Borer library
The two-toothed longhorn, New Zealand's native house borer
Most borer in Auckland houses is the small common house borer, but a native longhorn turns up in damp subfloors and old rimu, and it behaves differently enough to change what a finding means. If your holes are oval rather than round, a borer inspection is how you learn how far the tunnels go.
Updated
The two-toothed longhorn, Ambeodontus tristis, is a native New Zealand beetle whose larvae tunnel through both the sapwood and the heartwood of dead softwood, including untreated rimu, kahikatea and macrocarpa in buildings. Its exit holes are oval, about 3–7 mm long. Larvae stay in the timber for 2 to 11 years, and adults emerge in autumn or early winter.
Because it reaches heartwood, damage can be far worse than the holes suggest, which is why the timber is probed rather than judged by its surface.
How do you identify the two-toothed longhorn?
Start with the exit holes. The two-toothed longhorn leaves oval holes about 3–7 mm along the long side, clearly larger than the round 1–2 mm holes of the common house borer. The adult is a slim red-brown to black beetle up to about 25 mm long, named for two shiny raised areas just behind its head, though few owners ever see one.
Inside the wood, the tunnels are packed with a mix of wood scrapings and pellets, and the NZ Farm Forestry Association describes crescent-shaped marks across the tunnel walls. Adults shun light and emerge in autumn, so, unlike the house borer, they are rarely found on summer windowsills.
Is the two-toothed longhorn worse than common house borer?
Usually, where it is established. The common house borer stays in sapwood, so its damage tends to sit along the edges of a board or joist. The longhorn tunnels through heartwood as well, and badly attacked timber can be left as a thin outer shell. Consumer NZ notes that it is found in buildings less often than common borer, but that its damage is usually severe.
| Feature | Two-toothed longhorn | Common house borer |
|---|---|---|
| Exit hole | Oval, about 3–7 mm long | Round, about 1–2 mm |
| Wood attacked | Sapwood and heartwood | Sapwood, normally only |
| Time as a larva | 2 to 11 years | About 3 years in kahikatea, 4 in rimu or pine |
| Adults emerge | Autumn or early winter | Mostly November to January |
| Adult length | Up to about 25 mm | 2.5–5 mm |
| Where it builds up | Damp, poorly ventilated subfloors | Untreated sapwood throughout older houses |
The comparison is why the species line on a finding matters as much as the activity line. The same handful of holes means a sapwood edge in one case and possibly a hollow bearer in the other. The wider picture of which houses carry the common borer is in the guide to house borer by build year.
Where does the two-toothed longhorn turn up in a house?
Mostly underneath it. The NZ Farm Forestry Association reports that untreated rimu in buildings is often badly damaged when used for subfloor framing, bearers, joists and jack studs in damp, poorly ventilated places. Rimu, kahikatea and macrocarpa are attacked most often, with tōtara, mataī and Lawson cypress among the other recorded hosts. A manufacturer's technical sheet also singles out rimu floors.
That makes the subfloor the place to check first in an older house, especially where vents are blocked, the ground stays damp or the clearance is low. The guide to borer in piles, bearers and joists covers what is looked at down there and how areas that cannot be reached are recorded.
Why does probing matter with longhorn damage?
Because the surface can look almost intact while the inside has gone. Longhorn larvae can spend years working through the heartwood of a bearer or joist, and the only outward sign may be a few oval holes. Pressing the timber with a probe shows whether it is solid, soft or hollow, which no amount of looking at the surface can tell you.
A soft or hollow result moves the finding from treatment to repair. Like-for-like replacement of some damaged members in the same position is generally exempt from building consent, but complete or substantial replacement of structural framing is not. Borer damage repair explains where that line sits and when a builder needs to be involved.
How is two-toothed longhorn dealt with in a house?
Through the timber and the damp together. The NZ Farm Forestry Association lists removing affected timber and replacing it with preservative-treated timber, insecticide applied by brush or spray, and better subfloor ventilation to cut the risk of reinfestation. Treatment is timed ahead of autumn emergence, the opposite end of the year from the common house borer's early summer flight.
Which of those applies depends on how much strength the members have left. Sound timber with a few holes is a treatment and moisture job, set out under treating live borer; hollow timber is a builder's job. Fumigation appears in forestry guidance, but Consumer NZ notes it is not usually suitable for houses because the space has to be completely sealed.
Questions people ask
Is the longhorn beetle found in New Zealand?
Yes. New Zealand has native longhorn beetles, and the two-toothed longhorn is the one that matters in houses because its larvae develop in dead softwood, including untreated building timber. The huhu, the country's largest beetle, is also a longhorn but needs damp wood. If you suspect an exotic beetle, MPI takes reports on its pest and disease hotline, 0800 80 99 66.
How long does the two-toothed longhorn live inside timber?
Between 2 and 11 years as a larva, according to the NZ Farm Forestry Association, depending on moisture, temperature and how nourishing the wood is. Development is slowest in dry heartwood at low temperatures. That long and variable life inside the timber is one reason damage can build up for years before the first holes appear.
When do two-toothed longhorn beetles fly?
In autumn or early winter. The NZ Farm Forestry Association gives that season, a manufacturer's sheet puts emergence at about March to May, and Consumer NZ simply says autumn. That makes it the odd one out among house beetles: the common house borer and the huhu emerge in late spring and summer, so a summer-only check may miss fresh longhorn holes.
Should I buy a house with longhorn borer?
It depends on where it is and how far it has gone. Longhorn in a few replaceable joists is a repair cost to price in; longhorn through bearers or jack studs across the subfloor is a larger structural job. Get the extent in writing before you go unconditional, and see whether borer is a deal breaker for how buyers weigh it.
Read next
- Piles and subfloorBorer in piles, bearers and joists, damp subfloors, rot versus borer, and when replacement needs a consent
- House borer by build yearWhich New Zealand houses carry house borer, era by era, and why the framing timber decides it
- Borer damage repairWhen borer damage needs a builder, which repairs are exempt from consent, re-piling and the order of work
- Huhu in the houseWhy huhu in house timber points to wet wood, and where the moisture usually comes from