Hylotrupes bajulus
House Longhorn Beetle
Hylotrupes bajulus — rare across most of Britain, but one of the most destructive wood-boring beetles we deal with. Large 6–10mm oval holes and serious structural softwood damage make professional treatment urgent.
- Large oval exit holes, 6–10mm — far bigger than common woodworm
- Attacks structural softwood: rafters, joists, purlins and wall plates
- Building-regulation significance in parts of Surrey and north-west London
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What it is
A rare beetle that does outsized damage
The house longhorn beetle (Hylotrupes bajulus) is the heavyweight of British wood-boring beetles. It is uncommon — most homeowners will never encounter it — but where it does take hold, it can hollow out the structural softwood of a roof or floor faster and more completely than any other species in the UK. That combination of rarity and destructiveness is exactly why it deserves to be taken seriously.
The adult is large by woodworm standards, typically 8–25mm long, brownish-black, with long antennae and a flattened body. But you are far more likely to find the damage than the beetle itself. Unlike the common furniture beetle, which leaves neat 1–2mm round holes, the house longhorn produces large, ragged, oval exit holes of 6–10mm — and it confines its attack to one thing: seasoned structural softwood.
How to identify it
Four signs of house longhorn beetle
The oval holes are the giveaway, but the way it damages timber is just as telling. If you spot any of these in softwood, treat it as urgent.
Large oval exit holes
6–10mm ragged, oval holes — several times the size of the round 1–2mm holes left by the common furniture beetle. The shape alone is a strong identifier.
Softwood only
Attacks the sapwood of seasoned softwood: pine, spruce and fir. That puts roof rafters, floor joists, purlins and wall plates squarely in its path.
Blistered, rippling surface
The timber surface may bulge or ripple while the skin stays intact, because the larvae feed just beneath it. Tapping can reveal a hollow, papery shell over an empty interior.
Coarse, gritty frass
Bore dust is cylindrical and tightly packed into the tunnels — coarser than furniture-beetle frass and often hidden inside the timber rather than spilling out.
Why it is so destructive
Damage you cannot see from the surface
The reason the house longhorn beetle is so dangerous is the gap between what you can see and what is actually happening inside the wood. The female lays eggs in cracks and crevices in softwood, and the larvae that hatch can feed for between three and eleven years before emerging as adults. Throughout that time they tunnel through the body of the timber, leaving the outer skin almost untouched.
The result is a rafter or joist that looks perfectly sound from below but is hollow inside — sometimes reduced to little more than a thin shell over a mass of packed bore dust. Because the larvae prefer the sapwood of structural softwood, the timber under threat is precisely the load-bearing material that holds a roof or floor up: rafters, ceiling joists, purlins, wall plates and floor joists.
This is why a house longhorn infestation is never a cosmetic issue. By the time exit holes appear, the larvae have usually been at work for years, and the surrounding structure may already be significantly weakened. Left untreated, the damage can progress to the point where joists or rafters fail and need replacing — which is where structural timber repair becomes part of the job alongside chemical treatment.
Life cycle
How the House Longhorn Beetle develops
Eggs laid in cracks
Larvae tunnel (3-11 yrs)
Pupation near surface
Adult exits leaving holes
Where it lives & the law
The Surrey connection — and the building rules
For decades, the house longhorn beetle has had a stubborn stronghold in a relatively small part of southern England — historically centred on north-west Surrey and the neighbouring fringes of south-west London. The warm, sandy heathland around towns such as Camberley, Woking and parts of the Surrey–Berkshire border created conditions where the beetle established itself in domestic roof timbers, and it has persisted there long after dying out elsewhere.
That concentration is significant enough to be written into the Building Regulations. In the defined affected localities, the regulations require that softwood roof timbers in new buildings be treated with a suitable preservative against house longhorn beetle attack. It is one of very few cases where UK building rules name a specific insect — a measure of how much structural risk this one species carries.
If your property is in or near these areas, it is worth confirming the position with your local authority building control team or a qualified surveyor. The presence of the regulation does not mean every home is infested — most are not — but it does mean the beetle is established locally and that softwood roof timber should be specified and protected accordingly. For the wider technical background on timber decay and treatment, BRE (Building Research Establishment) and the Property Care Association are reliable references.
Treatment & assessment
Why this one needs a professional
A suspected house longhorn beetle infestation should always be handled professionally, and it should be handled urgently. There are two reasons. First, correct identification matters: the large oval holes are distinctive, but a proper survey confirms the species, establishes whether the attack is active and — crucially — maps how far the structural damage has spread inside timber that looks fine from the outside. Second, the structure itself must be assessed for safety before anyone decides what to do next.
Treatment normally combines two strands. Affected and at-risk softwood is treated with a preservative or insecticide to kill larvae and protect sound timber — our woodworm treatment service applies the right product for structural timber. Where the larvae have already hollowed out a joist, rafter or wall plate beyond safe load-bearing capacity, chemical treatment alone is not enough: the weakened section has to be spliced, strengthened or replaced through structural timber repair. A good survey tells you exactly which timbers can be treated and which must be repaired.
DIY surface sprays are not a sensible response here. They cannot reach larvae feeding deep inside structural timber, they do nothing for sections that are already structurally compromised, and they give a false sense of security while the damage continues. Given the species' long larval stage and its appetite for load-bearing softwood, the safe course is a qualified survey followed by guaranteed, certificated treatment and any repair the structure needs.
Related guides & services
Structural Timber Repair
Splicing, resin repair and replacement of joists, rafters and beams once beetle damage has gone structural.
Learn more →Woodworm Treatment
Surveyed, guaranteed eradication of wood-boring beetle, with the right method for your timber.
Learn more →Common Furniture Beetle
The UK's most common woodworm — and the species the house longhorn is most often confused with.
Learn more →Signs of Woodworm
Exit holes, frass, tunnels and weak timber — how to tell an active infestation from an old one.
Learn more →Frequently asked questions
Is the house longhorn beetle common in the UK?
Why does it have its own building regulation?
How dangerous is house longhorn beetle damage?
What should I do if I think I have it?
Suspect house longhorn beetle? Act now.
With this species, time matters. Book a free survey and a qualified surveyor will confirm the beetle, assess the structure and quote for treatment and any repair — with a 30-year guarantee on treated timber.
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