Woodworm

Every week 5000 British homes are sprayed with toxic chemicals to kill woodworm which probably left 100 years ago

The problem with woodworm is that when they leave the wood they also leave holes, and the holes are used by chemical salesmen to convince householders that they have a problem with insect infestation. But the truth is that the holes are probably 100 years old, and spraying them with chemicals is one almighty case of shutting the stable door after the horse has bolted.

There are many types of wood-boring insects, and in nature they play a vital role in the decomposition of forest timber, as well as providing a vital food source for other species such as birds. "Woodworm", or "common furniture beetle", are names given to Anobium punctatum, which, like many insects, spends most of its time as a maggot before pupating and emerging for a short adult life as a flying beetle.

So the "woodworm" is actually the larval stage of a beetle. The adult female beetle lays her eggs in an environment which will offer the best chances of survival for her offspring, which means moist sapwood. Moist, because the larvae need moisture to survive, and have no means of going off for a drink of water before returning to the wood. And sapwood (the outer, living part of the tree), because that is where the minerals and nutrients are, which the larvae need for their sustenance. So the ideal environment for woodworm is the outer layers of a freshly-felled tree. And that is what we find in practice in buildings - any woodworm damage is confined to the sapwood at the edges of floorboards and joists, and has not usually caused significant damage or weakening of timbers.

The woodworm larvae spend three to five years burrowing around in the sapwood before pupating, hatching out as adult beetles, and munching their way to the surface, where they leave the characteristic "flight hole". My own observations lead me to believe that most of this activity occurs in the first ten years after the house was built. After that, the sapwood will have dried out and become less appetising, and the adult female beetles will prefer to lay their eggs in a more promising location, such as dead trees in the garden. This is especially true in recent times, when most older homes have central heating, which dries timbers down to below the 11% moisture content needed to support insect life. Read on ...

Q. Can you suggest any safe woodworm fluid for an attic which has worm holes? We would be concerned for our children's health particularly. We have installed modern Velux windows and need to protect these from any live woodworm. What would you suggest?

A. I cannot recommend any woodworm fluids. They are all nerve poisons. Your worm holes are probably many years old. They are actually 'flight holes' of the adult beetles leaving the wood. There is no justification for using insecticides unless you have definitive evidence that there is a continuing active infestation, and that this cannot be dealt with by normal construction methods, i.e. central heating and ventilation. Your new Velux windows are unlikely to be affected by wood-boring insects - the timber in them is kiln-dried and sealed with a water-based varnish.

Q. I have noticed that an antique beechwood chair which I bought a few years ago has active woodworm - i.e. sawdust sort of dust coming out of new-looking holes. I treated the whole chair with an anti-woodworm treatment. Now I am wondering whether I need to treat the house timber (house built in 1880s) and the other furniture (mainly oak). I would hate to do that, partly because of the pollution, partly because I am asthmatic and just doing the one chair affected my breathing for a few days. On the other hand, everyone tells me that eventually the worms will eat half my house! I take great comfort from what you say about the worms preferring sapwood, but do wonder why they invaded the chair in the first place - it is definitely not a new chair. I note that I live in an area where there are lots of garden trees, and the window is often open, so any beetle wanting sapwood would have no trouble finding it.

A. Woodworm is not infectious, and there is no reason why it would spread from one piece of furniture to another or to house timbers. Each infestation results from the female adult beetle laying eggs in an environment that she thinks will be suitable for her offspring - i.e. moist nutritious wood. If you have moist nutritious wood anywhere in your house then adult female beetles - which are flying around everywhere between April and July - will lay eggs in it. If the wood is dry, as it will be if you live in a normal heated ventilated house, then they won't. The infestation in the beech chair probably happened five years ago when it was stored in damp conditions. Three to five years is the life cycle of the insects. Once this generation has hatched out and flown then that will probably be the end of it - regardless of how many chemicals you spray around the place.

Many more readers' questions like these are answered in my book, "The Sunday Telegraph Guide to Looking After Your Property" - click on books towards the bottom of the menu for more information...

"Safe? Of course it's safe. You could drink a glass of this and it wouldn't hurt you." The timber sprayer stood looking at me as though I'd just landed from Mars.

"Go on, then", I replied.

"Drink a glass of it", I said.

He looked at me with his head on one side. You could see he was thinking about it. "Get out of it. It's safe. I'm telling you it's safe. They wouldn't let you use it if it wasn't safe."

"But you just told me you could drink a glass of it", I said.

"So are you going to drink a glass of it or what?"

"Get out of my way", said the timber sprayer. "I've got work to do. I can't be standing around all day listening to your rubbish".

