How to tell if woodworm is active

You have lifted a floorboard, opened a loft hatch or pulled a wardrobe away from the wall, and there are small round holes in the timber. The question you want answered is not what made them. It is whether anything is still in there.

Put another way: how do you know if woodworm is active rather than long finished? Here is the short answer, before any beetle biology. Holes on their own prove nothing. Every hole you can see is an exit, chewed by an adult beetle on its way out, and that beetle may have left last June or in 1958. Three things tell you whether the infestation is live: the colour of the wood inside the hole, whether fresh bore dust appears after you have cleaned the timber, and the month you happen to be looking. A hole that is pale and clean inside with a sharp rim is recent. A hole that is grey, dusty and darkened inside with a soft rounded rim is old. Fine gritty dust that comes back within a week or two of being brushed away, especially between May and August, means live larvae underneath.

And one more thing, which most firms will not lead with: a great deal of the woodworm people find in Scottish houses is historic, finished, and needs nothing done to it at all. Old holes in a dry, sound joist are a record of something that stopped decades ago, usually when the damp that fed it dried out. If that is what you have, the correct amount of money to spend is nothing.

How to tell if woodworm active or dead: four checks

The question usually arrives as: how do I know if woodworm is active, in one test? There is no single test. Work through these four in order instead. None is conclusive alone, which is the whole trick to it: you are building a picture out of four weak signals rather than hunting for one strong one.

1. Look inside the holes. Colour and rim sharpness separate recent exits from old ones.
2. Find the frass, then remove it. What comes back is worth far more than what is already there.
3. Run the tissue paper test. It gives you a dated, physical record of a beetle emerging.
4. Check what month it is. Adults emerge roughly May to August, so a February inspection answers much less than a June one.

What active woodworm holes look like

The common furniture beetle, Anobium punctatum, makes a round hole about 1 to 2mm across, roughly the diameter of a pencil lead. Get a torch onto it and look into the hole itself rather than at the surface around it.

Active woodworm holes are pale inside. The wood the beetle cut through on its way out is the same clean colour as freshly sawn timber, because it has not been exposed to air, dust or light for long. The rim is sharp and slightly ragged, like a hole punched through card, and the fibres at the edge still stand proud. There is often a faint scatter of pale dust immediately below.

An old hole reads completely differently. The inside has gone grey or brown, the same shade as the aged surface around it, sometimes with cobweb or general household dirt in the mouth of it. The rim has softened and rounded off. On a waxed or polished piece of furniture the giveaway is better still: old holes fill with decades of polish and dirt, so a hole with a clean bore in an otherwise waxed surface stands out immediately as something that happened after the last polishing.

Two cautions. Holes noticeably bigger than a pinhead, around 3mm, in old oak or another hardwood may be deathwatch beetle, Xestobium rufovillosum, which is a different job and worth saying when you book anybody. And on timber that has been sanded, stained or painted since the beetles left, every hole will look wrong, because the finish has changed what you are looking at. On a floor that has been sanded and sealed, all bets are off until you get underneath it.

Frass, and the mistake nearly everybody makes with it

Frass is the bore dust the larva packs behind it and the emerging adult pushes out. In common furniture beetle it is pale cream to light brown, and the texture is the clearest thing about it: it feels like fine dry sand or table salt between finger and thumb, gritty rather than soft. Rub household dust between your fingers and it smears. Rub frass and it rolls. Under a hand lens the individual grains are tiny lemon shaped pellets with pointed ends, which is the detail that separates it from sawdust or plaster dust.

It collects in small cones directly under holes, along the top edge of a joist, in the angle between a joist and the floorboards above, on top of anything stored under a loft rafter, and on the shelf below an infested piece of furniture.

Now the mistake. Frass sitting on a surface does not mean the woodworm is active. Bore dust can sit in a hole or a gallery for years and then get shaken loose by somebody walking heavily across the floor above, by a door slamming, by a joiner lifting a board, or simply by you pulling the wardrobe out to look. Old timber that has just been disturbed sheds old dust. This is why a survey that hangs everything on “there was frass present” is not telling you much.

The frass that counts is the frass that appears after you have removed all of it. So remove all of it.

The tissue paper test

This is the closest a homeowner can get to proof, it costs nothing, and it is the single most useful thing to do before you ask anybody to quote.

1. Clean the timber completely. Brush and then hoover every trace of dust from the holes, the surrounding surface and whatever sits underneath. Be thorough. Anything you leave behind will confuse the result.
2. Cover a patch of holes with tissue paper. Ordinary white tissue or thin lining paper, taped down at the edges with masking tape so it lies flat across a cluster of holes. Do two or three separate patches in different places if the holes are spread out.
3. Put clean white paper underneath as well. A sheet of A4 or a strip of lining paper on the floor of the void, or on the shelf below the furniture, catches anything that falls.
4. Write the date on it and leave it alone.
5. Leave it through the emergence season. Ideally several weeks between May and August. A fortnight in November tells you very little.

An adult beetle emerging from a covered hole cannot get out without punching through the tissue, so what you are looking for on your return is a small ragged tear with pale dust around it. That is unambiguous: a beetle came out of that hole, after the date you wrote down. Fresh gritty dust on the paper underneath, when there was none when you left, is nearly as good.

What the test does not prove is the reverse. Intact tissue and clean paper are encouraging, but they are not a clean bill of health. The larval stage runs anything from two to five years inside the timber, so a grub three years deep is alive and producing nothing you can see. Your tissue also only covers the holes you found, in the timber you could reach. A negative result over one summer is a good sign. A negative result over two consecutive summers is close to an answer.

