Detecting Woodworm

Woodworm infested table?A couple of years ago I purchased a small table from a local antique shop. Rather than furnishing my cuppa the table is now a pedestal for birds in the garden. The reason for the eviction –  tiny round woodworm holes. So how can you tell if your furniture is going to infest the rest of the house?

In the garden I tapped and prodded the table. I’m not sure why… nothing crept from the holes. A brown beetle landed on the table but after capturing it and glancing through the pest guides on the Rentokil website I was satisfied it wasn’t a wood boring pest so let it free.

The reason for my panic over the potentially infested table is because it’s woodworm season. From May to October the woodworm beetle departs from its hiding place which typically might be roof timbers, furniture or flooring. It prefers dark, undisturbed places so admittedly it was somewhat baffling why my coffee table which sits in direct sunlight and is constantly disturbed by cups of tea should be the chosen object of infestation.

Woodworm holesOnce the beetle has chewed its way from its hiding place they instinctively head towards the light. They might be spotted near windows, windowsills or around loft hatches. No beetles lurking anywhere though. Thankfully the roof timbers had been treated for woodworm six years ago so I didn’t have the extra worry of my house collapsing.

It was time to call upon wood boring insect expert Matt Green. Apart from Superhero’s and Game of Thrones, there’s nothing Matt Green doesn’t know more about. This is what the wise Doctor (not a medical doctor but one who has studied insects for a very long time) said:

First of all we need to consider what we know about woodworm. Woodworm is a horrible term. It means ‘larvae of a wood-boring beetle that looks a bit like a worm (if you squint)’. Most commonly, ‘woodworm’ is applied to the larvae of common furniture beetleAnobium punctatum, an insect native to the British Isles and found in Hazels and other trees that are often dead or have bits of their stem dying.

Wood boring insect holesThere is a European Standard for breeding furniture beetles in laboratories (BS EN 48:2005) and there have been quite a few experiments on furniture beetles over the years; so quite a bit is known about their habits. The Rentokil Library included a book called: the Insect Factor in Wood Decay by Dr Norman Hickin. Written in 1963 it’s still the go-to resource on wood-boring insects. The book gives pretty clear environmental conditions in which furniture beetle are found which went on the influence that European Standard. It basically comes down to this:

Female furniture beetles will only lay eggs in wood where they think their larvae stand the best chance of pupating into adult insects. They tend to choose hardwoods with moisture contents of 28% or higher. The average moisture content in a house is nearer 10%, depending where it is. When the larvae hatch out it takes them between two and five years to chew enough nutrients from the timber to invest enough energy to pupate into adult beetles. All the while, the timber may be drying so they larvae can withstand those conditions and can be found living in timber down to 16-18% moisture content. It gets harder for them to feed as the timber gets drier and tougher and the resulting adult beetles will be smaller and less likely to produce offspring, so a female must choose a low risk place to lay her eggs.

It is also worth noting that furniture beetle will only eat the outer section of a tree (the ‘sapwood’ portion). There are two reasons for this: 1) there are more nutrient in this sort of wood ; and 2) the inner ‘heartwood’ of building timbers like pine and oak contains toxins that would poison them if they ate them. So: not all timber is equally susceptible to furniture beetle attack. If all your wood is good quality heartwood, your risk of furniture beetle attack is much reduced. Sapwood is usually lighter in colour than heartwood, so a trained eye can see where the high risk areas are.

Once pupated the adult beetle chews its way out of the timber leaving a 1-2mm exit hole that makes your furniture look secondhand (or ‘rustic’, if you shop in Chelsea).

What does this mean for your coffee table? Well, it looks like it’s oak and it looks like furniture beetle have bothered it at some point. Judging whether or not the damage is an active problem is a tricky task but there are a number of pointers that you can look for:

1) A difference in colour. Wood darkens noticeably under prolonged exposure to UV light, so the original colour of the timber would initially be visible in the exit holes of emerging adult beetles surrounded by the darken colour of the surface. this is a good indicator, but runs into trouble with timber kept in the dark.

2) Adult beetles. It might sound obvious but look for adult beetles around windows and doors in April to August when they emerge and fly around looking to start the next generation. Adult furniture beetles are attracted to UV light and can be found dead on windowsills in infested premises.

3) Steely blue beetle Korynetes caeruleus. This distinctive beetle is one of the main predators of furniture beetle and their presence is an indication they have found something to eat.

How To Tell If You Have A Woodworm Infestation

Fill (or cover) the exit holes and wait for a year to see what happens. There is an old technique of putting tissue paper over suspected activity for a year and seeing if any new adults bore through the paper in the next year. Which is fine as long as you don’t mind looking at the paper for a year. Another way would be to fill the holes with beeswax and look for any unfilled holes next year. A little easier on the eye.

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