Woodworm – The hole in the argument

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 …

Health dangers

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.

Woodworm in old furniture

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.