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 averages in length. Eggs are laid in dark crevices in old wood inside buildings, trees, or tunnels left behind by previous larvae. The larvae bore into the timber, feeding for up to ten years before pupating, after which they emerge 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, infestations by the beetles are often limited to historic buildings, because modern buildings tend to use softwoods for joists and rafters, rather than the 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. For that reason, the deathwatch beetle is associated with quiet, sleepless nights, so it was named for the vigil (watch) being kept beside the dying or dead. By extension, there exists a superstition that the sounds are an omen of impending death.

Taxonomy

The deathwatch beetle is part of the beetle family Ptinidae, formerly known as Anobiidae. This includes a number of subfamilies including Ptininae, the spider beetles which are mostly scavengers, Anobiinae, wood-boring beetles, and Ernobiinae, deathwatch beetles, also wood-borers. In 1912, Maurice Pic erected Ernobiinae for beetles previously classified under Dryophilini by Fall in 1905. White elevated this taxon to subfamily status in 1962 and 1971, and in 1974 included 14 genera in the subfamily.

Description

thumb|Adult in side view

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. 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. 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.

Ecology

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.

Host selection

An adult female deathwatch beetle is short-lived (1–2 months) and must find a suitable host in which to lay her eggs relatively quickly. She is capable of using odour to locate wood that has been decayed by fungi, which provides an excellent host. When selecting a host, old wood (more than a century old) is favoured, though it is unknown whether it is due to prevalence of fungus, general state of decay, or other factors.

Communication

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.

Identification of which insect is present in interior timbers is difficult; by their nature, the larvae are tucked away from sight in their galleries. The presence of wood-boring insects may be indicated by frass (fecal residue) and fresh dust. Recent exit holes often have bright rims, while the rims of older holes have become dull. The species of insects involved can sometimes be identified by examination of the fecal pellets in the frass. Adult beetles, alive or dead, may be present on the glass or the sills of windows, as may the specific enemies of the beetles in the same locations—a likely indication of specific wood-boring insects inside.

The larvae of deathwatch beetle feed deep within timbers. Recent studies have suggested that most of the previously accepted practices of external application of insecticides are largely ineffective. Only gas fumigation remains effective, but poses considerable practical challenges in effectively sealing the larger, historic types of properties that these beetles are mostly attracted to. External insecticide application may, in fact, do more harm than good by killing the natural enemies of the beetle. One way of dealing with the problem may be with the use of ultra-violet "insectocutors", to attract and kill the adults that emerge from the wood in the spring. If there is concern about the strength of structural timbers, a structural surveyor can drill core samples to determine the condition of the wood.

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.

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.

References

  • Death watch beetle tapping on wood
  • Museumpests.net Death watch beetle factsheet