Why the month matters more than people expect

Adults of the common furniture beetle emerge roughly between May and August. They live a couple of weeks, mate, lay eggs in cracks and open grain, and that is the extent of their public life. Everything else happens out of sight.

So the honest position on a winter inspection is that it can be genuinely inconclusive. In February there are no new holes because nothing has come out since August, no fresh frass for the same reason, and no beetles to find. Anyone who tells you in January that your floor is definitely clear is over-reading the evidence, and anyone who tells you it is definitely active is doing the same in the other direction. If you are in the middle of a purchase and cannot wait for June, that is a fair reason to have somebody lift boards and look at the timber itself rather than the signs on its surface.

What a torch tells you, and what a blade tells you

Two tools change what you can see, and neither costs much.

A torch held low. Do not shine it straight at the timber. Hold it almost flat to the surface so the light rakes across the grain from one side. Flat light hides everything. Raking light throws a shadow out of every hole and turns a scatter of frass you would never notice into an obvious pale drift. This one trick finds more fresh holes on a joist than anything else a homeowner does, and it works just as well on the underside of floorboards seen from a cellar or a void.

A bradawl, a penknife or a small screwdriver. Press the point into the timber at the joist end, along the bottom edge, and anywhere the surface looks darkened. Sound timber resists. Timber that takes the blade to a real depth with barely any pressure, or that crumbles rather than splitting, has lost strength. This is a different question from whether beetles are still eating, and it is the more important of the two. Beetles that left in 1970 having eaten most of the sapwood out of a joist end still leave you with a weak joist end today.

Beetles on the windowsill

Small brown beetles a few millimetres long, alive or dead, on an interior windowsill in early summer are strong evidence. Emerging adults fly towards light, so a sill is where they end up and where they die. Dead beetles in June, in a room where you have also found pale sharp holes, is about as clear as domestic evidence gets.

Keep two things in mind. First, check the timber in that room rather than assuming: a beetle that flew in from a neighbouring flat, or came in inside a piece of second hand furniture or a bag of logs, is telling you about somewhere else. Second, dead beetles that have been on the sill for years, dry and dusty behind the curtain, are historic like everything else. Sweep the sill and see what turns up next summer.

How can you tell if woodworm is active in a floor you cannot see into?

Honestly, often you cannot, and this is where the Scottish building stock makes the question harder than the textbooks suggest.

A tenement floor is suspended timber over a void you have no access to without lifting boards, joist ends built straight into solid sandstone with no cavity and no damp proof course between the wet masonry and the wood. The airbricks that were meant to keep a draught moving through that void have spent a century being blocked by flowerbeds, decking, a later back extension or a raised path. The result is timber sitting at a raised moisture content, in the dark, for decades.

That matters because Anobium punctatum strongly prefers damp timber, above roughly 12 per cent moisture content. Dry, heated, well ventilated wood is a poor home for it. A cold damp floor void, a cellar, the timber against a gable that takes the weather, or a loft with a slipped slate above it is a good one. This is exactly why woodworm and a damp problem so often turn up in the same report, and it is also why they usually have to be fixed in that order. Treating the beetles and leaving the water is how people end up paying twice, because the conditions that let the first infestation run are all still there for the next one. Where there is real damp there is often wet rot alongside the beetles too, and if you are trying to work out which is which, the difference between the rots is set out on wet rot vs dry rot.

So the practical answer to how to know if woodworm is active in a floor you cannot open up: take up two or three boards in the worst room, usually the one with the bathroom, the outside wall or the springy patch. Get the torch in at a low angle, clean everything, lay your paper, and look again after the summer. If lifting boards is beyond you, that is the point at which a look from somebody with a moisture meter is worth the money.

Most of it needs nothing done

This is worth repeating on its own, because it is the finding that saves people the most money. A large share of the woodworm found in Scottish homes is old, dead and structurally irrelevant. Holes in a joist that is dry, hard under a blade and doing its job are history, not a problem. There is no treatment that improves them, no reason to spray them and no reason to let anybody sell you a certificate about them. Knowing how to tell if woodworm is active is worth as much for the times it lets you walk away as for the times it catches something. If you have clean tissue paper after a summer, no fresh frass, grey holes with rounded rims and timber that resists a bradawl, the sensible answer is to leave it alone and look again in a year or two.

When it stops being a judgement you should make yourself

There is a clear line, and it is not about the beetles.

• The timber is structural. Floor joists, roof timbers, lintels, stair strings. Anything holding something up.
• A blade goes in. Timber that gives under a penknife has lost section, whether or not anything is still living in it, and the question has quietly changed from an insect one to a strength one.
• You cannot see the extent. Floor voids, the far side of a joist, timber buried in lath and plaster or built into masonry. Judging an infestation from the 10 per cent of it you can see is guessing.
• There is damp you have not traced. A live infestation is a symptom. Find the water first.
• A lender, a home report or a purchase deadline is involved. Your own careful tissue paper test, however well done, will not satisfy anybody who has asked for a specialist timber report in writing. That needs a timber survey.

If none of those apply, you are genuinely fine to make this call yourself, and if it turns out to be a small live patch in accessible timber, treating it is not difficult either. What that involves is on the woodworm treatment page.

If any of them do apply, stop testing and get it looked at. Somebody should be lifting a board, taking moisture readings, probing joist ends and telling you plainly, in writing, whether what you have is live or historic and whether the timber has lost strength. That is what a woodworm survey is for, and the answer “there is nothing here worth treating” is a perfectly good outcome for one.