Precision Treatment of Death Watch Beetle Attack
By Robert Demaus
Over the past few years it has become increasingly apparent that existing methods, both for the assessment of Death Watch Beetle attack and its treatment, are of limited success, and in some cases actually counter-productive. New non-destructive techniques for locating and quantifying the extent of infestation provide a far more accurate assessment of the structural implications of Death Watch Beetle attack: the precision of these techniques has in turn allowed a more conservative, effective and environmentally safer treatment to be developed. This has particular relevance to ecclesiastical buildings, which suffer more than most from continuing attack by the beetle.
Death Watch Beetle (Xestobium rufovillosum) is a native British insect, which naturally inhabits the dead wood of several hardwood species found in the United Kingdom. For the larvae to flourish, the heartwood is usually required to have been modified by fungal decay, making the timber more palatable. The vast majority of structural oak used in historic buildings was converted and assembled green, when the moisture content was still very high, and it is likely that some timbers used had already suffered minor fungal attack before felling. In larger section timbers, the moisture content would have remained high enough to sustain fungal attack for many years, and so a suitable environment for long term Death Watch Beetle infestation was present in the building from the outset, and the beetle larvae themselves were probably introduced into the buildings in the timber used in their construction. Lack of maintenance over the ensuing years inevitably allowed periods of water ingress, setting up new fungal attacks, and consequent fresh food sources for the infestation.
In many cases of active infestation, the environmental conditions allowing the beetle larvae to survive are only just met, so that the life cycle is continuing, but at a very slow rate; and structural damage occurs at a proportionally slow rate. A relatively small change to the environment can cause the attack to die out, or conversely, to become more active. At present it is thought that a moisture content of 14 per cent is the lower limit for a flourishing colony of Death Watch Beetle larvae, and if the moisture content drops below 12 per cent, the larvae will die. It therefore ought to be a simple matter of ensuring that the moisture content is below this level, and the infestation would cease to be a problem. Unfortunately, even in a fairly well ventilated roof space, the normal moisture content of structural timber averages 14-15 per cent and in many buildings in which this beetle is a problem (such as irregularly heated churches), condensation coupled with poor ventilation, can significantly increase this moisture level. In the long term, therefore, every effort should be concentrated on ensuring that the environmental conditions are adjusted, first to slow down, and ultimately to kill off, the beetle attack: this improved environment must then be maintained year after year. Even if these improvements can be achieved, it may still be necessary, over the short term, to introduce chemical control where the beetle is particularly active. It is of course essential that moisture levels in surrounding masonry are measured and reduced as necessary: if this is not practicable, the timber should be isolated from the damp masonry as much as possible.
There are some situations where sufficient improvement to the environment cannot be achieved, and in these situations more intensive chemical intervention may well be necessary over a longer term.
For many years it had been thought that the life cycle of Death Watch Beetle was a maximum of five to seven years, and that the adult beetle laid its eggs on or close to the surface of the wood. The hatched larvae then burrow into the timber and continue to feed on the wood until they have grown sufficiently to pupate: it is the larval stage that does most of the structural damage to the wood. The adult emerges during the spring following pupation, mates and renews the cycle. However, it is now established that the life cycle depends on the suitability of conditions, and that the larval stage may vary from one year in ideal conditions to 12 years or more, if conditions are not favourable. It has also been shown that the adults do not necessarily need to emerge, and can mate in cavities within the timber, and further, that adult females, if they have emerged to mate, sometimes re-enter existing flight-holes and lay their eggs deep in the timber, rather than on or near the surface. What is still unknown is whether some adults have always mated and laid their eggs without emerging, or whether this behaviour has evolved to counter surface chemical treatments. The results of these, and other observations, have highlighted the ineffectiveness of existing treatments.
Surface spraying will only penetrate a few millimetres into heartwood and then only if the surfaces are very thoroughly cleaned down before application. It has been argued that this is sufficient as it will kill the adult as it emerges, but what tends to happen is that the beetles avoid the treated areas and instead emerge, if at all, through joints and other untreated areas. The darkness and relatively stable environment of joints is in any case a favourite habitat of the insect, and any treatment that tends further to concentrate attack in joint areas should be avoided. No new flight holes appear, and the problem is thought to have been solved, but is in fact continuing, unobserved and unchecked. Further, by discouraging their emergence, the beetle's only natural predator within buildings, the spider, is prevented from exercising any control, if it has not already been killed by the spraying.
In an attempt to avoid the hazards associated with solvent-based chemicals, waterbased emulsion fluids have been introduced, but current evidence suggests that in oak, their depth of penetration is even less than the solvent-based fluids. Paste, commonly known as mayonnaise, which uses the same contact insecticide but in a thick emulsion carrier, does allow a slightly deeper penetration and greater effective concentration of chemical. This method of application still suffers the same limitations as surface spraying, and is even more difficult to apply where access is difficult. It also often leaves a waxy skin over the areas of timber treated.
Pressure injection or irrigation through one-way valves inserted into pre-drilled 10mm holes can be more effective in some cases, but there is no control over where the fluid is going, or how much is being used. It only needs one drill hole to enter a shake or mortice for gallons of fluid to run along the shake, emerging sometimes metres away from the injection hole, or into an unseen void, and not necessarily getting to the areas of larvae attack at all. Such uncontrolled use of large volumes of chemical (usually in a solvent carrier such as white spirit) introduces a number of potential hazards. First is the increased risk of fire; second, the risk of considerable damage and staining to plaster, decorative paint and other finishes; third is damage to electrical insulation; and fourth is potential damage to the health of those who inhabit the building. Water based emulsion cannot be used for injection or irrigation as the wood will swell, and there may also be a large amount of staining on decorative finishes.
