Illustration by Adelaide Tyrol
If, when quietly wandering around outdoors, you have ever heard and tried to track down a rhythmic, grating sound, thinking you were in search of a small rodent or perhaps a cricket, only to find that the noise comes from within a dead log, the noisemaker might well be the larva of the whitespotted sawyer. Its chewing noise is described as resembling a sawyer at work and is the source of its name. A small stream of finely ground, sawdust-like fragments can often be found coming from a hole in a noisy, dead, softwood log.
This wood-boring beetle, found throughout the Northeast and in the Northwest to Alaska, prefers to feed on white pine, but all the spruces and balsam fir can also serve as hosts.
In the spring, adult beetles emerge from their pupal cells in logs and, before they begin mating, feed on coniferous foliage and twigs, often killing small branches. Overall, damage by adult beetles is not usually significant.
After feeding on foliage for awhile, male beetles scout around for a suitable host tree, usually one that is dead or nearly dead, and defend a desired territory from other males. Larger beetles tend to occupy the best real estate and, perhaps for that reason, females have been found to prefer larger males. Piles of pine logs, waiting to go to the sawmill, are a whitespotted sawyer’s heaven.
In field-trapping experiments, female whitespotted sawyers have been found to be attracted by pheromones from bark beetles, suggesting that insect chemistry is capable of broadcasting to the whole tribe, not just one’s own species. Neither bark beetles nor whitespotteds are capable of surviving on a healthy tree, but when two or more insect species join the attack, it may put a stressed tree over the edge.
After mating, female whitespotted beetles chew slits into the bark of the host log and deposit one or more eggs in each slit. The larvae hatch out in about two weeks and begin feeding, making their way through the phloem and into the cambium, where they feed for several weeks. As the larva grows, its galleries become wider and deeper, forming grooves in the surface of the wood.
When young, the larvae do not penetrate deeply into the wood, but they do introduce blue-stain and other fungi, which either have been delivered by the adult along with her egg or have landed on the holes the female excavated in the bark. Except for stain, little harm is done in the first few months of beetle occupancy, because the bark is removed at the sawmill.
As the larvae grow, they head towards the center of the log, where their larger tunnels are most unwelcome in any normal wood product: at up to two inches long and 3/8 of an inch in diameter, they leave quite a tunnel behind. Dead wood is not the most nutritious food, and in the North it normally takes two years for a whitespotted sawyer to complete its life cycle. Cannibalism is commonly engaged in by larvae, an interesting way to improve the quality of your diet.
Although adult beetles are most active on sunny days, the egg slits are cut on the sides and bottoms of logs, away from direct sunlight. Still, they strongly prefer logs in the sun, so if you need to leave a pile of pine logs out in the summer, covering them with a nice layer of slash to shade them will reduce their appeal. Logs piled in the shade have far fewer beetle eggs as well. Shade, together with slash covers, is even better.
At up to one inch long, the adults are big, clumsy fliers with oversized antennae and a tendency to plop down for a rest just about anywhere, making them easy to spot and catch on sunny summer days. Though smaller than the dreaded Asian longhorned beetle (ALB), they are in the same family – the Cerambicidae – and share the same general shape and format. Fortunately, whitespotted sawyers have a nice little diagnostic feature that can bring instant reassurance: they have a small white spot at the very front edge of their wings, right in the middle, just aft of the thorax. Both the whitespotted and the ALB may have numerous other spots on their outer wings, but the position of the spot on the less-threatening species allows for instant ID. Another beetle sometimes confused with the ALB, the northeastern sawyer (Monochamus notatus), is as large as the ALB, but it is light in color, mottled, and has no distinct spots.