The deathwatch beetle (Xestobium rufovillosum) is a species of woodboring beetle that sometimes infests the structural timbers of old buildings. The adult beetle is brown and measures on average 7 mm (0.3 in) long. Eggs are laid in dark crevices in old wood inside buildings, trees, and inside tunnels left behind by previous larvae. The larvae bore into the timber, feeding for up to ten years before pupating, and later emerging from the wood as adult beetles. Timber that has been damp and is affected by fungal decay is soft enough for the larvae to chew through. They obtain nourishment by using enzymes present in their gut to digest the cellulose and hemicellulose in the wood.
The larvae of deathwatch beetles weaken the structural timbers of a building by tunneling through them. Treatment with insecticides to kill the larvae is largely ineffective, and killing the adult beetles when they emerge in spring and early summer may be a better option. However, infestation by these beetles is often limited to historic buildings, because modern buildings tend to use softwoods for joists and rafters instead of aged oak timbers, which the beetles prefer.
To attract mates, the adult insects create a tapping or ticking sound that can sometimes be heard in the rafters of old buildings on summer nights; therefore, the deathwatch beetle is associated with quiet, sleepless nights and is named for the vigil (watch) being kept beside the dying or dead. By extension, there exists a superstition that these sounds are an omen of impending death.
The tapping sound of the deathwatch beetle has long been associated as a harbinger of death, being most audible on quiet nights in the rafters of old houses, and in silent bedside vigils for the dying.
The English writer, physician, and naturalist Thomas Browne (1605-1682) attempted to correct misconceptions about the deathwatch beetle as an omen of death in his encyclopedic catalog of common errors, Pseudodoxia Epidemica:
Its notoriety as an ill omen is alluded to in the fourth book of John Keats' 1818 poem "Endymion":
The term "death watch" has been applied to a variety of other ticking insects, including Anobium striatum; some of the so-called booklice of the family Psocidae, and the appropriately named Atropos divinatoria and Clothilla pulsatoria (in Greek mythology Atropos and Clotho were two of the three moirai (Fates) associated with death).
In 1838 Henry David Thoreau published an essay mentioning the deathwatch beetle. It is possible that this essay influenced Edgar Allan Poe's 1843 short story "The Tell-Tale Heart" and that the sound the protagonist was hearing at the end of that story was that of a beetle tapping inside the wall, not the beating of the (dead) victim's heart. However, it is more likely that it was the metronomic ticking of a booklouse rather than the groups of six to eight taps made by the deathwatch beetle.
The beetle was referenced in Mark Twain's 1876 The Adventures of Tom Sawyer: "Next the ghastly ticking of a deathwatch in the wall at the bed's head made Tom shudder – it meant that somebody's days were numbered."
Even Beatrix Potter references the beetle in her children's book The Tailor of Gloucester (written 1901, published 1903) when the mice under the tea-cups start up "a chorus of little tappings, all sounding together, and answering one another, like watch-beetles in an old worm-eaten window-shutter—".
In Dorothy L. Sayers' Gaudy Night (chapter 17), the mechanism of the ticking of the death-watch beetle is discussed, and it is compared with a clicking sound made by an ill-fitting hard shirt front.
In 1988, Linda Pastan wrote a poem entitled "The Deathwatch Beetle". In 1995, Alice Hoffman made reference to the deathwatch beetle in her novel Practical Magic, using it as an omen of death; the main character hears it shortly before her husband dies.
The eggs are white, slightly pointed at one end and sticky. Eggs measure on average 0.7 mm (~0.03 in) in length and 0.5 mm (~0.02 in) in width.
The larvae are creamy-white with six legs, black jaws, a pair of eyespots on either side of the head. They grow to about 11 mm (0.4 in) long making them the largest Ptininae found in Britain. These larvae are distinctive due to a swollen thoracic region and multiple golden setae.
The pupa, when newly formed, is shiny and milky white in colour. They will gradually darken as they mature and produce eyes, tarsi, and “teeth”. During this stage of development, they will completely change appearance by forming a head, complete eyes, mouthparts, antennae, and legs. The pupa measures 7–8 mm in length and around 3 mm in width.
The adult deathwatch beetle is cylindrical measuring on average 6 to 7.5 mm (0.24 to 0.30 in) long. The head is largely concealed by a brown thoracic shield. The shield and elytra are dark brown or reddish-brown, with a patchy felting of yellowish-grey short hairs. The antennae have eleven segments, the distal three segments are somewhat enlarged.
