(De Geer, 1774)
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 about 7 mm (0.3 in) long. Eggs are laid in crevices in old wood inside buildings, and the larvae bore their way 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. They obtain sufficient nourishment by using a number of 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 tunnelling 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 largely limited to historic buildings, because modern buildings tend to use softwoods for joists and rafters, while the beetle prefers aged oak timbers.
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, a superstition has grown up that these sounds are an omen of impending death.
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, 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.
The adult deathwatch beetle is cylindrical and some 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. The larvae are pale yellow with black jaws and a pair of eyespots on either side of the head. They grow to about 11 mm (0.4 in) long. The eggs are white, slightly pointed at one end and sticky.
Distribution and habitat
This beetle is found in Europe, including the United Kingdom, and eastern North America. Its natural habitat is dead hardwood, especially where the timber has been softened by fungal attack. 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.
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. Mating takes place in a concealed location and lasts for about an hour. The beetles communicate with each other by tapping their heads on the timber, making a characteristic, rhythmic, clicking sound which can be heard in the summer. These are sexual signals and facilitate finding a mate. 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 60 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).
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 tunnelling 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.
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 (faecal 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 faecal 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.
Direct examination of the interior of the timber by destructive means is often not acceptable, and non-invasive means are required. Other means of identifying the wood-boring insects include pheromone traps; these are effective for the common furniture beetle and the house longhorn beetle (Hylotrupes bajulus) but not for the deathwatch beetle. However, adults of the deathwatch beetle are attracted to light. The sounds of the feeding larvae can be heard either unaided or with the help of a stethoscope, and X-ray scans and computer tomography can also be used. Similarly, active larvae may be identified by vibrations in the ultrasound range. The exit holes of deathwatch beetles are 2 to 3 mm (about 0.1 inch) in diameter, larger than those produced by the common furniture beetle.
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.
Modern techniques of ultrasound examination now allow the extent and localisation of an attack within timbers to be determined with great accuracy, and, for historic properties where damage to ornate plasterwork must be avoided, can be followed by micro-drilling and highly targeted injection of insecticide via hypodermic needle. Alternatively, where a degree of damage to the fabric of a building is acceptable, larger 6mm holes can be drilled deep into the timbers, and a thick, insecticide-laden paste introduced which does not seep out into surrounding areas. In all situations, any structural damage which has permitted water to ingress and moisten the timbers now being attacked should be addressed in order to slow down the life cycle of the insects, and thus minimise their spread.
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.
"...within ye hear
No sound so loud as when on curtain'd bier
The death-watch tick is stifled."
("Stifled" because the death it was portending has taken place.)
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."
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; whenever anyone hears its clicking, that person soon dies.
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