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Beetles & Hardwood Floors

- How does wood become infested? -

Summary: Wood flooring can become infested with beetle larvae from eggs laid on unfinished wood surfaces. Hatching larvae bore into the wood and can result in extensive damage. Even kiln-dried wood can become infested. Wood flooring can be protected by sealing the surface and/or treating it with borate insecticide.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Wood floor infestations

There are a number of insects that develop in recently dead trees or milled lumber. The larvae of these wood borers feed on starch and other nutrients in the wood that were stored when the tree was alive. Most of these insects cannot infest live trees because of the tree's natural defenses. While most wood boring insects need relatively fresh wood, certain ones, like powderpost beetles, can re-infest older, seasoned wood.

Powderpost beetles (see What are Powderpost Beetles?) lay their eggs on the bare wood surfaces and tiny beetle larvae (sometimes called woodworms) bore into the wood through surface cracks. Extensive internal damage can be done by the larvae during the months or years in takes to complete development (see A Picture of a Lyctid Powderpost Beetle Larva). Adult beetles emerge through circular emergence holes (right).

lyctid beetle emergence hole in oak

Round emergence hole in oak flooring. These holes often have fine, powdery wood particles around them. The ruler marks are 1/16" apart.

Wood may even be infested even after it has been kiln-dried (heat dried) and milled into lumber. Infestation occurs when dried wood is improperly stored near an active infestation (see below). Kiln-drying alone does not protect against infestation, it only eliminates larvae that are present at the time the wood entered the dryer.

Is the infestation still active?

This is one of the first questions usually asked. Since emergence holes are permanent it is impossible to know for certain if the infestation is active if you find holes in the floor as pictured above. The best indication of an active infestation is the presence of fine, powdery boring dust associated with emergence holes. Another good indicator is the condition of the emergence holes themselves.

Use your hand lens or magnifying glass to look carefully at the emergence holes under a good light. If the walls of the hole are covered with floor finish (below right) you can conclude that the holes were made before the finish was applied and may indicate a old, inactive infestation. On the other hand, if the walls appear fresh you might conclude that beetles emerged recently indicating an active infestation.

Elongated emergence holes (right) also indicate that the holes were made before the wood was cut into flooring. Emergence holes are normally round, elongated holes occur when a round log is cut at an angle to the original surface. Another way to distinguish active from inactive infestations is to mark all existing holes with a grease pencil so that new ones can be spotted. You probably won't find beetles flying around the room.

Control of wood destroying insects

There are two products that can be used to protect new floors from wood destroying insects and fungi. These products are based on insecticidal borates (see What are Borate Insecticides?). Bora Care and Shell-Guard (see this article) are glycol-based borate products while Timbor (see this article) is a pure borate powder without any glycol additive. Timbor is mixed with water to apply. Some studies indicate that glycol helps to carry the borate into the wood, other studies found no difference.

lyctid powderpost beetle emergence holes in oak flooring

Elongated emergence holes in oak flooring. These holes are made when the log is cut at an angle to the original surface to make flat boards- this shape indicates that the holes were present when the wood was milled into lumber. Note also - shiny floor finish inside holes, another indication that holes were already present when floor finished was applied. Ruler marks are 1/16" apart.

All borate products should be applied before wood is stained and sealed for best results. Bora Care is a 40% solution whereas Shell-Guard is a 25.3% solution. Keep this difference in mind when comparing costs. Timbor is cheaper than either of the glycol-based borates but does not have the potential benefits of glycol.

Borate solution should be applied to the unfinished back and to the tongue and groove edges with a small brush or paint roller. Apply solution to the top only if it is unfinished. After the solution dries the wood will be impregnated with borate which will prevent beetle larvae from boring into the underside, or edges, once the floor is installed. Don't get the wood too wet as the flooring may warp. And, allow it to dry completely before installation. This procedure won't stop beetles already in the wood floor from emerging but it will prevent new attacks.

For infestations in existing floors you'll need to first determine if the infestation is active (see discussion above). If the infestation is active see Treating Active Beetle Infestations in Wood Flooring.

Related Articles

Other Insects That Damage Wood

What is Timbor Insecticide?

What is Bora Care and Shell -Guard?

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