Woodworm larvae are the immature grub stage of wood-boring beetles, and they’re responsible for virtually all the damage associated with woodworm. The adult beetles live only days to weeks, but their larvae spend two to five years tunneling through timber, quietly eating it from the inside out. If you’ve spotted tiny holes or fine dust near wooden furniture or structural beams, larvae are almost certainly the cause.
What Woodworm Larvae Look Like
Woodworm larvae are small, creamy-white grubs with a curved, C-shaped body. They have a slightly darker head and visible ridges along their segments. Depending on the species and how far along they are in development, they range from about 1 mm when newly hatched to around 6 or 7 mm near the end of their feeding stage. You’ll rarely see them unless you break open infested timber, since they spend their entire larval life buried inside the wood.
The Life Cycle Inside Your Timber
The cycle starts when an adult female beetle crawls into small natural cracks in timber and lays her eggs just beneath the surface. This protects the eggs and gives the larvae immediate access to food when they hatch a few weeks later.
Once hatched, the larvae begin eating. This is the destructive phase, and it lasts between two and five years. During this time, the larvae chew a network of tunnels through the wood, gradually hollowing it out while leaving the surface mostly intact. When they’re finally ready to mature, they move toward the surface and form a small chamber where they pupate and transform into adult beetles. The new adult then chews through the final thin layer of wood, creating the characteristic round exit hole, and emerges to mate. Female beetles live 10 to 14 days after emerging. Males survive only 3 to 4 days. In that brief window, they mate and lay eggs in new timber, restarting the cycle.
Species That Cause the Most Damage
Several beetle species produce wood-boring larvae, and each has preferences for different types of timber.
- Common furniture beetle: Responsible for roughly 80% of all woodworm damage in homes. Its larvae eat both softwoods and hardwoods, targeting floorboards, roof timbers, furniture, and old plywood.
- Deathwatch beetle: Possibly the most destructive species overall. Larvae prefer hardwoods like oak, ash, and sweet chestnut that have already been softened by fungal decay. This makes historic buildings particularly vulnerable.
- Powderpost beetle: Infests the starch-rich sapwood of wide-pored hardwoods such as oak, elm, and walnut. It cannot attack softwoods because the pores are too small and lack starch. Typically found in timber under 15 years old.
- House longhorn beetle: Targets the sapwood of most softwoods. Its larvae can grow considerably larger than other species, and the structural damage can be severe.
How to Tell If Larvae Are Still Active
The tricky thing about woodworm is that exit holes can persist in timber for decades after an infestation has ended. Old holes alone don’t mean you have a current problem. What you’re looking for is evidence that larvae are still feeding inside.
Fresh frass is the most reliable indicator. Frass is the fine, powdery dust that larvae produce as they eat through wood. If you see small piles of pale, gritty dust beneath holes or along timber surfaces, that suggests active tunneling. You can test by cleaning the area thoroughly and checking again after a few weeks. If new dust appears, something is still feeding. Placing the wood on a white cloth for several days can also reveal fresh frass as it falls.
Adult beetles typically emerge between May and September. If you spot small, brown beetles near windowsills or around wooden furniture during these months, that’s another strong sign. The exit holes left by newly emerged beetles tend to look clean-edged and pale inside, while old holes darken over time and may fill with dust or debris. Crumbling timber, especially if it feels soft when you press a screwdriver into it, indicates significant internal damage from prolonged larval feeding.
Moisture and Why It Matters
Timber moisture content plays a major role in whether larvae can survive and thrive. The common furniture beetle generally requires a relative humidity above 60% for eggs to hatch and for pupation to occur. In practical terms, if the deep moisture content of timber stays below 12%, conditions are too dry for new infestations to establish and progress through a full life cycle.
That said, larvae already present in wood can be surprisingly resilient. Active larvae have been found feeding in timber with moisture levels as low as 9% to 12%, which is common in well-ventilated buildings. So while keeping timber dry is an excellent preventive measure, it won’t necessarily kill larvae that are already embedded. Drying alone is not a guaranteed cure for an existing infestation.
Structural Risks of Long-Term Infestation
Because larvae feed for years with little visible damage on the surface, infestations can silently weaken load-bearing timber. Over time, the network of internal tunnels reduces the wood’s strength and load-bearing capacity. In bad infestations, enough timber gets eaten away to cause noticeable problems: sagging floors, bouncy stairs, weakened roof timbers, and ultimately the need for expensive structural repairs.
A floor that feels springy or bouncy underfoot is one warning sign that internal damage may be extensive. By the time the surface starts crumbling or breaking apart, the structural compromise is already significant.
Treatment Options
Treating woodworm means killing the larvae deep inside the timber, which presents an obvious challenge since they’re hidden beneath the surface.
Chemical treatments are the most common approach. These typically use permethrin as the active ingredient, applied as a spray or brush-on liquid that soaks into the wood. The treatment works through contact when larvae encounter the chemical in their tunnels, or through ingestion as they consume treated wood. Permethrin doesn’t dissolve well in water on its own, so effective water-based products use a co-solvent to carry it deeper into the timber. Solvent-based formulations generally penetrate further.
For larger-scale or harder-to-reach infestations, heat treatments offer an alternative. Microwave treatment uses electromagnetic energy to heat the wood internally. Larvae and eggs die when their temperature reaches around 55°C (smaller larvae at 52 to 53°C), while the wood itself stays cooler at roughly 45°C. Hot air treatment works differently, using generators that produce air at 80 to 100°C, piped into a sealed space until all the timber reaches 55 to 60°C throughout. This method is commonly used for wooden attics, though the space needs to be sealed with thermal blankets to retain heat during the process.
Preventing Infestations
The most effective prevention comes down to controlling moisture and limiting access. Keeping timber moisture below 16% for sustained periods makes the environment inhospitable to most wood-boring beetles. Good ventilation, proper damp-proofing, and addressing any sources of moisture penetration are the core strategies. Standard building maintenance, fixing leaks, ensuring air circulates around timber, goes a long way.
Surface finishes also help. Varnishes, paints, and sealed surfaces physically prevent adult beetles from depositing eggs into the wood grain. Timber that has a smooth, finished surface offers fewer of the cracks and crevices that female beetles seek out for egg-laying. UV light traps and pheromone traps can supplement drying efforts by capturing emerging adults before they have a chance to lay eggs in new timber.