Monochamus is a genus of longhorn beetles known more grimly as sawyers—a name earned by the fierce work of their larvae. Found worldwide, these beetles target the wounded: fire-scorched, diseased, or storm-battered conifers like pine, Douglas fir, spruce, and true firs. The adult beetles carve slits into the bark of these dying giants to deposit their eggs. From each slit hatches a pale, legless grub—blind and insatiable—that begins to bore deep into the heartwood. With no eyes to guide them, the larvae chew blindly through the tree’s flesh, spiraling inward in a slow, relentless assault. As they tunnel, they pass through multiple growth stages, or instars, before sealing themselves into pupal chambers to complete their transformation. When feeding, these grubs make a clicking or rasping noise that can be heard several feet away.
While splitting some rounds of fir
into cordwood, I encountered several sawyer grubs. In the natural environment,
they help break down trees and return nutrients to the soil. Fortunately, they
do not infest dry, seasoned lumber or cured wood. They’re only active in green
or recently felled or dying trees. I’m sharing photographs—including one of a
grub posed alongside a Cold Smoke beer—so you have a reference for size.
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—Mitchell Hegman
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