Deathwatch Beetle, (Xestobium rufovillosum), is a woodboring insect which has the potential to carry out extensive damage to timber which it may affect. These particular insects are renowned for making a tapping or knocking sound, which it does by bumping its head or jaws against the sides of the tunnels as it is creating bore tunnels in timber; which is usually in old hardwood timber. With regards to its rather macabre name, there is, of course folk-lore as to how this beetle was named.
According to some superstition, the sound, which is actually a mating call, was believed to forecast an approaching death. Some stories suggest it was given this name during the times of the great plague of London in 1665-66, where properties would have been built using hardwood timber framing. Its name is derived from the credence that it was often heard by the people “on watch” with an ill person, on the verge of death. These beetles tend to be relatively small, although larger than the more known Common Furniture Beetle. They tend to be between 1 to 9 mm long and cylindrical. When disturbed, they usually pull in their legs and play dead; maybe that is really what the name refers to, although, we think the folk-lore stories are far more interesting.
One of the biggest issues with regards to Deathwatch Beetle is the very difficult task of eradicating an attack. It has, over a number of years, become increasingly apparent that most methods, both for the assessment of Deathwatch Beetle attack and its treatment, are of limited success.
Deathwatch Beetle is a native British insect, which naturally inhabits the dead wood of several hardwood species found in the United Kingdom. For the larvae to survive, the heartwood is usually required to have been affected by fungal decay, which, it is believed, makes it easier for the insect to bore through. This is generally how attacks start in older, timber framed properties i.e. structural timbers being affected by fungal decay, which tends to be the Wet Rot fungus (Coniophora puteana). The vast majority of structural oak used in historic buildings would generally have been converted to usable timber sections and assembled “green”; this is un-seasoned timber when the moisture content was still very high. This means that there was a high probability that some of the timber used would already have been suffering from a degree of fungal decay before actually being used for construction.
In timber framed properties, some of the timbers used were extremely large (depth and width) and due to the density of the materials, lack of any heating in properties, it is likely that the moisture content, deep within the timber, would have remained high enough to sustain fungal attack for many years after construction. Therefore, creating a perfect environment for long term Deathwatch Beetle infestation. Again, this meant that it would have been highly likely that the larvae of Deathwatch Beetle was probably introduced into the buildings in the timber at construction stage. Continued water ingress into such buildings and a lack of knowledge – your children may be surprised to know that the internet was not available then – meant that the environment within properties, especially to confined, built-in structural timbers, was ideal to allow Deathwatch Beetle infestations.
This beetle has an incredibly long life-cycle in the right conditions. For a long time, it was thought that the life-cycle was in the region of five years, which is still about twice that of Common Furniture Beetle. However, in reality the life-cycle generally depends on the suitability of conditions and not just the various stages of the beetle’s life i.e. laying its eggs, hatching into the larval stage, pupating and turning into the adult beetle. It is conceivable that the larval stage may vary from one year to 12 years or more in ideal conditions. We know that the larval stage does most of the damage. It is also known that the adults do not necessarily need to emerge and they can mate in cavities within the timber. This means that the life-cycles can continue, without being broken, as a result of the adult beetle emerging through the outer timber shell, which has been treated with an insecticide.
Knowing these things means that treating timbers, in an effort to eradicate Deathwatch Beetle is going to be problematic. Also, we must take into account that the timbers affected are likely to be dense hardwood and not softwood, which is far more absorbent. Insecticides applied by spray onto the surfaces of hardwood timber will tend to have little, or no effect as the absorbency rate is so low. Gels, or deep penetrating paste, may be a better option as it will absorb slightly more than sprayed insecticide. Injection of insecticide through fixed injection nozzles has been tried but the problem here tends to be that there is very little “spread” of the insecticide around the injection nozzle. Smoke treatments are unlikely to be particularly effective as it is not really possible to contain the smoke and the percentage of active ingredient (of insecticide) is usually very low.
Given that these insects are usually going to be affecting hardwood timbers in older, historic buildings, the other point of consideration is whether or not the exposed beams have been waxed, varnished or painted. With any surface decorative finishes such as this, treatment would not be possible without having the decorative / sealing finishes removed first.
Knowing that insecticide treatment – which would generally not be a guaranteed treatment – may not eradicate an attack, there are thoughts any attack may be better dealt with by making environmental changes. A relatively small change to the environment i.e. lowering the moisture content of the timber over a period of time, can cause the attack to die out. A moisture content of around 14 per cent is the about the lower limit for a flourishing colony of Deathwatch Beetle larvae. However, if the moisture content drops below 12 per cent, the larvae will die. However, achieving these changes in old, hardwood timbers, however, is never going to be straightforward and, in many cases simply not possible.
When a live infestation is found, a lot of factors need to be taken into consideration. Rather than a “one-off” treatment, which we know is going to have limited success, it would be far better to think of managing the infestation, which may mean carrying out targeted treatments to the timbers over a number of years. During this time it is highly probable that adult beetles will continue to emerge for many years (10 to 15 years).
All of this sounds very concerning but the other thing to bear in mind is that the great majority of Deathwatch Beetle attacks found in historic buildings died out many years – even centuries – ago. The holes in the timbers will always remain and can, where there has been a particularly heavy and widespread infestation, look worrying. However, it does not mean that further treatment is required. An unscrupulous person may inspect the evidence and suggest further insecticide treatment is necessary, when in fact, it will provide no benefit and, used where it is not needed, contravenes the COSHH regulations we must all abide by.
If you do own an historic, timber framed property, which you believe may be affected by Deathwatch Beetle and have concerns as to whether or not it is an active infestation, it is important to have it inspected by an experienced Surveyor, who will be able to ascertain whether or not the infestation is active and whether or not further remedial treatments will be necessary.