Building Pathology: Understanding Woodworm
Insects which cause issues for timber are often referred to as ‘woodworm’, this can be misleading as they can fly and are in fact beetles. The beetles use wood as a food source and habitat and cause damage by eating away at the timber. The sight of pinholes in timbers will often send a tenant into a state of anxiety, I think it is down to the thought of creepy crawlies running around as well as a fear that the floor or roof may collapse.
I will be discussing the background of insect attacks in this blog post and describing in some detail the most common form of insect to attack timber in this country – the common furniture beetle or known in Latin as Anobium Punctatum. The purpose of this blog is to give guidance on determining if you have an active insect attack.
Understanding the life cycle makes it easier to understand the problem and remedial treatments. Eggs are laid by female insects in cracks, splits and rough surfaces of the timbers. Eggs hatch and emerge as larvae. The larvae will then burrow into the wood to obtain food and shelter, there will be no blatant external damage at this point as the activity will be taking place within the wood. Depending on the species of beetle, the length of time spent within the timber can vary from 2 – 10 years. The insect then ceases to feed and goes into metamorphosis. Upon completion the adult insect will emerge from its pupal skin and burrow its way out leaving an exit hole in the timber.
A good reference source for the identification of the type of beetle is BRE 453 ‘Recognising wood rot and insect damage in buildings 3rd edition’. In Scotland you are more likely to see the common furniture beetle or the wood boring weevil, the house longhorn beetle and the deathwatch beetle are more likely to be seen further down south.
A practical way to establish the type of beetle present is by looking at the spiders webs for checking for dead beetles in the vicinity. This will tell you whether there is a high volume of insect activity. The next thing is looking for frassing (bore dust) of the timber where the outer layer of the timber has been attacked. At this point you may decide to remove a small area of the outer layer to inspect the heartwood, this will indicate the extent of the infestation and whether treatments are necessary and determine the structural integrity of the timber.
The size and shape of the exit holes will give a useful indicator of the type of insect involved as does the bore dust. The adult common furniture beetle is about 3-5mm long and will leave a hole of around 1-2mm in diameter. It will attack softwood and hardwood in many situations. They usually only attack the sapwood although will attack heartwood after fungal decay. Thoroughly removing all traces of frass and checking for reappearance is one way to monitor for an active infestation.
Another practical way of determining if it is an active timber attack of wood boring insects is to place a piece of tissue over an area of timber and seal it with water soluble glue (lining paper, masking tape or painting timbers with emulsion paint will also work), leave it between the months of around April to July, and then come back and monitor this regularly and any beetles that are boring their way out of the timber will need to pass through the tissue and pierce the tissue and in some cases may get trapped by the tissue. The common furniture beetle will lay around 30 eggs, the larval stage will last 2-5 years with the adult beetle will normally emerge between May and August.
The ‘tissue test’ can be utilised in several locations and this will give you a cheap and effective monitoring tool for determining whether there is an active beetle attack and what the extent of that problem may be. If it found to be a historic issue and there are no concern for structural defects and no moisture related issues the timber could be left and monitored. Insect attacks can die out naturally for an array of reasons such as; attack by parasites, lack of nutrients or the desiccation of the larvae where moisture content has been lowered in the timber. Controlling the moisture content can be helpful. Sustaining moisture below 18% in timber can help to control the presence of the wood boring insects and can kill off larvae. This being said some wood boring insects will still attack dry timbers.
The treatment of infested timbers is beyond the scope of this blog but if a live infestation is found then a specialist contractor may advise insecticides which can be applied as a liquid or paste. Permanent tags should be applied to record the treatment to avoid unnecessary repeat treatments.
The treatment used in my case from the pictures used in this blog was to remove and dispose of the damaged timber floor boards and apply an insecticide. The insecticide had an active ingredient of Permethrin which can be applied as a preventative or curative measure dependant on the amount used.
For more on this topic you can search for Mike Parrett’s building pathology DVD’s, the RICS also do a great online CPD course on timber decay and infestation, in terms of books, the BRE handbook 453 would be a good place to start or my personal favourite book ‘Understanding Housing Defects’.