Active woodworm holes are clean, sharp-edged, and surrounded by fine powdery dust called frass. If the holes in your timber look dark, dusty, or filled with paint or varnish, the infestation is almost certainly old. The distinction matters because only active holes mean larvae are still tunneling through the wood and causing ongoing damage.
What Active Holes Look Like
The exit holes left by woodworm beetles are small, typically 1 to 2mm in diameter for the common furniture beetle, which is responsible for most infestations in the UK and Europe. When they’re active, these holes have a few consistent characteristics that set them apart from old damage.
Fresh holes have crisp, light-colored edges. They look like someone pressed a pin cleanly into the wood surface. The surrounding timber hasn’t darkened or weathered around the opening. By contrast, old holes tend to be darker, sometimes clogged with dust or grime, and may show signs of having been filled or treated in the past. If you run a fingertip near the hole and the wood feels soft or crumbly, that’s another sign of ongoing activity beneath the surface.
The most reliable indicator is frass: a fine, gritty powder that collects beneath or around exit holes. Fresh frass looks like sawdust and its color matches the interior of the wood. If you brush it away and it reappears within a few weeks, you have an active infestation. Old frass tends to be darker, clumped together, or absent entirely.
When New Holes Appear
Woodworm beetles emerge from timber in a seasonal pattern. The larvae spend months (sometimes years) eating tunnels through the wood before pupating near the surface and chewing their way out as adult beetles. This emergence typically begins in March or April once temperatures warm up and stay consistently mild. Activity peaks through spring and can continue into early summer.
This is why you’re most likely to spot fresh holes, new frass, or even small beetles near windows and sills between late spring and early summer. If you’ve noticed holes during winter months, they may well be old. That doesn’t guarantee the wood is clear, though. Larvae could still be feeding deeper inside, with new beetles not due to emerge until the following spring.
The Masking Tape Test
If you’re unsure whether your holes are active, there’s a simple monitoring technique that gives a definitive answer. Cover the exit holes with strips of masking tape, pressing them down firmly so the tape sits flush against the wood surface. A paintbrush handle works well for smoothing the tape into place. Don’t use paint or varnish for this, as you’ll need to remove the covering later if treatment is necessary.
Leave the tape in place through spring. If beetles are still living in the timber, they’ll chew through the tape as they emerge, leaving small punctures that are easy to spot. If May or early June arrives with the tape still intact, those particular holes are inactive. If you find punctures, you have confirmed active woodworm that needs treatment.
This test costs almost nothing and removes the guesswork. It’s especially useful when you’ve bought a property with visible holes and aren’t sure whether a previous treatment was effective.
Where Active Holes Are Most Concerning
A few exit holes in a dry skirting board or a piece of furniture are mainly a cosmetic problem. The same holes in a floor joist, roof rafter, staircase timber, or any load-bearing beam are a different situation entirely. The key question is whether the affected wood carries structural weight.
Timber specialists assess damage by considering the role of the timber, how deep and widespread the tunneling is, and whether moisture is present (damp wood attracts and sustains woodworm far more than dry wood). If you can push a screwdriver into the timber with little resistance, or if the wood feels spongy near the holes, the internal damage may be significant. Joists, rafters, lintels, and landing timbers all deserve closer inspection if they show signs of active boring. A structural timber survey can determine whether reinforcement or replacement is needed.
Common Hole Sizes by Species
The common furniture beetle produces the small round holes most people recognize, at 1 to 2mm across. These are by far the most frequently encountered. But other species leave different signatures:
- Deathwatch beetle: Holes around 3mm in diameter, typically found in old hardwoods like oak, often in historic buildings. These beetles prefer damp, partially decayed timber.
- House longhorn beetle: Much larger oval holes, roughly 6 to 10mm across. This species attacks softwood roof timbers and can cause serious structural damage relatively quickly.
- Powderpost beetle: Very small holes, around 1 to 2mm, but the frass is talcum-powder fine rather than gritty. Found mainly in hardwoods with high starch content.
Identifying which species is responsible helps determine how urgently you need to act. House longhorn beetle infestations, for instance, are treated as a more serious structural threat than common furniture beetle.
How Active Infestations Are Treated
Treatment for active woodworm targets the larvae inside the wood, since the adult beetles that create exit holes only live a few weeks and are mainly focused on mating. The most widely used approach involves liquid treatments containing permethrin, an insecticide approved specifically for wood preservation.
For accessible timbers like exposed joists or roof beams, the product is sprayed at low pressure across the wood surface, sometimes followed by brushing to ensure even coverage. If exit holes are already present, a syringe can be used to inject the treatment directly into the tunnels, pushing the product deeper into the galleries the larvae have created. Smaller items like furniture legs or picture frames can be treated by brushing alone, or even by immersing the piece in the solution.
The treatment works by soaking into the wood and poisoning larvae as they feed. How deeply the product penetrates depends on the solvent it’s carried in and the density of the timber. For serious infestations in thick structural timbers, professional treatment ensures adequate penetration. Surface-level sprays from hardware stores can work for minor problems in thin or non-structural wood, but they won’t reach larvae tunneling deep inside a substantial beam.
Microwave treatment is an alternative used for delicate or valuable pieces. It heats the wood from within, killing larvae, eggs, and pupae in timber several centimeters thick without introducing chemicals. This approach is more common in restoration work on antiques or heritage timbers.
Preventing Reinfestation
Woodworm beetles seek out timber with higher moisture content. Keeping indoor humidity low and ensuring good ventilation in roof spaces, subfloor voids, and basements is the single most effective prevention measure. Timber with a moisture content below about 12% is far less attractive to egg-laying beetles. A cheap moisture meter can tell you where problem areas are.
Fix any sources of damp: leaking pipes, blocked gutters, rising damp, or condensation in poorly ventilated spaces. Once the wood dries out, existing larvae often fail to complete their lifecycle, and new beetles are unlikely to choose that timber for egg-laying. Treating wood with a preservative after resolving the moisture issue provides an additional layer of protection that lasts for years.