How do I tell if I have woodworm, and does it need treating?

Woodworm is the collective name for the larvae of several wood-boring beetles, and the common furniture beetle accounts for most of what people find. The important judgement is not whether there are holes — most old houses have them — but whether anything is still alive in there, because dead damage needs no chemicals at all.

Safety

What you'll need

• A torch and a magnifier
• A bradawl or a penknife, to test timber softness
• Black paper or a dark cloth, to show frass (optional)
• A moisture meter (optional)

Step by step

1. Look at the holes themselves.Common furniture beetle exit holes are 1-2mm, round and sharp-edged. Fresh holes look pale and clean inside; historic ones are grey, dusty and often painted or polished over. Colour and edge condition are the first read.
2. Look for frass, the giveaway.Fine, gritty, creamy dust below the holes or on the surface beneath — like coarse talc. Wipe the area clean, lay dark paper underneath and check again after a fortnight in late spring or summer. Fresh frass means active.
3. Look for the beetles themselves in season.Adults emerge roughly May to August. Small brown beetles on a windowsill under an infested timber, or on the floor below a loft hatch, are direct evidence of an active infestation.
4. Identify the species from the hole size.1-2mm round: common furniture beetle, the usual find. 3-4mm oval: house longhorn, rare and serious, mainly in parts of Surrey and neighbouring counties. Larger holes in hardwood with bore dust like fine flour: deathwatch beetle, usually in old oak with damp.
5. Test the strength of the timber.Press a bradawl into the wood in an affected area. If it sinks easily, or the surface crumbles, the section has lost strength and that changes the question from treatment to repair or replacement.
6. Find the moisture, because it is usually the driver.Common furniture beetle thrives in timber above roughly 12 per cent moisture content. A leaking gutter, a failed roof, poor subfloor ventilation, a plumbing leak or a blocked airbrick is very often the real cause, and drying the timber makes it unattractive on its own.
7. Check the loft for bats before anything else.Droppings, staining at a gap, or a smell in a roof space. Bats and their roosts are legally protected, timber treatments are lethal to them, and any work in a roost requires advice and often a licence. Stop and take advice if there is any doubt.
8. Get an independent survey rather than a free one.A free survey from a company that also sells the treatment has an obvious incentive. An independent surveyor charging a fee will often conclude that the infestation is historic and nothing is needed, which pays for itself immediately.
9. Treat only what is active, and where it matters.Treatment is normally a boron or permethrin-based fluid applied to accessible surfaces, sometimes with injection into structural members. Furniture and small items can be treated individually or, in some cases, dealt with by freezing or heat.
10. Deal with structural loss properly.Where a joist end or a rafter has lost section, chemical treatment does not restore strength. That is a repair — splicing, sistering or replacement — and needs a structural view.
11. Recheck the following summer.Mark the holes you know about with a pencil ring, or fill them, and look for new ones and fresh frass next season. It is the cheapest and most reliable test of whether treatment worked.

Tips

• Old houses are full of historic woodworm holes and most need nothing at all. Treatment for damage that stopped fifty years ago is a common and expensive mistake.
• Second-hand furniture is a very common way infestation arrives. Inspect the underside and the back of anything before bringing it in.
• Fixing the damp is usually cheaper than repeated treatment and it addresses several other problems at once.

Common mistakes

• Treating on the basis of holes alone — Holes are exit holes, meaning beetles have already left. Without fresh frass, new pale holes or live beetles, there is no evidence of anything alive to treat.
• Spraying loft timbers without checking for bats — Roosts are legally protected and standard timber treatments are lethal to bats. Checking first is both a legal and a practical requirement.

If it doesn't work

Questions people ask