Borer in houses

Borer can be destructive to the structural timber in houses. Highly destructive over time.

There are a few species of borer in New Zealand. Borer are small beetles that have a long life cycle where the larval stage can spend many years attacking untreated timber, such as the sapwood of rimu that was commonly used in the construction of houses in Nelson/Tasman prior to about 1970. Common Borer, Anobium punctatum, and Native Borer, Leanobium flavomaculatum, are the most often found causing damage.

Borer are more prevalent with damp timber. That means the sub-floor is more commonly attacked than the drier roof space structure. Timber flooring, and weatherboards, as well as untreated timber furniture can be attacked. If not identified the timber may become damaged to the point of requiring replacement.

The best method to inhibit borer infestation in the sub-floor area of the house is to limit the humidity by having adequate sub-floor ventilation, and installing a polythene ground covering sheet if the ground beneath the house is often damp.

The adult insects lay about 30 eggs in summer on the surface of damp timber, or in old flight holes. The eggs hatch after a month or so and the larvae bore through the wood breaking down the cellulose as they go. After 3 to 5 years the larvae pupate near the surface and a month or two later create a 1–2 mm round flight hole which is what is visible in the affected timber. The adult insects are airborne for about a month, mate and lay eggs to continue the cycle.

The larvae need other micro organisms to help break down the cellulose in the timber, which is why the dampness is an important part of borer infestation.

Borer infested timber can be treated by the application of water soluble sodium borate that diffuses into the wood and acts as an insecticide, fungicide and preservative. Boron doesn’t break down over time. Boron kills an adult insect as it chews towards the timber surface. Killing adults is the usual way to break the life-cycle because the larvae are safe deep inside the timber. Surface treatment prevents the re-infestation of the timber for 10 years or more under normal conditions. The best time for spraying the affected timber is in spring prior to the adult insects emerging.

Treatment is usually a soaking spray, a two hour break and a second application. The manufacturers of the chemicals are reducing their guarantee from lifetime, to 10 years, or 5 years. The treatment needs to be effective for longer than the usual 3 to 5 year life of the larvae to eradicate all borer.

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