Woodworm killer is a liquid, gel, or paste treatment applied to timber to eliminate wood-boring beetle larvae living inside the grain. Most retail products use one of two active ingredients: permethrin (a synthetic insecticide) or boron (a mineral salt). Both work, but they differ in toxicity, cost, and how deeply they penetrate, so choosing the right one depends on how severe your infestation is and what’s living in your home.
Before you buy anything, though, the first step is confirming you actually have an active problem. Many homes have old woodworm holes that haven’t housed a living larva in decades.
How to Tell if Woodworm Is Active
Woodworm larvae spend two to five years tunneling inside timber before emerging as adult beetles, so the damage you see on the surface is always old news. The key question is whether anything is still alive in there. Fresh exit holes look clean-edged and light-colored, as if someone pressed a pencil tip into the wood. Old holes darken over time and their edges soften.
The fine powdery dust (called frass) near exit holes is the classic sign people look for, but it can be misleading. Frass is produced by larvae chewing through wood, not by adult beetles emerging. In some cases, tiny ants forage through abandoned tunnels and push old dust out, creating the appearance of activity long after the beetles are gone. If you tap the timber and no fresh dust falls, and the holes look dark and weathered, the infestation is likely historical.
To confirm activity, try this: brush away all visible dust from the affected area in autumn or winter, then check again in spring and summer (beetle emergence season, roughly April through September). If new dust appears or you spot fresh, pale holes, you have a live problem worth treating.
Permethrin-Based Treatments
Most off-the-shelf woodworm killers sold in hardware stores contain permethrin, the same compound found in household fly spray. It works by disrupting the nervous system of insects on contact. Permethrin is considered low-toxicity for humans. The UK’s Health and Safety Executive has approved water-based permethrin treatments with an eight-hour re-entry time, meaning you can return to treated rooms after eight hours of ventilation. At that point, airborne permethrin levels drop well below the HSE’s safety threshold of 125 micrograms per cubic metre.
That said, safety standards vary internationally. German health authorities have flagged permethrin as a potential concern for up to two years after application in indoor spaces. If you’re treating a bedroom or a room with poor ventilation, factor in extra airing time beyond the minimum eight hours.
The major risk is to pets, specifically cats. Cats lack the liver enzyme needed to break down permethrin efficiently, making them far more sensitive than dogs or humans. If you have cats, either keep them out of treated areas until the product is fully dry and the room is ventilated, or choose a boron-based alternative instead. Permethrin is also highly toxic to fish, bees, and aquatic invertebrates, so avoid any runoff near ponds, streams, or beehives.
Boron-Based Treatments
Boron (sold under names like Boracol) is the low-toxicity alternative. Its toxicity to mammals is roughly comparable to table salt, it produces no odor, and it doesn’t evaporate out of the wood over time. That last point is a genuine advantage: once boron penetrates timber, it stays there as a long-term preventative. It also doubles as a flame retardant.
Boron treatments selectively kill wood-boring insects without harming spiders, birds, or bats, which makes them the standard choice when treating roof timbers where protected species may be roosting. The tradeoff is cost. Boron products tend to be more expensive than permethrin equivalents, and coverage rates are moderate. A 5-litre container of ready-to-use boron solution at 5% concentration covers roughly 10 square metres with the required two coats. A 25-litre container handles about 50 square metres, enough for two average rooms or a small roof space.
For areas where timber is damp, shows heavy attack, or exposes end grain (the cut cross-section of a beam), a boron-based brushing gel provides deeper penetration than a liquid spray. End grain absorbs treatment more readily, so gel on those surfaces combined with spray on flat faces gives the most thorough coverage.
How to Apply Woodworm Killer
Surface application with a brush or low-pressure garden sprayer is the standard method for most infestations. Spraying is faster, but brushing lets you work treatment into crevices and rough surfaces more thoroughly. Either way, the liquid only penetrates a few millimetres into the timber surface, which is enough to catch larvae tunneling near the outside but won’t reach deep-seated infestations.
Before treating, prepare the area properly:
- Strip the wood. Remove paint, varnish, or any surface coating that would block absorption. Bare timber is essential.
- Clean the surface. Brush away dust, cobwebs, and loose debris so the treatment contacts the wood directly.
- Wear full PPE. That means a P3-rated respirator mask, eye protection, chemical-resistant gloves, and clothing that covers all exposed skin. This applies to both permethrin and boron products.
- Apply two coats. Most products require two coats for adequate coverage. Allow the first coat to soak in before applying the second.
Coverage rates for boron products run about 4 square metres per litre with two coats. Permethrin products vary by brand, so check the container, but expect similar ranges. Ventilate the space during and after treatment by opening windows and doors.
When Surface Treatment Isn’t Enough
Surface spraying works for common furniture beetle (the small, 1-2mm exit holes most people encounter). But some species tunnel deep into structural timber where a surface spray can’t reach. Death watch beetle, which targets older hardwoods in period properties, is the main culprit. If you’re seeing larger exit holes (3-4mm) in oak beams or structural joists, surface treatment alone won’t solve the problem.
Deep infestations require injection. You can use a nozzle to squirt liquid treatment directly into exit holes, but a gel or paste product penetrates further and stays in place rather than running back out. Some professionals drill into the affected zone and fill the interior with gel to ensure full saturation of the damaged area.
For floorboards with deep infestations, both sides of each board need treatment. That means lifting the flooring entirely, treating the underside, then treating the top face after reinstallation. This is disruptive work, and if the timber has lost structural integrity from years of tunneling, some boards may need replacing rather than treating.
Any infestation that has compromised the structural strength of load-bearing timber (joists, beams, lintels) warrants professional assessment. A surveyor can probe the wood to determine how much sound timber remains and whether treatment alone is sufficient or if reinforcement or replacement is needed.
Choosing the Right Product
For a small, confirmed active infestation in furniture or non-structural timber, a ready-to-use permethrin spray is the cheapest and simplest option. Pick a water-based formula, apply two coats, ventilate for at least eight hours, and the job is done.
If you have cats, bats in the roof, or concerns about chemical sensitivity, go with a boron-based treatment. It costs more but stays effective in the wood indefinitely and poses minimal risk to mammals or protected wildlife. For damp timber or heavy infestations, add a boron brushing gel to your order for the worst-affected areas and any exposed end grain.
For deep structural infestations, particularly in older properties with large oak timbers, a gel injection product gives you the penetration that surface sprays can’t. If the damage is extensive or you’re unsure about structural integrity, this is the point where professional treatment pays for itself in avoided mistakes.