DIY treatment
Treating borer yourself, and what each product actually reaches
What borer bombs, surface sprays, hole injectors and home remedies such as vinegar or kerosene can reach inside timber, and when treating borer yourself is enough.
The short answer
Most of what you can buy for borer works only on the surface. A borer bomb kills beetles flying at the moment it is set off, a surface spray or brush-on kills beetles as they chew out of bare timber and hatchlings as they try to bore in, and an injector reaches the tunnel behind one hole. None of them reaches a larva deep in a joist, which is why professional borer treatment relies on timing and residual products.
At a glance
How far each borer product gets into the timber
| Product | What it reaches | How long it acts | Worth using for |
|---|---|---|---|
| Hole injector | The tunnels behind each hole it is pushed into | Stays in those galleries | Painted or polished timber with a limited number of holes |
| Residual spray or brush-on | The surface of bare timber, and the beetles crossing it | Years, on bare timber under cover | Roof framing, subfloors, unfinished backs of furniture |
| Borate preservative | Into unfinished wood, further when the wood is damp | Long lasting on bare timber | Bare structural timber and new repairs |
| Borer bomb or fogger | Adult beetles in the air or on surfaces that day | Hours | A supplement in the November to January flight only |
| Vinegar, baking soda, kerosene | Nothing that has been shown to work | Not applicable | Nothing; kerosene is also a fire risk |
How do you inject borer holes in painted or varnished timber?
Where timber is finished, an injector puts a small amount of borer fluid into each flight hole, and from there into the tunnels behind it. It reaches the galleries connected to that hole rather than the whole piece, so it suits door frames, skirtings, furniture and turned joinery with a countable number of holes, worked through methodically, one hole at a time, and repeated if new holes appear the following summer.
Once the fluid has dried, plug each hole with tinted wax or a wood filler. Beetles emerging later will lay eggs in their own old exit holes if they are left open, and sealing them takes that option away. Wipe any spill off the finish straight away, because some products mark polish and shellac, and test on a hidden spot first. Pieces of furniture bring their own questions about finishes and freezing, which we cover in our guide to borer in furniture.
What is the best spray for borer?
The products that genuinely work are residual treatments applied to bare, unpainted timber: an insecticide or a boron-based preservative that stays active in the surface for years. They kill each beetle as it chews out and each newly hatched larva as it bores in, so over a few seasons the population runs out. Consumer NZ describes this as the only long-term treatment, provided it outlasts the borer's life cycle.
The catch is the bare timber. Paint, varnish and polish block both kinds of product, and borates penetrate only unfinished wood, going deeper when the wood is damp. Retail residual sprays, including the one many people search for by name as NO Borer, are of this type, and the label sets out the surfaces and rates.
Coverage is where DIY usually falls short. A roof space means every face of every rafter and the underside of the sarking, often with little headroom; a subfloor means piles, bearers, joists and the underside of the boards, in dust, on your back, wearing a respirator. Missed faces are where the next generation comes out, and that is the honest line between doing it yourself and paying someone.
Do borer bombs work?
Only on adult beetles. Sold as a borer bomb, the can is a total release fogger that fills a closed roof space or subfloor with insecticide, and it kills beetles in the air or on surfaces during the hours it is active. Larvae inside the timber are untouched, and in the common house borer, Anobium punctatum, they do the damage over three to four years. Consumer NZ makes the same point about bombs, misting and fogging.
That limits a bomb to one job: set off during the November to January flight of the common house borer, repeated as the label allows, it removes some of the beetles that would otherwise lay the next generation. A single can in July does next to nothing. You will find them in New Zealand hardware stores, and the label decides how long the space stays closed, how it is aired, and what has to be done about smoke alarms, pilot lights, pets and fish tanks.
Do home remedies like vinegar or kerosene kill borer?
We could find no published research showing that vinegar or baking soda kills borer larvae inside timber, and no account of how either would get to them. Kerosene, the old remedy, may kill what it soaks, but it is flammable, it smells for weeks, it can stain and soften finishes, and it is not registered as a timber treatment. None of them leaves the lasting barrier that actually ends an infestation.
The one natural ingredient with real science behind it is boron. Borates are the basis of genuine timber preservatives, and they work on bare wood. A kitchen mix of borax and water, though, is not a tested formulation, and nobody can tell you how deep it goes or how long it lasts, so use a registered product instead. For small loose items there is also a chemical-free option, which is freezing.
Can you treat borer yourself, or do you need someone?
Doing it yourself is enough when the timber is accessible, the area is small and you know the borer is live: a piece of furniture, an exposed beam, a run of skirting or a garden shed. It is not enough for a whole roof space or subfloor, where coverage decides the result, for longhorn in damp bearers, where moisture is the real fix, or when you are not sure it is active at all.
In that last case the right first step is checking, not spraying, and we have explained how to tell whether the borer is still active before you buy anything. If timber crumbles under a thumbnail or a floor has gone springy, no product will help; that is a builder's job, and fumigation is not something a householder can do either, since only licensed operators may carry it out.
Questions
Questions about DIY borer products
How long does a borer bomb take to work?
The fog acts during the hours the space stays closed, and the label sets that time, the airing and when you can go back in. It kills only beetles exposed in that window. That is why timing matters more than the product: set off in the November to January flight it catches emerging adults, set off in winter it catches almost nothing, and the larvae inside the timber are never reached.
How long does borer spray last?
A residual product on bare timber stays active for years under a roof or a floor, and it has to, because it must outlast the three to four years larvae spend inside the wood. Sunlight, weather, dust and later painting all shorten that. The label gives the reapplication interval for each product, and that is the figure to follow rather than any general rule.
Can I just paint over borer holes?
Paint, varnish and wax stop beetles laying eggs on that surface, and filling the holes stops them laying in their own exit holes, so finishing bare timber is a sound preventive step. It does nothing to larvae already inside, and they will bore straight out through a fresh coat. Paint over holes once you know the borer is dead, or after the timber has been treated.
Is it safe to use borer products indoors?
Registered products are approved for the uses on their label, and the label sets the precautions: ventilation, gloves, keeping children, pets and fish tanks away until dry, and never fogging an occupied room. If anyone in the house has asthma, an injector or a brush-on is a better choice than an aerosol. If someone is exposed and feels unwell, call the National Poisons Centre on 0800 764 766 or see a doctor.
If you need someone
When it needs more than a can
More advice
Other things worth knowing
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.
Read this FurnitureBorer in furniture, and how to keep the piece
How to tell whether borer in a chair, dresser or antique is still active, which beetle made the holes, and how injection, freezing and refinishing deal with it.
Read this Timber pestsBorer or termites?
Borer and termites both hollow out timber, but the dust, the holes and the next step differ. How to tell them apart in a New Zealand house, and when to call MPI.
Read thisNot sure what you are dealing with?
Describe it, or send a photograph. Identifying it correctly is our job, and it is what decides the treatment.