PEST CONTROL THE DEATH-WATCH BEETLE – ACCOMMODATED IN ALL THE BEST PLACES Steven Belmain from the Natural Resources Institute, Monique Simmonds from the Royal Botanic Gardens, Kew, and Brian Ridout from Ridout Associates in the UK report on the final frontier of Integrated Pest Management – our cathedrals and listed buildings Introduction beetle laid its eggs on or close to the surface of the wood. Preservation of historical buildings is a hot topic, and g e t t i n g The hatched larvae then burrow into the timber and hotter if predictions about global warming and climate continue to feed on the wood until they have grown suffi- change impact upon the built environment in the way we ciently to pupate. The adult emerges during the spring, think they will. Invasions of exotic insect species and extre m e mates and renews the cycle. It is now established that the life population fluctuations of indigenous species are widely cycle depends on the suitability of conditions, and that the re p o r ted around the world. Timber pests are no exception, larval stage may vary from one year in ideal conditions to and we have already seen termites establish themselves in 12 years or more, if conditions are not favourable. Death- Southwest England and observed an increase in the watch beetle larvae develop more rapidly when there is a p r evalence of the house longhorn beetle, H y l o t r upes bajulus. high relative humidity and the presence of fungal decay in This may be partly due to climate change, but it is also the timber. Whereas the short-lived adult does not directly p r obably related to changes in lifestyle. Central heating feed on timber, the larva causes considerable damage as it systems are now present in most historical buildings. And digests its way through the wood, creating structural and coupled with reduced ventilation, it can lead to condensation aesthetic damage to our buildings. New research has shown and warm e r, more humid environments inside buildings, that the adults do not necessarily need to emerge from the c r eating a more conducive environment for timber pests. timber and can mate in cavities within the timber. Further- It is also feared that another European timber pest, the more, newly mated adult females have been shown to re- death-watch beetle (Xestobium rufovillosum), is on the enter existing flight-holes and lay their eggs deep in the increase, which is a particular worry for architectural timber, rather than on or near the surface. These observa- conservationists as the beetle has a preference for ancient tions have highlighted why existing treatments are often oak timber found in cathedrals, palaces and stately homes ineffective. (Belmain et al., 1998). Historically, beginning with attempts to treat the roof timbers of Westminster Hall at the beginning Tapping of the 20th century, surface treatment with chemicals has The death-watch beetle has been living in our buildings for been employed as the treatment method of choice. Surf a c e centuries and was first noted for the tapping sound the adult t r eatment has proved, however, of very limited effect in controlling the death-watch beetle in such historic buildings. As a result, between 1993 and 1997 the European Commission funded the international collaborative research project Woodcare, led by English Heritage, to understand the interaction between beetle behaviour, timber and fungus with a view to understand why surface treatments so often fail, and to evolve alternative environmentally acceptable treatment methods (Ridout, 1999). This short article outlines the problems involved in the effective control of death-watch beetle and some of the research which has been carried out to discover why it is so problematical and to develop better methods of control. Figure 1. The death-watch The death-watch beetle (Figure 1) beetle, Xestobium rufovil- losum, one of the primary Life cycle pests affecting hardwoods in For many years it had been thought that the life historical buildings across cycle of the death-watch beetle was a maximum Europe. Adult (left) and of 5–7 years (Fisher, 1940), and that the adult Larva (right). DOI: 10.1 0 3 9 / b 0 0 9 2 7 0 n Pesticide Outlook – December 2000 2 3 3 This journal is © The Royal Society of Chemistry 2000 PEST CONTROL makes. When people usually died at home in ancient structural oak used in historic buildings was converted and Europe, the death vigil, or death watch, would have allowed assembled green, when the moisture content was still very this tapping noise to be clearly audible when the house was high, and it is likely that some timbers used had already quiet, the noise emanating from the structure of the house. suffered minor fungal attack before felling. In larger section The tapping noise subsequently became associated with bad timbers, the moisture content would have remained high omens, implying that when the tapping noise was heard a enough to sustain fungal attack for many years, and so a loved one would soon die. We now know the tapping is a suitable environment for long term death-watch beetle in- form of communication employed for finding mates, festation was present in the building from the outset. Many through research conducted by Martin Birch and colleagues have argued that the death-watch beetle larvae, themselves, at Oxford University (Goulson et al., 1994; Birch and were introduced into the buildings within the timber used Keenlyside, 1991). for 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 Environmental conditions the infestation. Our Irish collaborators based at University In many cases of active infestation, the environmental College, Dublin have studied the chemistry of Donkioporia conditions allowing the beetle larvae to survive are only just expansa which has helped us understand the complex rela- met, so that the life cycle is continuing, but at a very slow tionship between the death-watch beetle, timber, and fungus. rate; and structural damage occurs at a proportionally slow Research has shown that the death-watch beetle uses rate. However, a relatively small change to the environment fungally-produced compounds to ‘home in’ on suitable areas can cause the attack to die out, or conversely, to become of timber for infestation. more active. Moisture Chemical control At present it is thought that a moisture content of 14% is Since the advent of chemical pesticides, they have been the lower limit for a flourishing colony of death-watch increasingly used in timber pest control. Various nasty beetles, and if the moisture content drops below 12%, the chemical concoctions were developed from the beginning of larvae will die. It, therefore, ought to be a simple matter of the 20th century and applied as blanket treatments ensuring that the moisture content is below this level, and whenever pest problems were identified in buildings. For the infestation would cease to be a problem. Unfortunately, example, a formulation developed by Harold Maxwell- even in a fairly well ventilated roof space, the normal Lefroy of Imperial College, London, consisting of 50% moisture content of structural timber averages 14–15%, and tetrachloroethane, 40% trichloroethylene, 6% cedarwood in many buildings in which this beetle is a problem (such as oil, 2% solvent soap and 2% paraffin wax, was used for the irregularly heated churches), condensation coupled with treatment of the roof timbers of Westminster Hall in central poor ventilation can significantly increase this moisture London in the 1920s. In many countries, lindane and level. As temperatures within buildings can wildly fluctuate dieldran continue to be used for timber treatment. However, between summer and winter, causing subsequent changes in toxicity concerns are resulting in their replacement with moisture content, it is likely that the beetle larvae can safer pyrethroid alternatives such as permethrin. tolerate periodic drops in moisture below their optimal requirements. In the long term, Figure 2. The nave of therefore, every effort should be concentrated on Salisbury Cathedral (left) ensuring that the environmental conditions are which has a relatively low population of death-watch adjusted, first to slow down, and ultimately to beetles thanks to building kill off, the beetle attack. The improved, drier works aimed at reducing environment must then be maintained year after damp and humidity in the year (Figure 2). Even if these improvements can building. The roof area be achieved, it may still be necessary, over the above the nave (right) was one of the trial sites used short term, to introduce chemical control where to test the efficacy of the beetle is particularly active. It is of course differently coloured sticky essential that moisture levels in surrounding traps for pest monitoring. 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. Fungal decay For the death-watch beetle to flourish, timber is usually required to have been previously modified by fungal decay (often by the oak rot fungus Donkioporia expansa), making the timber more easily digested. The vast majority of 234 Pesticide Outlook – December 2000 PEST CONTROL The situation of blanket chemical treatments has become larvae can take upwards of 10 years to complete their institutionalised through the property market and insurance development (Fisher, 1940), implying that surface treat- company requirements for timber treatment certificates. ments with pesticides would have to be either very long However, more recently, several forces have been at work lasting or repeatedly applied to the timber over a number of which call into question the use of pesticides, especially with years to have any impact upon pest populations.