Firewood Insects

By Kurt Jones, Chaffee County Extension Director 

That fire in the woodstove feels good on these cooler fall evenings, but the firewood can be a source of nuisance insects being introduced into the home. Though most insects will not attack home furnishings, these insects can be troublesome for the diligent housekeeper. Fortunately, I am not a diligent housekeeper. 

There are literally hundreds of insects that can attack our native trees, however several common ones can be found associated with firewood. Wood borers are the most frequently observed insects infesting firewood and house logs. Most common are roundheaded borers, also known as longhorned borers or sawyers. Adults are medium to large beetles (1/4 to 2 inches), often with long antennae that may exceed the body length. Common roundheaded borers are gray-brown with black speckling (sawyers) or deep blue-black (black-horned pine borer). 

Adult flatheaded borers, also called metallic wood borers (see picture), generally are smaller than roundheaded borers. Flatheaded borers commonly are gray, bronze or blue-green with a metallic sheen and have inconspicuous antennae.

Borer larvae are slender, white, segmented grubs with brownish heads and rather prominent jaws. These larvae produce the chewing noises and piles of wood-colored sawdust that frequently cause alarm. This sawdust material may be relatively fine or coarse and fibrous. These borers also are responsible for the wide zigzag or meandering tunnels seen beneath the bark and deep in the wood. The tunnels of both groups are oval in cross-section, not perfectly round. 

Wood borers are primarily a nuisance. The noise and sawdust they produce is suggestive of termites and, thus, disconcerting. Because of their long life cycle, borers may be present in wood for a year or longer. They do not emerge and attack healthy trees. Furniture, wall framing or other seasoned woods are not suitable for wood borer attack. Despite producing what may seem like great quantities of dust, borers rarely tunnel extensively enough to cause structural failure. Adult borers found inside the home may look ominous and pinch the skin if handled, but are not dangerous. 

Bark beetles commonly infest dead or dying trees and then appear in firewood from such trees. Several well-known tree killers and disease vectors are the mountain pine beetle, European elm bark beetle and Ips beetles. Adult bark beetles are small (1/16 to 1/4 inch), dark and bluntly cylindrical. Infestation on conifers usually is marked by a glob of pitch (pitch-tube) at the point of attack. Eggs are laid in central pathways (egg galleries) constructed under the bark. The larvae feed on wood as they chew at right angles from the central gallery. 

Most bark beetles have a one-year life cycle, but a few can complete generations in two-month intervals. Bark beetles cannot reproduce in household wood products. 

Problems with firewood insects emerging in the home are best handled by storing firewood outdoors until needed. Outdoor storage will greatly slow insect development during the winter and limit the opportunity of insects to emerge inside a home. Vacuuming can control the occasional insects that do manage to emerge indoors. To limit firewood insect infestations, stack wood so air readily flows through the pile. Well-dried wood will not invite bark beetle attack. The drying process can kill many developing bark beetle larvae already present in the wood.