This beetle is found in Europe, including the United Kingdom, as well as North America, Corsica, Algeria, and New Caledonia. Its natural habitat is dead or decaying hardwood, or in some cases coniferous wood, especially when the timber has been softened by fungal attack. This may be due to the way fungal decayed wood affects nitrogen metabolism in the deathwatch beetle. Decayed wood is also much easier for the larvae of the deathwatch beetle to bore into which allows them to develop at a faster rate. The sapwood is more nutritious and is usually attacked first, followed by heart wood that has been softened by decay. Oak (Quercus spp.) is the main host, with American oaks being more susceptible than European oaks. Pollarded willow is also attacked in the United Kingdom. The beetle does not infest wood that has recently died; about sixty years must pass for dead oak to reach a suitable condition for attack. These beetles tend to stay on the same piece of wood for several generations until resources are used up and the piece of wood is no longer sufficient.
In buildings, deathwatch beetles infest old oak timbers, especially those that have been the subject of fungal decay, usually by the fungus Donkioporia expansa. This fungus affects damp timber, often gaining entry where rafters or joists are embedded in stone walls, or in the vicinity of leaking roofs or overflowing gutters. It is not the adult insects that cause structural damage to the building, but rather their larvae tunneling through the wood.
Wood is difficult to digest, but as long as the wood has been softened by fungal decay, the enzymes in the guts of the larvae are able to digest the cellulose and hemicellulose forming the cell walls; this enables the larvae to make use of the protein, starch, and sugars found within the cells.
The steely blue beetle (Korynetes caeruleus) is a predator of the deathwatch beetle and of the common furniture beetle (Anobium punctatum). The adult female blue beetle lays her eggs in the exit holes made by the emerging borers, and the carnivorous larvae wander through the galleries made by the wood-borers, feeding on their larvae. The adult deathwatch beetles are weak fliers and may run over the surface of the timber, rather than fly. They are sometimes caught by spiders, their silk-encased husks being found on webs.
A deathwatch beetle communicates by hitting its head on a substrate to create a noise, a method called tapping. Males and females differ in that males usually tap first, and females tap only in response to males. A female responds within 2 seconds of a male tap. After the female responds, a male will tap again from 2 to 30 seconds later. The taps create a substrate-borne vibration. This long-distance communication mode differs from that of most wood-boring beetles, which use pheromones.
To locate females, males will walk a short distance, stop and tap, orient themselves towards a female's response, and repeat. If females respond they advertise their receptivity. Recently mated females will not respond. Each tapping bout contains between 4–11 taps at an average frequency of 10 Hz. Females will only respond to tapping bouts with 6 or more taps and only bouts with a frequency of 4–20 Hz. Males with higher frequencies are more likely to obtain a mate than males with lower frequencies.
In Britain, the adults emerge in April, May or June. The males emerge first, and the females are willing to copulate as soon as they emerge, often in the afternoon. Emergence only occurs in temperatures above 10 °C (50 °F). Mating takes place in a concealed location, mainly on surface wood, and lasts for about an hour. Females lay eggs in crevices in the wood or in the holes left by emerging beetles, The adults do not feed, and so die within a few weeks, by which time the female may have laid 40 to 80 eggs in small batches.
The eggs hatch after about a month. The newly hatched larvae are tiny and chew their way into the timber, feeding on the wood. Their growth is slow and it may take from two to ten years, or even more, for them to reach their full size. At this stage they pupate in a chamber close to the wood surface, and either emerge through a newly created hole after twenty to thirty days, or else emerge in the following spring (about eleven months later).
Females have been shown to be selective of which males they mate with. During mating, males give up a significant fraction of their body mass, an average of 13.5%, via ejaculation of the spermatophore. This is a nutritional nuptial gift to the female. Although females cannot tell the mass of the male by looking at them, females can instead determine the mass of the male when the male tries to climb on the females back and mount them. Since male deathwatch beetles do not feed, their resources for the gift have been stored from the larval stage. Males who are heavier in mass are capable of donating a larger mass to the female than lighter males which results in females choosing heavier males and rejecting lighter males. By giving up this much body weight, males are reducing the likelihood that they will mate with an additional female due to a lack of resources for a further gift.
