Identification

Is the borer in your house still active?

Most older Auckland houses have borer holes. How to tell live borer from old damage by the dust and the hole edges, and what it means if you are buying one.

The short answer

Holes alone tell you borer was there at some point, not that it is there now. Live borer shows itself as fine, pale dust the colour of freshly cut wood, sifting from holes with sharp clean edges, usually in early summer when the adults emerge. Old damage has darkened holes and dust that has caked, yellowed or gathered grime. When the evidence is unclear, a season of watching settles it before anyone pays for borer treatment.

At a glance

Live borer or old damage: what to compare

Live borer or old damage: what to compare
What to look at	Live borer	Old, inactive damage
Bore dust	Pale, the colour of fresh-cut wood, loose	Darkened, yellowed, caked or filmed with grime
Where the dust sits	Small piles directly under holes, or on the floor below	Spread thin, mixed with ordinary dust, or none at all
Hole edges	Sharp and pale, brighter than the surface around them	Weathered to the timber's colour, often painted over or cobwebbed
New holes	Appear in a surface you know, mainly after the summer flight	No change from one year to the next
Beetles	Small brown beetles of 3–4 mm on sills or in cobwebs, November to January	None seen in season

What does active borer look like?

Look for three things together: dust the colour of freshly cut wood, small piles of it directly beneath the holes or on the floor below, and hole edges that are sharp and pale rather than weathered to the colour of the timber around them. Any one of these alone can mislead, because footsteps, wind or building work shake old dust out of abandoned tunnels, and that dust can look fresh for a while.

The dust itself also tells you which beetle you have. The common house borer, Anobium punctatum, leaves loose bore dust with a sandy, gritty texture between finger and thumb, while the powderpost beetle, which only attacks hardwoods, leaves dust as fine as talcum powder. The full comparison, with hole sizes, is in the identification table on our borer treatment page.

The other live sign is the beetle. Common house borer adults are small brown beetles of about 3–4 mm, and between November and January they turn up on windowsills, in cobwebs and on the floor below the timber they came out of. Finding several of them indoors in early summer is strong evidence that something nearby is still producing them.

What old, inactive borer damage looks like

Old holes have edges the same colour as the surface around them, often with paint, dust or cobweb across them. Any dust has darkened, clumped or gathered a film of grime, and no new holes appear in a surface you know well. In a villa roof space that has stayed dry for decades, this is the most common thing we find, and it needs nothing beyond being noted.

Damage often stops at a clear line, and that line is usually a renovation. Original framing in older houses went in untreated, while timber added in later extensions and reroofs is generally treated, so the borer had nowhere to go. It is worth knowing that historic damage can still matter if it is heavy. Edges that crumble, timber that dents under a thumbnail or a floor that has started to spring are questions for a builder, whether or not anything is still alive in it.

How to check over one flight season

When you cannot tell, test it. Before November, vacuum away all the dust in a sample area, then mark or seal every existing hole with a dab of wax, filler or masking tape, and photograph it. Check again in February, after the November to January flight. New holes or fresh dust mean live borer; a clean, unchanged surface means you can stop worrying about that area.

Choose the worst places for the test: the roof framing near the eaves, subfloor timbers by a damp corner, the back of a skirting along an outside wall. The two-toothed longhorn is the exception to the timetable, because its adults emerge in autumn and early winter, so recheck any subfloor bearers with oval holes in May or June as well.

Keep the photographs and the dates. A builder, an insurer or a property manager will take a dated record far more seriously than a recollection, and if you own a rental, it belongs in the maintenance file.

Should borer holes stop you buying a house?

Rarely on their own. Borer holes are expected in villas and pre-war bungalows, which were built from untreated timber, and most of them are historic. What matters is whether the borer is active, how much of the structural timber is affected, and whether the longhorn is working in damp subfloor members. A building inspector's report should answer each of those, and the answers belong in the price, not in the decision.

Questions worth putting to the inspector: is the damage active or historic, which members are affected, how damp is the subfloor and how well is it ventilated, and is there any structural concern. If the report is vague on borer, a timber pest inspection before you go unconditional settles it, and our pricing page sets out what treatment would cost if it does turn out to be live.

Questions

Questions about borer activity

How long do borer larvae live in the wood?

For the common house borer, three years from egg to adult in kahikatea sapwood and four in rimu and radiata pine, according to the Scion leaflet on the beetle. The two-toothed longhorn may spend between 2 and 11 years inside, depending on moisture, temperature and the timber. That is why one quiet year proves little, and why any treatment has to outlast the cycle.

When do borer beetles come out in New Zealand?

Common house borer adults appear mainly from November to January, live up to about four weeks and lay in bare timber or old holes. Powderpost beetles emerge in October and November, the two-toothed longhorn in autumn and early winter, and huhu from November to early March. A check for fresh activity belongs straight after the window for the beetle you suspect.

Can I see borer larvae?

Not without opening the timber. Common house borer larvae are creamy, C-shaped grubs up to about 7 mm long that spend their whole lives inside their tunnels, so what you see is the holes the adults leave and the dust pushed out behind them. A grub large enough to spot easily in house timber is more likely a huhu, and that points to damp, rotting wood.

Is borer dust harmful?

Bore dust is wood that has passed through the larvae, and it is not known as a particular health hazard. In a roof space, though, it mixes with insulation fibres, old droppings and general dust, so wear a dust mask and gloves when you clean it up, and vacuum it or wipe it with a damp cloth rather than sweeping it dry into the air.

Sources

If you need someone

If it turns out to be live

Borer

Borer Treatment

Most borer holes in an Auckland house are older than the people living in it.

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DIY treatment