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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, blackpoplar, 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 the timbers, at 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 a week or two later the females lay eggs, usually about fifty, 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 ten weeks, and males eight or nine weeks, at a temperature of about 20°C.

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


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