What is this very large beetle?

Whether a bird, plant or insect, identifying one I haven’t seen before is always interesting.

So when I received an email that Cal Russell was displaying a big beetle on his counter at Boundary Tractor, I hurried down to take a look at the mystery beetle.

I’ve seen big beetles before but not this big. The roughly two-inch long, reddish-brown beetle had long antennae, gnarly-looking legs and short spines on its neck.

Partly due to the long antennae, the beetle is classified in the long-horned beetle family. Like some other species, this beetle has many common names: ponderous borer, pine sawyer, western pine sawyer, spined woodborer and ponderosa pine borer.

Even scientifically the beetle has two names because the classification has changed: Ergates spiculatus and Trichocnemis spiculatus.

As the names suggest, this beetle is closely associated with pine trees. Adults lay eggs in bark crevices of mainly ponderosa pine and Douglas-fir. Once the eggs hatch, the larvae (called round headed borers) bore tunnels through the sapwood and heartwood.

With the drive to bore deep into the tree, ponderous borers are considered wood borers. While they may tunnel under the bark, they don’t exclusively stay there like bark or engraver beetles.

The tunnel pattern under the bark helps distinguish the two: wood borer galleries are unbranched while bark beetle galleries are branched.

As with the adults, the larvae (also known as timber worms) are quite big and can be almost three inches long. The creamy white grubs have dark mandibles that gnaw through wood and this gnawing action actually inspired the invention of the chainsaw.

Larvae make round tunnels in the wood that are packed with frass, a combination of sawdust and waste.

The tunnels help break down the wood of dead trees and helps the nutrients to cycle back into the system since the larvae turn solid wood into sawdust.

Unlike bark beetles, ponderous borers target stressed, dying or dead trees, which includes trees defoliated by insects, weakened by root disease, killed by fire or a bark beetle outbreak.

Ponderous borers don’t attack live trees but can infest logs stacked in a log deck in the woods or at a lumber mill, often diminishing the value. They also infest firewood which is one reason the transportation of firewood long distances is discouraged so that the beetles are not transported to a new area.

Unlike some insects, round headed borers may take two to three years to mature and reach the point in metamorphosis to begin pupating.

When ready to pupate, the larvae tunnel back towards the outside of the tree and create a large chamber near the bark. Then when the adults mature, they gnaw their way out with their large mandibles.

Adults emerge midsummer to fall and move at dusk and during the night. They are drawn to light which helps disperse them especially on full moon nights.

Adult ponderous borers can make a squeaking or hissing sound if picked up because of the release of air from the breathing ports.

To complete the cycle, the adults mate and the females lay eggs. Not all larvae will survive because they are a tasty meal for woodpeckers and cold winters can take a toll.

Ponderous borers that manage to evade the woodpeckers emerge to give us a chance to look at one of the largest beetles in western North America.

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