The idea that a man could safely drink a pint glass of woodworm fluid is preached on the industry training courses. So, naturally enough, the salesmen and sprayers repeat it, parrot fashion. But, for some reason, I have never had one comply with my request.

The origin of the "drink a glass of it" boast is that permethrin, like all pesticides, is tested for toxicity by feeding it to rats. When half of your rats have died - usually by nerve collapse causing paralysis and suffocation - you see how much pesticide they've had, divide it by their body weight, and write it in the column marked LD50. This stands for Lethal Dose 50 percent. The rats who haven't died, but who are, presumably, starting to feel a bit off-colour, are bashed over the head.

The LD50 is used to calculate the safe level for human exposure. People weigh more than rats, so it is argued that a dose of permethrin that would kill a rat could not possible kill a person. And a smaller dose, which would make a rat ill, would not make a human ill. The only trouble with this logic is that there is no such thing as an average rat, nor an average person. Hundreds of people claim to have become ill following permethrin woodworm treatment in their homes -including readers of this column who have written in to tell me about it.

Some of these tales are very sad, such as that of a woman who bought an old house in 1985, had it sprayed with permethrin at the behest of her mortgage lender, and has been ill ever since. Her baby daughter was also affected, and has suffered a lifetime of illness and incapacity. Now 16, she would like to go to university, but has had so much time off school that she is struggling to cope with her GCSEs. I copied this reader's letter to the director of the timber treaters' trade association, and he responded in standard chemical industry style; timber treatment chemicals are licensed as safe by the Health and Safety Executive, he said, so anyone who thinks permethrin has made them ill must be wrong, and simply trying to start a witch hunt.

You might think this is pretty rich, coming from an association whose members were recently shown on BBC television to be systematically defrauding the public and even, in one case, assaulting the TV presenter.

But the timber treaters have the law on their side. Permethrin is approved by the HSE, using tests such as the LD50. And the British government refuses to acknowledge that woodworm treatment could make householders ill.

That's why I always issue the "drink a glass of it" challenge.

Try it for yourself – none of them will ever do it.

I have come across an interesting passage in a decorating book called "Period Finishes and Effects". It reads, "Evidence of woodworm infestation can be effectively simulated by making clusters of small holes with a nail or the point of a compass". So now it becomes clear - half the population is trying to make new wood look old by poking holes in it, and the other half is spraying the holes with nasty chemicals in case something pokes its head out and bites them. You couldn't make it up.

As it happens, a friend called last week to ask what to do about his woodworm. He's a Building Surveyor by profession, so naturally he doesn't know anything about timber infestation.

Like most people, he assumed the little holes in the edges of his floorboards were evidence of wood-boring insect attack. I told him that the holes may well be evidence that the boards once had woodworm, but since they are known in the trade as flight holes, what they actually indicate is that the insects have now scarpered. It is the larvae – the maggot stage – of Anobium punctatum that do the munching, and they do it below the surface of the timber. After a couple of years they pupate into little chrysalises, and then they hatch out as adult beetles, chomp their way to the surface and take off into the wild blue.

So why do surveyors think flight holes – or compass holes, as we shall now call them – are evidence of continuing insect attack? After all, active infestation is easy to confirm – the munching larvae produce faeces which consist largely, you will not be surprised to learn, of wood. It looks like sawdust, feels gritty when rubbed between the fingers, and can be collected on a sheet of paper under the suspect area.

I have made one or two observations about woodworm: first, Anobium's staple diet is sapwood – the outer, growing, part of the tree. They like the sapwood because it is moist and nutritious, so they hang out in places where there is plenty of fresh sapwood available - forests, for example. Now, while they are in the egg stage, they may suffer the indignity of being in a tree which is sawn up and carted off to be used as building timber. For as long as the timber retains enough moisture, they will carry on with their munching and burrowing, complete their normal life cycle and then buzz off, leaving flight holes. But by that time the timber will have dried down to an unpalatable level, especially if it is in a centrally-heated building, so there is little attraction for the female beetles to lay their eggs in the same spot; they'll be off outside looking for a nice fresh juicy tree. I suspect that most woodworm damage occurs in this fresh timber, probably in the first couple of years after the house is built; and since it is confined to the sapwood at the edges of boards and joists, there is rarely any structural weakening, and so no real problem.

Also, the term timber "infestation" is sometimes confused with "infection". It makes woodworm sound like some kind of contagious disease – I've known people who've bought old furniture at the auctions get paranoid that they've imported woodworm into their homes – they think the little blighters are going to spread out and eat the whole house, so they've sprayed chemicals everywhere. Left to themselves the woodworm will probably clear off – it's the long-term effects of the chemicals that are starting to look like the real problem.