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 Preservation of historical buildings is a hot topic, and g e t t i n g hotter if predictions about global warming and climate change impact upon the built environment in the way we think they will. Invasions of exotic insect species and extreme population fluctuations of indigenous species are widely reported around the world. Timber pests are no exception, and we have already seen termites establish themselves in Southwest England and observed an increase in the prevalence of the house longhorn beetle, Hylotrupes bajulus. This may be partly due to climate change, but it is also probably related to changes in lifestyle. Central heating systems are now present in most historical buildings. And coupled with reduced ventilation, it can lead to condensation and warmer, more humid environments inside buildings, creating a more conducive environment for timber pests. It is also feared that another European timber pest, the death-watch beetle (Xestobium rufovillosum) , i s o n t h e increase, which is a particular worry for architectural conservationists as the beetle has a preference for ancient oak timber found in cathedrals, palaces and stately homes (Belmain et al., 1998). Historically, beginning with attempts to treat the roof timbers of Westminster Hall at the beginning of the 20th century, surface treatment with chemicals has been employed as the treatment method of choice. Surface treatment 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.
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. The adult emerges during the spring, mates and renews the cycle. 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. Deathwatch beetle larvae develop more rapidly when there is a high relative humidity and the presence of fungal decay in the timber. Whereas the short-lived adult does not directly feed on timber, the larva causes considerable damage as it digests its way through the wood, creating structural and aesthetic damage to our buildings. New research has shown that the adults do not necessarily need to emerge from the timber and can mate in cavities within the timber. Furthermore, newly mated adult females have been shown to reenter existing flight-holes and lay their eggs deep in the timber, rather than on or near the surface. These observations have highlighted why existing treatments are often ineffective.
Tapping The death-watch beetle has been living in our buildings for centuries and was first noted for the tapping sound the adult
Figure 1. The death-watch beetle, Xestobium rufovillosum, one of the primary pests affecting hardwoods in historical buildings across Europe. Adult (left) and Larva (right).
The death-watch beetle (Figure 1) Life cycle For many years it had been thought that the life cycle of the death-watch beetle was a maximum of 5–7 years (Fisher, 1940), and that the adult DOI: 10.1039/b009270n
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PEST CONTROL makes. When people usually died at home in ancient Europe, the death vigil, or death watch, would have allowed this tapping noise to be clearly audible when the house was quiet, the noise emanating from the structure of the house. The tapping noise subsequently became associated with bad omens, implying that when the tapping noise was heard a loved one would soon die. We now know the tapping is a form of communication employed for finding mates, through research conducted by Martin Birch and colleagues at Oxford University (Goulson et al., 1994; Birch and Keenlyside, 1991).
Environmental conditions 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. However, a relatively small change to the environment can cause the attack to die out, or conversely, to become more active.
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. Many have argued that the death-watch beetle larvae, themselves, were introduced into the buildings within the timber used 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 the infestation. Our Irish collaborators based at University College, Dublin have studied the chemistry of Donkioporia expansa which has helped us understand the complex relationship between the death-watch beetle, timber, and fungus. Research has shown that the death-watch beetle uses fungally-produced compounds to ‘home in’ on suitable areas of timber for infestation.
Moisture
Chemical control
At present it is thought that a moisture content of 14% is the lower limit for a flourishing colony of death-watch beetles, and if the moisture content drops below 12%, 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%, 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. As temperatures within buildings can wildly fluctuate between summer and winter, causing subsequent changes in moisture content, it is likely that the beetle larvae can tolerate periodic drops in moisture below their optimal requirements. 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. The improved, drier environment must then be maintained year after year (Figure 2). 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.
Since the advent of chemical pesticides, they have been increasingly used in timber pest control. Various nasty chemical concoctions were developed from the beginning of the 20th century and applied as blanket treatments whenever pest problems were identified in buildings. For example, a formulation developed by Harold MaxwellLefroy of Imperial College, London, consisting of 50% tetrachloroethane, 40% trichloroethylene, 6% cedarwood oil, 2% solvent soap and 2% paraffin wax, was used for the treatment of the roof timbers of Westminster Hall in central London in the 1920s. In many countries, lindane and dieldran continue to be used for timber treatment. However, toxicity concerns are resulting in their replacement with safer pyrethroid alternatives such as permethrin.
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
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Figure 2. The nave of Salisbury Cathedral (left) which has a relatively low population of death-watch beetles thanks to building works aimed at reducing damp and humidity in the building. The roof area above the nave (right) was one of the trial sites used to test the efficacy of differently coloured sticky traps for pest monitoring.
