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Southern England, with its fine heritage of medieval churches, is a rich hunting ground for those interested in the history of ancient buildings. These buildings often play a large part in village life, with parishioners struggling to raise money for the church restoration fund. Usually, it is the heavy oak timbers in the roof which need urgent attention. For these are the haunt of the deathwatch beetle, Xestobium rufovillosum.

The larvae of the deathwatch beetle damage the structure of timbers. The soft, white grubs eat their way through the wood, taking between 2 and 10 or more years to complete their development (see Box 1 for more details of the life history). Often they go undetected for many years, until the strength of the timber is undermined. Costly rescue work or replacement of the timbers is then unavoidable.

Although the larvae may not be detected for years, the adults can be heard in spring and early summer. Once they have emerged from the timber, they make a tapping noise by knocking the front of their heads against the timber.

The beetle got its grisly name in the days when the British usually died at home in their own beds. During the death vigil, or deathwatch, when the house was still, the tapping of the beetles was clearly audible. When a beetle taps, it sounds as though the noise comes from within the very structure of the house, and people regarded it as the countdown to death – the sound, perhaps, of the grim reaper, tapping impatiently with a bony finger waiting for the loved one to die.

But this superstition was questioned as early as 1695, when Benjamin Allen reported in the Royal Society’s Philosophical Transactions that: ‘Yet I have known (the deathwatch) to be heard by many, where no mortality follow’d; and particularly by myself, who have taken Two of the same, Seven Years since, without any Death following that year.’ Allen’s description of the beetle is sufficiently detailed for us to be certain that the beetle he studied was the same species that, in the British Isles at least, is known as the deathwatch beetle. John Wilkins, the Dean of Ripon, had 27 years earlier described the tapping of an insect which he called the deathwatch beetle. This was in his comprehensive book on animals, plants and minerals, entitled An Essay Towards a Real Character and a Philosophical Language. He likened the beetle’s noise to the ticking of a watch and claimed that the insect accomplished this ‘by striking the bottom of his breast against his belly’.

We now know that the deathwatch beetle makes its tapping noise by knocking the frons, or front of the head, against the timber. Both sexes do it and it has always been assumed that the tapping has something to do with sexual communication. Much of our present-day knowledge of these insects comes from studies by Ronald Fisher in the Forest Products Research Laboratories at Princes Risborough, Buckinghamshire, during the 1930s and 1940s. But despite his classic work, our understanding of the beetle’s behaviour has not progressed much beyond conjecture and anecdote. Recently, we have studied its life history, evolution and adult behaviour.

The deathwatch beetle’s name is not entirely misleading, at least as far as the decay of mature trees is concerned. The insect exploits the resources locked up in sapwood and heartwood, after the tree has died, been seasoned and then weakened by fungal decay. In the absence of human intervention, the beetles attack only dead standing wood, not trees felled while alive. Over many years, the larvae weaken a dead tree’s structural support, making it more likely to fall.

The fact that the insects can live in buildings, in wood too dry for fungi to grow, shows that the species is remarkably flexible. Because building timbers are sheltered from the weather, and so decay slowly, most buildings are a poor habitat for developing larvae, compared with the wood in decaying trees. But when leaking water or condensation gathers on timbers, fungal decay allows the beetles to breed. In some buildings, populations of beetles have apparently become extinct spontaneously. But many may have survived for long periods, probably for as long as the building has been standing (see Box 2 for further details on the dispersal of beetles).

A well-known example is the splendid Westminster Hall in London, built in 1394 under the auspices of Richard II. When the roofing timbers were inspected in 1913, Sir Frank Baines found that generations of deathwatch beetles had reduced some major structural timbers to a ‘thin shell’. Only emergency action prevented the roof collapsing. The timbers harboured an enclosed population, which may have been living there for 500 years.

Damage by beetles can be limited by maintaining roofs in good repair, and eliminating the humid, still air that encourages fungal decay. But in the short term, only long-acting pesticides can eradicate the insects. No other way of controlling the beetles is available at present.

There is no effective ‘natural method’ of eradicating the beetle because a suitable predator or parasite has not been found. Two species feed on deathwatch beetles: the metallic blue clerid Korynetes caeruleus and a Pyemotes mite sometimes referred to as P. venticosus (the taxonomy is currently under review). The mite will bite humans, and in severe cases produces flu-like symptoms. The unpleasant itching and swelling that accompanies the bite of these mites makes their use as a natural control agent inadvisable. Another problem is that any potential predator would find it difficult to reach its prey: during development, deathwatch beetle larvae are usually hidden deep within oak timbers.

An alternative approach would be to interfere with the beetle’s reproduction. The deathwatch beetle, a member of the family Anobiidae, is a relative of the woodworm or common furniture beetle. But unlike the woodworm, it does not seem to make use of scent or pheromones in its sex life. Experiments with deathwatch beetles in a wind tunnel failed to yield any evidence for a sex pheromone. Nor is it possible to exploit wood smells as cues to deceive the beetles and prevent egg laying on the timber, for the beetles appear not to rely on these types of odours either.

The infamous deathwatch tap has long been assumed to be a sex call central to the beetle’s mating game. Perhaps by understanding how males and females identify and find each other, and so mate and lay eggs, we could learn how to limit damage by deathwatch beetles to Britain’s unique historic buildings. One of us (MB), working with Julian Keenlyside in Oxford, set out to test whether tapping really is a means of sexual communication.

A church roof is not an easy place to study these beetles, even if it is a good place to find them, so we brought them into the laboratory. We placed a pair of beetles, either a male and a female, or two males or two females, 30 centimetres apart on a small strip of an artificial roof beam. The wood was marked at intervals so that we could measure how far an individual walked.

