Facts About Old House Borers

Old house borers are typically found in residential settings in wood that is less than 10 years old. Although, they do prefer the protein-rich sapwood of younger wood. Often, what happens is that wood harvested from a woodlot had old house borers, and they develop in the lumber if the treatment at the log yards did not remove them all. Those larvae stay hidden in the lumber for years, until they emerge as adults leaving large exit holes that are quite distinct. And that is when they are noticed.

The heartwood portion of the wood is less desirable as the protein declines as wood ages. Consequently, infestations are most common in structures less than about 10 years old. Old house borers only infest softwoods (needle-bearing conifers) such as pine, fir, and spruce — they will not attack hardwoods such as oak, ash, maple, or poplar. Consequently, infestations within buildings are usually associated with structural components (studs, joists, beams, etc.) rather than manufactured items such as furniture or flooring.

Female old house borers lay their eggs (about 50- 200) in cracks, crevices, and joints between wood members. The eggs hatch in a few weeks and the tiny larvae bore into the wood to feed. The larvae grow slowly, inflicting progressively more damage as they tunnel within the wood. Feeding and damage tend to be greatest closer to the surface where the protein content of the sapwood is highest. Under optimal conditions, larvae may mature in as little as 2 years, but the average development time for the old house borer is about 3 to 6 years. In cooler locales, or if conditions within the wood are less favorable, development can be prolonged 10 years or longer.

Moisture has an important effect on this — higher wood moisture contents (15-25 percent) is optimal conditions for larvae maturation. When wood moisture levels are below 10 percent, the larvae are unlikely to survive. Once mature, they tunnel to the surface, cut an oval-shaped emergence hole, and temporarily plug the hole with bits of wood. Some weeks later, the insect completes its development and emerges as an adult beetle. Old house borers are highly destructive pests in log homes, which often sustain more serious damage than conventional frame construction.

There are different options for managing old house borers. Choosing the best approach depends on factors like the degree of damage, likelihood of re-infestation, and effort/expense. As with other wood-infesting pests, old house borers typically enter buildings in infested lumber. Controlling moisture helps. Infestations usually originate where wood is processed and stored, or at the construction site. The most vulnerable time for exposed lumber to become infested is when adult beetles are emerging and laying eggs, especially June through August. Before wood is used for construction, most of the water should be removed by air-drying or kiln drying, which is typically done by the silvicultural industry. Silviculture is the practice of controlling the growth, composition/structure, as well as quality of forests to meet values and needs, specifically timber production. Wood that is suspected of harboring beetles should not be used, especially if emergence holes are present. Some of the most serious infestations occur from using repurposed lumber from a barn, etc. to panel a room or build an addition.

The best way to avoid such problems is early detection and employment of other corrective measures. However, if you suspect old house borers, a structural fumigation may be warranted when infestations have spread into walls, between floors, and other areas where access for surface treatment or wood removal is impractical. Portable wood material such as a stack of lumber can be fumigated more efficiently and economically under tarps or in trailers.