A. Beetles are indeed a serious problem, mainly because they spend most of their life tunneling inside the wood, making the wood very weak. Of course, the holes they make to the outside are not very pretty.

Actually, you did not give me enough information to make an educated guess as to what insect you actually have in the wood. The best advice is to have the insect positively identified by an expert; oftentimes your local county extension office has contacts at the state university that will be able to identify the beetles for certain.

However, there are three insects that are quite common in lumber and wood products in North America: lyctid powderpost beetles, anobiid powderpost beetles (sometimes just called anobiid beetles) and old house borers. A fourth group are the bostrichid beetles (includes the false PPB), which we see abundantly in the tropics, but this group are not too important or common in the dry wood industry in North America.

 

Key Questions

If you want to try to identify the beetles yourself, here are four key questions:

1. Are the insects in hardwoods or softwoods, and in the sapwood or heartwood?

2. Is the infestation active (new piles of sawdust or powder seen) or is the damage from a previous, but now inactive, insect infestation?

3. What is the moisture content of the wood when the insect was spotted(the wettest, average and driest MC)?

4. How large are the holes on the surface (to the closest 1/32 of an inch)?

 

Before looking at these questions in more detail, it is important to understand that the egg will hatch into a small worm-like phase of the insect called a larva (plural is larvae). The larvae can be active inside the wood for a few weeks (like the anobiid), or may take years before the larvae become beetles and bore to the surface and leave the wood. So, finding no exit holes when visually inspecting the lumber will not rule out the possibility of active infestation by the larvae of these beetles. However, if the kiln drying process heated the lumber to over 133F, we do know that the wood is free of living insects and their eggs.

1. Hardwoods vs Softwoods

The lyctid PPB only attacks hardwoods and the old house borer only attacks softwoods. The anobiid and the bostrichids can attack either, but the anobiid prefers softwoods and the bostrichids, hardwoods. Note that all four prefer sapwood, but the anobiid does work in heartwood as well.

A few female adult insects will lay their eggs in a hole that they bore into the wood, but many, including the lyctid PPB, lay eggs in the grain cavities of wood; so they prefer rough sawn lumber or grainy woods like red oak. They cannot lay eggs in wood with a smooth finish (varnish, for example).

2. Active or Inactive

The beetles, except for the lyctid PPB, do not like dry (under 15 percent moisture content or so). Therefore, with well dried wood, it is common to find that the remnants of a previous infestation is what is being seen. With an active infestation, we look for fresh dust or powder coming from the holes, and also look for new holes developing overnight. (Although the beetles do fly, they are nighttime movers, so are seldom seen, although they are attracted to lights.)

In most cases, the origin of an infestation is from wood brought into the manufacturing facility from an outside source, especially foreign sources. In a manufacturing facility with dry kilns (which will sterilize the lumber, as mentioned) and with short storage time, conditions are not favorable for starting a new infestation. Nevertheless, when dealing with both local wood and foreign wood, it is best to keep the two separate during storage. Also, always clean up any wood debris or dust in the storage area.

Note that if lumber does have some insects laying eggs on the surface, and if storage is short, the planing operation will eliminate the eggs before they hatch.

3. Moisture content

Although many insects inhabit the growing tree, there are only a few that like air-dried or kiln-dried wood. The preferred moisture content (this is the moisture content in the region where they are active) for lyctid PPB is generally 8  to 25 percent MC. The anobiid PPB likes 15 to 30 percent MC. The old house borer likes 12 percent or wetter wood. Bostrichids prefer wood over 12 percent MC.

Special note: All of these beetles, especially the anobiid beetle, can be found in air drying yards. Another common air-drying beetle is the ambrosia beetle.

4. Hole Diameter

The holes we see in wood are exit holes…that is, the larvae have been living in the wood for months to several years, boring and eating. Finally, they decide to exit and look for a mate. As the insects are different sizes, these exit holes will vary as well. The smallest holes (often 1/32 inch in diameter with none over 1/16 inch) are the lyctid PPB. The anobiid PPB is 1/16 inch and slightly larger. The old house borer is ¼-inch holes that are slightly oval. The bostrichids are about 1/8 to ¼-inch diameter holes.

 

Control

As mentioned, the larvae can do a lot of damage inside the wood during their life (weeks, months or a years). So in addition to eliminating the larvae, it is prudent to check the wood’s strength. Therefore, any infestation should be treated rather quickly, even though the larvae do move slowly.

Basically, there are two treatment choices, but neither treatment prevents future infestation if conditions for insect growth are adequate. The choices are to use an approved insecticide that will penetrate throughout the piece of wood (fumigation with a lethal gas is most common; a surface treatment is not sufficient), or heat the wood to 133F throughout.

As DIY chemical treatments do not get deep enough or are not powerful enough to stop existing activity, heat is the only reasonable approach for most woodworking operations. To avoid drying the wood out during heating, add moisture to the heating chamber to achieve 35 percent RH, or encapsulate the wood item in a plastic bog (that will not be affected by the heat) so that the moisture cannot leave.

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