PEST CONTROL The situation of blanket chemical treatments has become institutionalised through the property market and insurance company requirements for timber treatment certificates. However, more recently, several forces have been at work which call into question the use of pesticides, especially with regard to the death-watch beetle. There is an increasingly widespread concern about the safety of pesticides, with consumers in many countries demanding reduced usage in agriculture and in urban environments. This is affecting the alternatives available to the timber preservation industry. More importantly, with the death-watch beetle there are many reported incidents of poor efficacy of pesticide treatments, most notably in Westminster Hall (see above). The Hall experienced massive death-watch beetle damage after the Second World War initiated by ingress of rain water through bomb holes in the roof, and despite repeated pesticide treatments over a number of years, the Hall continued to have high infestations for decades until the timbers slowly dried out. The death-watch beetle is particularly ill-suited for control by pesticides for a number of reasons related to its biology and habitat. Perhaps one of the most difficult problems is related to pesticide application. The larvae of the death-watch beetle feed on hardwoods such as oak, often deep within large timbers hewn from entire trees. Because of the size of the timbers and the inaccessibility of the pest, even pesticide injected into affected timber under pressure often fails to reach its target. Other forms of pesticide delivery such as fumigating fogs or liquid and paste treatments on the surface of the timber are even worse at reaching larvae deep within it. Of course the adult deathwatch beetles must eventually emerge from the timber to mate and lay eggs. However, killing the adult is also no easy task. Adults usually emerge simultaneously in the spring within a fairly narrow window of time, and our research has shown that their contact with the treated surface is limited by their behaviour in seeking out cracks and crevices and their preferences for dark areas (Belmain et al., 2000). Poor treatment success is further compounded by the fact that the
larvae can take upwards of 10 years to complete their development (Fisher, 1940), implying that surface treatments with pesticides would have to be either very long lasting or repeatedly applied to the timber over a number of years to have any impact upon pest populations.
Figure 3. Research using UV insectocutors placed in roof spaces has found them to be effective tools in monitoring for the presence of timber pests. As the UV light is attractive to insects, it may also be contributing towards pest population reduction.
Figure 4. Coloured traps can be placed in relatively inaccessible roof spaces or other areas suspected of deathwatch beetle infestation. Based on the number of insects caught on the traps, appropriate remedial decisions can be made.
Monitoring Better prospects for timber pest control are now becoming a reality. Research within the Woodcare project has aimed at discovering potential solutions to control the death-watch beetle and preserve cultural landmarks such as cathedrals and palaces from continuing damage. The first step in designing an integrated approach to pest control is to ensure that there are effective monitoring tools to detect pest populations. The most common and perhaps most obvious method usually employed to identify actively infested timber has been the presence of insect emergence holes. However, most timbers in historical buildings are riddled with insect holes, and distinguishing between currently active and historical infestation is difficult. Our research has shown that holes and the presence of frass or wood dust around the holes do not necessarily indicate an active infestation and that death-watch beetles can also emerge out of pre-existing holes leaving no new tell-tale signs (Ridout, in press). Monitoring for pest infestation can be more effectively done using light traps (Figure 3) and sticky traps (Figure 4), which we have found to give a good indication of active infestation (Belmain et al., 1999). Another new monitoring tool is being developed by our collaborators in The Netherlands (TNO Building and Construction Research) using ultrasonic sensors. When these small sensors are placed on the surface of the wood, they can detect the sounds the insects make when eating the wood, thereby pinpointing areas of a building that are infested. This will allow the pest control operator to focus pesticide applications on specific areas, increasing efficacy and cost-effectiveness, as well as to monitor the success rate after treatment.
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PEST CONTROL Integrated control Effective monitoring and targeted application of pesticides is, however, only part of the new integrated approach being developed for timber pests.
Biological control Biological control, now widely used in agriculture, can also work in buildings. Predators of the death-watch beetle such as spiders and a small blue beetle, Korynetes caeruleus (Figure 5), are naturally found in buildings. The predatory beetle is particularly interesting because it seeks out its prey inside the timber providing a means of controlling pest larvae deep inside timbers where pesticides do not normally reach. Biological control alone is unlikely to rid our buildings of pests, but if pest managers promote conditions which protect predators, such as more targeted pesticide applications, predators can help keep pest populations in check. Studies we have conducted in infested churches have shown that changes in the spider population follow changes in their prey population of death-watch beetles. However, it remains to be seen whether proprietors of historical buildings can overcome their arachnophobia to control the death-watch beetle.
Heat sterilisation Heat sterilisation is currently receiving a lot of attention. It is claimed that a temperature of 52–55°C 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 much longer than one hour if this temperature is to be achieved throughout a 300 × 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.