Typically, a male deathwatch beetle begins a series of tapping bouts, each separated by a short pause. A female responds with tapping bouts of her own, usually within 2 seconds of the male’s tapping. The male then moves towards the female, stopping now and then to tap. The female responds again by tapping. In this way, the beetles eventually meet and mate. This communication conveys three kinds of information: the presence of sexually mature, mate-seeking males, the presence of responding females, and the direction of a potential mate.

Males initiate more tapping bouts than females. Females rarely tap on their own, but respond quickly to taps from either sex. They make many more taps than do males. Although a male may tap on his own, he does not respond to tapping bouts as quickly as a female. When male and female beetles are paired on the experimental strip, the males always move about more than the females. So the usual pattern is that the male begins the ‘conversation’, the female quickly replies, and the male walks towards the female.

Do beetles perceive the taps through airborne vibrations or through the wood? To find out, we placed male and female beetles close to each other on the strip but isolated inside a shallow Perspex ring. We then placed a second female, slightly closer to the male, on an adjacent strip. When the two strips were separate, the female on the same piece of wood as the calling male responded to his tapping bouts, while the female on the separate strip did not. But when the strips were pushed together, both females responded to the tapping male. So vibrations through the wood seem to be is the main channel of communication.

Vibrations are picked up by a dense pad on the tarsal segments of each of the beetle’s six feet, and we are study-ing these pads with electron microscopy. The six tarsi form a ‘sensory field’ of receptors around the beetle which the male uses to guide himself towards the taps made by a receptive female.

Unfortunately, we are not yet close to a feasible form of biological control. At first, we thought we might be able to use the deathwatch beetles’ tapping to disrupt their mating, but this notion now strikes us as fanciful. One idea was to create an artificial ‘siren’ female which would respond to tapping males and lure them to ‘her’, preventing them from finding real females. Another idea was that setting up continual vibrations through the timbers, at the same frequency as the taps, would mask the beetle’s tapping and prevent mating. The trouble is that every susceptible timber, including church pews, would have to carry the vibration. The adult beetles are active for two or three months each year, and if the vibrations stopped for services, the beetles might seize the opportunity to mate.

We need to learn the whole communication ‘language’ of the beetle to find a behaviour that we can exploit. Meanwhile, our discoveries about its sexual communication could lead to a better way of detecting infestation in timbers. It might be possible to use recordings of tapping beetles to generate answering vibrations in the timbers, and so detect them before they have done a great deal of damage.

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1: Hidden history: the beetle’s secret cycle of life

The deathwatch beetle is thought of as the devil’s pest in churches and old houses, but in natural habitats it infests a wide range of decaying hardwoods. It has been found in hornbeam, sweet chestnut, hawthorn, beech, ash, black poplar, elm, larch, spruce and yew, but the two most commonly infested species in Britain are oak and willow.

In buildings, oak timbers are usually the focus of attack by the beetle, but alder, walnut, elm, larch and Scots pine can be affected too. Deathwatch beetles attack wood that has been decayed by fungi, so it is the damp-prone parts of timbers, the ends and near leaking gutters and enclosed spaces, that are normally attacked first.

Adult beetles emerge from holes in the timber in spring, or occasionally in autumn. They breed once and die. Mating lasts between half an hour and one-and-a-quarter hours. A week or two later the females lay eggs, usually about 50, in small cracks on the surface of the wood. Adults depend on stored reserves; they do not feed. So the adult lifespan is largely determined by body size and metabolic demands. Emergent females rarely live for more than 10 weeks, and males 8 or 9 weeks, at a temperature of about 20 °C.

The eggs hatch after 2 to 5 weeks, and the larvae then wander across the wood to find suitable entry points through which to bore into the timber. They then take between 2 and 10 years to complete their development. The larvae pupate in late summer to early autumn, each individual having constructed a pupal cell just below the surface of the wood. After 2 or 3 weeks, the immature beetle emerges from the pupal skin, but then remains torpid inside the chamber until the following spring or early summer. Then the mature beetle cuts a perfectly round hole, 3 to 5 millimetres in diameter, and emerges covered in a fine layer of wood powder.

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2: How beetles find their timbers

Adult beetles can fly, although few people have seen them do so under natural conditions. Some naturalists have suggested that they walk from tree to tree, but that fails to explain their presence in isolated country churches or in ancient cathedrals in city centres. It is inconceivable that they walk from one old building to another. Most of the populations of deathwatch beetles in old buildings have probably been isolated since they arrived with the timber used to build or repair them.

No doubt some of the timber used to build the great cathedrals and halls of Britain contained deathwatch beetles. The timber often came from royal forests that must have grown the ancient trees that the beetle favours.

Nobody knows how deathwatch beetles disperse in the wild. Although adults are usually inactive, they can walk quite fast when stimulated. When trees are far apart, the beetles are unlikely to walk from one to another, and they can fly only at high temperatures. The beetles can take off from a surface warmed to 30 to 40 °C in an ambient temperature above 22 °C. But such temperatures are rarely reached during spring and early summer in Britain when the beetles are active. Also, because the beetles do not appear to respond to potential pheromones or dead wood extracts, no one knows how they find suitable wood to exploit.

Paul Buckland, working from the University of Birmingham in the 1970s, suggested that populations of deathwatch beetles living in buildings in the north of England are isolated relics. If the beetles evolved in the now rare habitat of mature lowland deciduous forest, the countryside of today may have created islands of wild populations that have no nearby old trees to colonise. Britain might now support two kinds of populations: one in the wild, which may well be in decline, and another made up of a set of isolated populations in old buildings.

Martin Birch lectures in the Department of Zoology at the University of Oxford, and Guy Menendez was a research student in the Department of Pure and Applied Zoology at the University of Reading