Wood-boring insects

It can be difficult to prove whether you have an active infestation of wood-boring insects, at most times of the year. Adult insects mainly emerge in the spring.

An active outbreak can sometimes be identified by holes with sharp (rather than rounded) edges, alongside bore dust. The interiors of the holes ('frass') are not dark, but the colour of freshly-cut timber. Otherwise, holes may indicate an old infestation, that's no longer active. In these cases, no action is normally needed.

In the UK, the most common wood-boring insects are furniture beetles and death watch beetles. Furniture beetles target both soft and hardwoods, and produce 1-2mm wide holes with lemon-shaped frass.

Death watch beetles might be less obvious, as adults sometimes emerge from cracks in the wood, rather than creating holes. However, they can sometimes be discovered by the distinctive sound they make when they tap their heads on timber, during their courtship period in the spring.

If you are not sure whether you have an active presence of beetles, you should monitor the situation for a year. Water-soluble glue can be used to cover a group of holes with acid-free tissue paper. If there are beetles, they will punch their way out.

You can also clean the surface underneath a suspect area - as well as nearby window sills - and document any beetles that accumulate in the space.

Wood-boring insects are a symptom of excess damp in a building (normal temperatures, and humidity, in a house creates a hostile environment for timber pests. The infestation is likely to continue until humidity levels are reduced.

This damp is typically caused by the addition of inappropriate, modern materials to a historic building.

The most common cause of damp in old buildings is impermeable outer finishes - such as cement render, or modern paints. If these materials are present, an insect infestation may persist until they are replaced with breathable alternatives.

Beetle infestations in roofs might be caused by rain somehow getting into the space, or by condensation - for example, if the roof has an old felt layer under the tiling (which would prevent airborne-moisture from escaping).

In these cases, the preferred solution would be the introduction of vents in the eaves, or creating gaps where the felt layers overlap. A more drastic solution would be to lift and re-lay the roof covering on a breathable felt.

If you have an active infestation, you might need to apply treatment straightaway.

A paste treatment is the preferred option, as it will penetrate the holes better than sprayed chemicals and is generally considered more appropriate.

We typically do not support spray treatment, as it is harder to control. Any treatment used should be limited to areas of active infestation only.

For a comprehensive spray treatment, you will need listed building consent. We would also expect to see an independent assessment of the buildings, which justifies its necessity (we do not consider treatments written by preservation treatment firms to be independent). We recommend that you use a surveyor, or an engineer who is accredited to work in historic buildings.

Spray treatment can also have a more limited effect on the insects, as it may not penetrate into the holes. It is more likely to kill spiders, which are predators of wood-boring insects.

Drilling holes into timbers to get the spray treatment closer to the insects, can be very damaging to historic fabric and is unlikely to be supported.

It might take some time for the last insects to leave, upon application of any treatment or after addressing the cause of any damp.

For more information, please read SPAB's advice on wood-boring insects ⧉.