As temperatures drop, many of us are warming our homes by burning wood in wood stoves and fireplaces. To make it easier to start that fire first thing in the morning and keeping wood dry and ready to burn, it is common to store some firewood inside. When this wood warms up indoors, insects that have been overwintering or feeding on the cut wood often emerge, causing curiosity and sometimes concern among residents. The following information will help you identify and manage these insects that often move in with our firewood.
With few exceptions, insects found in firewood will not infest household furnishings.
The best way to avoid insects emerging in the home is to store wood outdoors until needed.
Some bark beetles in firewood, such as the mountain pine beetle and elm bark beetle, can infest nearby healthy trees.
Some exotic wood borers are a potential threat to Colorado’s forests.
Hundreds of insect species potentially can inhabit the wood of our native and ornamental trees. However, the great majority of cases involve a few basic groups: round-headed and flat-headed wood borers, bark beetles, carpenter ants, and powder post and anobiid beetles.
With few exceptions, insects found in Colorado firewood will not survive indoors and are capable of infesting only well-dried logs with intact bark. The primary problems with firewood insects involve a few species of bark beetles that can develop in firewood and later infest healthy trees.
By far, the most important of these insects is the mountain pine beetle, which kills large numbers of trees (primarily ponderosa pine) in natural forest areas. Elm bark beetles and, rarely, Ips beetles may also threaten healthy trees after emerging from firewood. Simple precautions can prevent injury by these firewood insects.
Common firewood and house log insects
Wood borers are the most frequently observed insects infesting firewood and house logs. Most common are round-headed borers, also known as long-horned 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 round-headed borers are gray-brown with black speckling (sawyers) or deep blue-black (black-horned pine borer).
Adult flat-headed borers, also called metallic wood borers, generally are smaller than round-headed borers. Flat-headed 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 boring dust that frequently cause alarm. This boring dust 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 boring dust 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. No western species normally re-attack in the same wood that produced them. 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 Dutch elm disease 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, or boring dust in bark crevices.
Eggs are laid in central pathways (egg galleries) constructed under the bark. The larvae feed on inner bark 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 product; they need dying or recently dead logs to reproduce and feed.
Powderpost and anobiid beetle infestations of structural wood and furniture are not common in Colorado, but can be serious. Native species do occur naturally in dead tree limbs and dry, seasoned wood. However, problems with these insects in Colorado appear to be associated with the introduction of infested wood products from eastern states. Fresh piles of fine boring dust and small, round holes (1/32 to 1/8 inch diameter) are possible signs of infestation.
Intact, sound logs are not used by carpenter ants (camponotus spp.). These ants nest in rotting, water-damaged wood and such logs are rarely used for firewood. Native populations of carpenter ants may develop within old rotting wood that has been stored improperly for long periods.
There is a widespread but unfounded concern about transporting termites in firewood or other wood products. Colorado termites nest underground. Under natural conditions, they rarely infest firewood and timber products. Occasional termites found in this wood are not in the reproductive stages. Furthermore, the low humidity in houses causes any incidental termites in firewood to quickly dry out and die. Colorado termites do not produce boring dust.
Firewood insects do not normally pose any hazards to people, household furnishings or plants. This is particularly true for the wood borers, the most conspicuous group of firewood insects.
It is hard to witness the activity of borers without feeling a need to take action, but, in reality, borers speed up the drying process and promote better burning.
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. The occasional insects that do manage to emerge indoors can be controlled by vacuuming.
If buying firewood, buy properly seasoned wood from local sources. Properly seasoned wood is characterized by being very dry, with loose bark, and many holes on the bark, indicating many insects exited the wood.
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.
When collected firewood is known to harbor mountain pine beetle or other undesirable species, the best option is to burn the wood before adult beetles begin to emerge in mid-July. Elm bark beetles emerge in mid-May.
To avoid wood infested by these insects, choose trees that have been dried for at least one year or that have noticeably loose bark. Check local ordinances, as it is illegal to store certain types of firewood (elm, for example).
If log piles are small and located in a sunny area, firewood insects can be killed because the wood and bark would dry quickly. The high temperatures produced will kill many insects inside the wood. Control of insects in logs at the top of the pile may exceed 50 percent, but insects in lower logs generally are not affected.
A more difficult but highly effective means of killing most firewood insects is to remove the bark. Debarking also prevents reinfestation and speeds drying. Also, kiln-drying logs kills any stage of bark or wood boring insects; this method is cost-effective if the logs are to be used as finished wood products.
Chemical controls may be needed in some situations to protect house logs. At present, insecticidal fumigants are not available for general use on firewood. Currently, no insecticides are registered for use in control of insects that infest firewood.
Exotic insects are those that are not native to Colorado. Some exotic insects may cause potential threat to our native vegetation. One of many ways exotic insects can arrive or be brought into Colorado is by human transportation of infested logs or unprocessed wood products (this includes wooden crates, pallets and packing material) from their native place or from quarantined areas. Because these insects would be new to the area, they probably would not come with their parasitoids and predators to keep them in check; thus, they can become a problem to Colorado’s vegetation and these infestations would possibly severely affect rural and urban forests.
Some of the insects that may be transported in firewood or logs can be:
• Asian longhorned beetle (anaplophora glabripennis).
• Emerald ash borer (agrilus planipennis).
• European sirex woodwasp (sirex noctilio).
Information provided by A. Leatherman, entomologist, Colorado State Forest Service; and W.S. Cranshaw, Colorado State University Extension entomologist and professor, bioagricultural sciences and pest management. Reviewed and revised by I. Aguayo, forester, Colorado State University.