Light traps Traps which use an appropriate wavelength which attracts beetles during the emergence season (late-March to late July) can be effective in controlling beetles. The simplest and least expensive form of such traps is the Beetle Screen which utilises replaceable sticky sheets of paper, and is hung in a roof space where a population of death-watch beetles is suspected. The limitation of these traps is that they rely upon the ability of the death-watch beetle adults to fly towards the light and become trapped. Our research has shown that ambient temperatures must exceed 19°C in order for adult beetles to readily fly. As such temperatures can not be guaranteed during a British spring, it may be necessary to artificially increase the temperature within the infested area using space heaters which would help ensure that the maximum number of beetles are caught.
Outlook Further efforts are being made to develop traps which are so attractive to the death-watch beetle that newly emergent adults go to their death before they have had the chance to 236
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Figure 5. The predatory beetle, Korynetes caeruleus. The adult eats the eggs of timber beetles, and its larval stage hunts down pest larvae deep within the timber.
mate and lay eggs, breaking the infestation cycle. No one has yet discovered sex pheromones produced by the deathwatch beetle, but both males and females are attracted to a range of compounds found in its preferred food, i.e. timber. It is hoped that, eventually, traps laced with the right compounds will provide yet another tool for the integrated management of timber pests. Through our research, several essential facts about the death-watch beetle are now better understood. This information can be used to develop cost-effective and environmentally sustainable control strategies for historical buildings with timber pest problems. The main hurdle now faced is one of putting theory into practice. For IPM in buildings to work, the pest control industry requires better education so that pest control operators can recognise active infestations through monitoring and implement targeted treatments. The general public should understand that a few insects inside their buildings does not necessarily mean the building is structurally undermined, but if neglected, could result in severe problems. The cornerstone of IPM in buildings will rely upon good building management. A healthy building is a sound and dry building, and good health means fewer pest problems.
References and Further Reading Belmain, S. R.; Blaney, W. M.; Simmonds, M. S. J. (1998) Host selection behaviour of deathwatch beetle, Xestobium rufovillosum de Geer (Coleoptera: Anobiidae): Oviposition preference choice assays testing old vs. new oak timber, Quercus sp. Entomologia Experimentalis et Applicata, 89, 193–199. Belmain, S. R.; Simmonds, M. S. J.; Blaney, W. M. (1999) Deathwatch beetle, Xestobium rufovillosum, in historical buildings: monitoring the pest and its predators. Entomologia Experimentalis et Applicata, 93, 97–104. Belmain, S. R.; Simmonds, M. S. J.; Blaney, W. M. (2000) Behavioral responses of adult deathwatch beetles, Xestobium rufovillosum de Geer (Coleoptera: Anobiidae), to light and dark. Journal of Insect Behavior, 13, 15–26. Birch, M. C.; Keenlyside, J. J. (1991) Tapping behavior is a rhythmic communication in the death-watch beetle, Xestobium
PEST CONTROL rufovillosum (Coleoptera, Anobiidae). Journal of Insect Behavior, 4, 257–263. Fisher, R. C. (1940) Studies of the biology of the deathwatch beetle, Xestobium rufovillosum de Geer. Part III. Fungal decay in timber in relation to the occurrence and rate of development of the insect. Annals of Applied Biology, 27, 545–557. Goulson, D.; Birch, M. C.; Wyatt, T. D. (1994) Mate location in the death-watch beetle. Xestobium rufovillosum de Geer (Anobiidae) – orientation to substrate vibrations. Animal Behaviour, 47, 899–907. Maxwell-Lefroy, H. (1924) The Treatment of the Deathwatch Beetle in Timber Roofs. Journal of the Royal Society of Arts 72, 260-270. Ridout, B (1999) Timber Decay in Buildings: A Conservation Approach. E & FN Spon. Ridout, B. (in press) Woodcare and the deathwatch beetle. English Heritage Research Transactions.
Dr Steven Belmain (
[email protected]) is a pest ecologist leading various research projects from developing control strategies for rodent problems in rural Mozambique to the use of insecticidal plant materials to control insect pests in Ghana. His PhD was conducted on the biology of death-watch beetle on research conducted within the Woodcare project whilst employed at Birkbeck College, University of London. Professor Monique Simmonds is head of the Biological Interactions group of the Jodrell Laboratory at the Royal Botanic Gardens, Kew. She is a world expert on insect-plant interactions using tools such as electrophysiology to understand how plant chemistry affects insect behaviour and physiology. Dr Brian Ridout is one of the foremost timber preservation experts in Europe. His expertise in the field led him to set up his own firm, Ridout Associates, offering advice, consultancy and remedial treatment for timber decay problems. He works closely with English Heritage and other bodies to assess, monitor and treat timber problems in historical buildings.
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