Woodworm — active or historic?

What woodworm is

“Woodworm” is not a worm. It is the larval stage of any of several wood-boring beetles. An adult lays eggs on or in timber, the larvae tunnel through it for anything from two to five years, and then emerge as beetles — and it is that exit that makes the round flight hole you can see. The damage is done inside, out of sight, by the larvae.

This matters for one practical reason: the holes are where beetles left, not where they are. A ceiling of old flight holes may be evidence of an infestation that died out decades ago when the roof was repaired and the timber dried.

Telling live from historic

• Fresh frass. Live activity produces gritty bore dust, the colour of clean new timber, in small cones below the holes or caught in cobwebs. Old frass is grey, dusty and settled.
• Sharp-edged holes. Recent exits are clean and pale inside. Old holes are darkened, dusty and often painted over.
• Beetles. Dead adults on window sills below infested timber, generally in late spring and summer when they emerge.
• Moisture. Most of the species that matter need timber above roughly 12–14% moisture content. Genuinely dry timber does not sustain the common furniture beetle for long, which is why woodworm and a damp problem so often turn up together.

Which beetle

The species changes the seriousness considerably.

Common furniture beetle

By far the most frequent, and responsible for the great majority of the small round holes about 1–2mm across found in joinery, floorboards and roof timbers. Usually a nuisance rather than a structural threat, though decades of unchecked activity in a small section can weaken it.

Deathwatch beetle

Larger holes, around 3mm, in old hardwood — oak beams in particular — usually where there has been fungal decay first. Slow, persistent and much more serious structurally. Found in older and historic buildings.

House longhorn beetle

Rare in most of the country but very destructive, with large oval exit holes in softwood.

Wood-boring weevils

Associated with timber that is already decayed and wet. The presence of weevils is telling you about the moisture and the rot more than about insects.

What we recommend, and when we don't

Where activity is genuinely live and widespread, insecticidal treatment of the affected timber is appropriate, along with removing the reason the timber was damp enough to sustain it. Where the damage is historic, the right recommendation is often no treatment at all — just fix the moisture problem and check again.

Where larvae have removed enough section to matter, the answer is not chemical but carpentry: splicing, strengthening or replacing the weakened timber. We assess that as part of a timber survey, because insect attack and fungal decay commonly sit side by side in the same joist end.