Monochamus scutellatus, the Whitespotted Sawyer Beetle


Monochamus scutellatus
Canoe Island
August 12, 2019
Photo by Chase Stoddard
Monochamus scutellatus
Canoe Island, August 12, 2019

I received this photo yesterday of a very cool Cerambycid (Long-horned) beetle from Chase S. on Canoe Island, WA. This is a Whitespotted Sawyer beetle (Monochamus scutellatus). These beetles are found in forested areas across the continent. The first time I saw one on San Juan Island, I almost mistook it for a giant black spider. Its long antennae give it the appearance of having eight legs, like an arachnid.

The larvae (known as sawyers) of Monochamus beetles develop in the wood of fire-scorched, diseased or injured, dying pine, Douglas fir, true firs, and spruce trees. The adult beetles lay eggs in slits in tree bark. Larvae (grubs) develop through several instars before pupating. The younger larvae feed on wood beneath the bark and filling the void with long, fibrous borings, then move into the wood, forming holes that will become almost perfectly round as the tunnel grows deeper. The tunnels can eventually reach the heartwood of the tree.

Prior to pupation, the larvae will pack the deep gallery with grass, leaving a space at the end for the pupal cell or chamber. Adults will emerge by gnawing through the cell and a thin layer of wood and bark to the surface. Telltale signs of boring damage is the accumulation of debris along infested logs. As they can attack and damage recently felled trees, they are considered a pest in logging areas. According to Furniss and Caroline’s 1977 Department of Agriculture Forest Service Publication No. 1339, utilization of preventative methods that avoid leaving logs exposed during the beetles’ egg laying period ( July-September ) is the best approach.

Monochamus scutellatus White-spotted Sawyer
August 30, 2010
Photo by Cynthia Brast
San Juan Island, WA

References:

Furniss, R. L. (Robert Livingston)., Carolin, V. M. (Valentine M.)., United States. Forest Service. (1977). Western forest insects. [Washington]: Dept. of Agriculture, Forest Service : for sale by the Supt. of Docs., U.S. Govt. Print. Off..

Haggard, P. and Haggard, J. (2006). Insects of the Pacific Northwest. Timber Press Field Guide.

Peterson, M.A. (2018) Pacific Northwest Insects. Seattle Audubon Society.

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