfly infestations, while dramatically defoliating, generally do not kill the tree.

Sawflies are related to wasps, bees, and ants, and adults look something like a wasp, although they cannot sting. The common name “sawfly” comes from the saw-like ovipositor females use to cut into plant tissue and deposit eggs. Larvae often look like hairless caterpillars, but can be distinguished by their greater number of prolegs (caterpillars typically do not have more than five pairs).


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