Smoke treatments, set off around emergence time, are particularly ineffective, generally killing more spiders than beetles.
Gas fumigation can be effective, but it is extremely difficult adequately to seal a building or area of a building, of the type typically attacked by Death Watch Beetle. This, coupled with the hazards generally involved with using toxic gas (usually methyl bromide), render it impractical for use in buildings.
Heat sterilisation is currently receiving a lot of attention. It is claimed that a temperature of 52-55șC maintained for 30-60 minutes will kill all wood-boring insects. Given that live Death Watch Beetle larvae have been found in the middle of large, recently fire damaged timbers, the duration of treatment would need to be very much longer than one hour if this temperature is to be achieved throughout a 300 x 250mm oak member, for example. The potential effects on delicate finishes, oak panelling and other fragile fabric of such a temperature for a prolonged period are likely to be considerable.
IDENTIFICATION OF DEATH WATCH BEETLE ATTACK
The identification of the beetle itself is amply covered in most building surveying books. The adults are 6-9mm long, dark brown with patches of yellow hair: the larva are up to 9mm long, cream and slightly curved, covered in fine yellow hairs. The flight holes and tunnels are circular and 3mm in diameter. The bore dust is cream coloured with bun-shaped pellets.
It is important to confirm whether a beetle attack is active or dead. It should always be borne in mind that the great majority of Death Watch Beetle attacks found in historic buildings died out many years - even centuries - ago. However, this has not stopped the unscrupulous from treating the attack by one system or another, and hailing the subsequent status quo as a success.
The extent of the attack within the timber is not always proportional to the number of flight holes visible, and the structural integrity of the timber should always be checked. Many visible attacks affect only the sapwood areas left on the outside of the timber after conversion, which has no structural significance: surface treatment will normally deal with this, but the attack has usually died out years ago.
The presence of fresh, brightly coloured bore dust and clean dust-free flight holes certainly indicates that the attack is active, but their absence may not necessarily mean that the attack is dead. It is quite possible that a previous unsuccessful remedial treatment has discouraged any flight holes in the visible area, but allowed the attack to continue within. Moisture content of the timber is a useful indicator: if it can be shown that the moisture content within the timber is below 14 per cent, then it is very unlikely to be active; between 14-17 per cent there is a good chance there will be some activity, and over 17 per cent the colony is likely to be thriving. It is essential that the moisture content is measured deep within the timber, not on, or near, the surface (as with most proprietary moisture meters), where daily or seasonal variations, condensation and other factors may well give misleading information.
One of the reasons why such a hit and (more often) miss, 'carpet bombing' approach has been so widely used is that, until relatively recently, there has been no way of accurately assessing the internal condition of large section timbers where Death Watch Beetle attack was suspected. New diagnostic techniques, using a combination of ultrasound and micro-drilling, allow very precise location of cavities and tunnels within the cross-section of the timber.
Ultrasound is a very quick and totally non-destructive method of locating areas where significant internal degradation of the timber has occurred. Microdrilling allows a very accurate measurement of the size of cavities and the depths at which they, and tunnels, occur. The microdrill leaves a hole of about 1mm diameter (it looks very like the exit hole left by Anobium punctatum, the common furniture beetle), and testing can be carried out through ornate plaster, panelling and other decorative finishes. This in itself is a great bonus, as it reduces or eliminates the damage and cost of stripping out. Because the depth at which the cavities and tunnels within the attacked timber occur can be so accurately measured (2mm), it is then possible to insert a 0.81mm x 200mm long hypodermic needle through the hole left by the micro-drill, and inject fluid precisely into the cavities and tunnels, and in controlled measured volumes. The coverage within the cavities depends on the size of the cavity, the design of the spray head on the needle, and the injection pressure used, but normally a spacing of around 150mm is required for adequate coverage. This may seem very close centres, but it should be remembered that the overall area being treated is vastly reduced by knowing exactly the extent of the attack before treatment is started. In situations where a small amount of damage to the fabric is acceptable, it may sometimes be more effective to drill 6mm holes and inject a bodied emulsion paste deep into the timber. It is essential that the timber is assessed first, to confirm exactly at what depth, and in what volume, the paste should be injected. The paste can be introduced using a caulking gun with extended delivery tube. Research and development of a new bulked paste is currently underway. This paste is specifically designed to fill internal cavities, but not to spread further.
Almost all problems of decay in timber structural components originate from faults in the design or maintenance of other components of the building: remedy of the timber problems must be considered as an integral part of a building's repair and maintenance programme and not in isolation. Architects and others involved in the care of buildings must maintain their control over the methods used in Death Watch Beetle treatment, and should not merely pass the responsibility to a remedial treatment specialist, who has little or no control over the faults that set up the decay problem in the first place. The amount of treatment necessary, and certainly the volume of toxic chemicals used, can be vastly reduced, and the effectiveness of the treatment greatly improved, but only if a detailed assessment of the severity of the attack, and the mechanisms that allow that attack to continue, is carried out first.
About the Author:
ROBERT DEMAUS specialises in the detection and assessment of decay, weakness and fire damage in structural timber using non-destructive techniques. He is a partner in The Demaus Partnership.
This article is reproduced from The Conservation and Repair of Ecclesiastical Buildings 1995.
© Cathedral Communications Limited 2004
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