Borer is unsightly and can sometimes affect a house’s structure.
Borer damage may be found in art deco houses. It is more likely to occur where sapwood has become damp in service, but it can be found in may native timbers that have become damp.
There are two common types of borer:
- the common house borer (Anobium punctatum)
- the long-horn or two-tooth borer (Ambeodontus tristis).
The easiest way to identify which one is affecting timber is to look at the holes they leave: the common house borer leaves a small round hole, the two-tooth long-horn borer an oval hole up to 5 mm long.
Both types of borer lay their eggs on a rough timber surface or in cracks or holes. The larvae bore into the timber, sometimes for up to three years. As adults, they bore their way back to the surface.
Fresh holes and dust that appear between November and March indicate that the infestation is still active.
Infestation by the common house borer generally tends to cease before timber becomes structurally unsound because these borer attack only the sapwood in the timber – most framing timber has sufficient heartwood to avoid serious weakening.
Infestation by two-tooth borer is far more serious as they can attack heartwood as well as sapwood. Structural failure sometimes occurs before their presence becomes evident.
For structural timber such as rafters and floor joists, the difficult decision is determining the extent of the damage and its effect on the structural performance of the framing. If two-tooth borer is present, the timber should be replaced. If the common house borer is present, the timber can often be left, strengthening is generally adequate.
For internal finishing timbers, the decision to replace infested timber is usually based on aesthetic reasons – whether the appearance of borer holes is acceptable or not. As the borer holes in finishing timbers are generally caused by the common house borer, major damage is unlikely to be a problem.
Precautionary measures against both types of borer are similar. Treat infested timber with a residual insecticide by brushing on or spraying surfaces, ensuring that flight holes are drenched or preferably injected. Using a small spray nozzle to inject the liquid into the holes is effective, but wear eye protection as the spray may shoot out another hole towards you.
If the infestation is extensive or fumigation is being considered, consult a specialist pest destruction firm that is a member of the Pest Management Association of New Zealand.