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Huhu in house timber: a moisture problem first

In the bush, huhu larvae feed in dead and rotting wood, and they belong there. Inside a house the meaning changes: it means some of the timber is staying wet. A borer inspection records the moisture as well as the beetle.

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Huhu, Prionoplus reticularis, are New Zealand's largest beetles, and their larvae need damp wood. Young larvae establish only in moist timber, and older larvae keep developing in seasoned timber only while its moisture content stays at about 25% or more. Huhu in a house is therefore a moisture finding first: sound, dry framing is not where they live.

How do you recognise huhu in a house?

By size. Huhu adults are up to about 50 mm long, the largest beetle in New Zealand, and the grubs grow to 50–70 mm. Their exit holes are large and oval, 10–20 mm at the widest, far bigger than any other borer hole in a house. Adults fly at night from November to early March and are strongly drawn to lights.

Exit-hole size guide for four New Zealand wood-boring beetles Holes drawn at six times life size beside a millimetre scale: powderpost 0.8 to 1.6 mm round, common house borer 1 to 2 mm round, two-toothed longhorn oval 3 to 7 mm long, huhu oval 10 to 20 mm. Exit holes at 6× life size 0 10 20 mm Powderpost beetle Lyctus brunneus talc-fine dust, hardwood only smallest largest round, 0.8–1.6 mm Common house borer Anobium punctatum gritty dust, sapwood smallest largest round, 1–2 mm Two-toothed longhorn Ambeodontus tristis packed dust, reaches heartwood smallest largest oval, 3–7 mm long Huhu beetle Prionoplus reticularis damp or decaying wood smallest largest oval, 10–20 mm Source: NZFFA Forest and Timber Insects in New Zealand leaflets. Hole sizes vary; use with the dust and the timber type.
Huhu holes dwarf the other three beetles' holes. A hole this size in house timber is a reason to look for damp.

Why does huhu mean the timber is wet?

Because the larvae cannot develop in dry wood. Newly hatched huhu larvae need moist wood to get established, and the NZ Farm Forestry Association says older larvae carry on in seasoned timber only while its moisture content stays at or above about 25%. Framing in a dry, ventilated house should sit well below that, so huhu activity marks a place where water is getting in or staying.

In damp conditions the damage can be serious. The Association records rimu framing destroyed by huhu and describes structural timber reduced to a thin outer shell. A huhu finding is therefore both a timber question and a building question: the wood may need replacing, and the source of the water needs finding first.

Where does the moisture in house timber come from?

In older Auckland houses the usual sources are slow roof or plumbing leaks, failed flashings or cladding, and subfloors that stay damp because of blocked vents, poor drainage or low clearance. Timber in contact with damp ground, such as old piles, is another. Each can keep a section of wood wet long enough for huhu, and often for rot as well.

  • Subfloor. Damp ground, blocked vents and timber piles. See borer in piles, bearers and joists.
  • Walls and cladding. Lower boards and the framing behind failed paint or flashings. See borer in weatherboards.
  • Roof and plumbing. A slow leak above a ceiling or beside a bathroom can keep framing wet for a long time without showing inside.

What should you do about huhu in house timber?

Find and fix the water first, then deal with the timber. Once the moisture source is stopped and the wood dries below about 25%, it no longer suits huhu larvae. Damaged members are then checked for strength: some can stay, and soft or hollow ones are replaced with treated timber. Whether that needs a building consent depends on how much structural timber is involved.

Treated timber is not a perfect barrier, but it is a poor host: the Association notes larvae may bore a short way into preservative-treated wood before they die. Where damp subfloor timber also shows smaller oval holes, the guide to the two-toothed longhorn covers the other beetle that favours those conditions.

Questions people ask

Are huhu beetles a pest in houses?

Only where timber stays damp. Huhu larvae are a normal part of how dead wood breaks down in the bush and do not infest sound, dry framing. When they turn up in a house they usually point to a leak, a damp subfloor or timber in contact with wet ground, and the moisture is the problem to solve.

Why are huhu beetles drawn to my house at night?

Adult huhu are strong night fliers and are attracted to lights, so they often bump into windows and porch lights from November to early March. A beetle at a lit window in summer does not mean it came out of your house. Large oval holes in the timber itself are the sign that matters.

How long do huhu grubs live in timber?

Usually two to three years, according to the NZ Farm Forestry Association, though the time varies with conditions. Grubs grow to 50–70 mm before the adult chews out through a large oval hole. Because they need damp wood throughout, grubs in a house have been living in wet timber for some time.

Do huhu attack treated timber?

Not successfully. The NZ Farm Forestry Association notes that larvae may start boring into preservative-treated timber, getting up to about 10 mm in before they die. Treated framing can show shallow marks, but sustained huhu damage needs untreated wood that stays damp.

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