INSECTS

Death-watch beetle continued

Eggs

Larva

Reproduction
Between April and May the female beetle lays up to 50 eggs. The lemon-shaped eggs measure less than a millimetre and are pearly-white with a circular translucent patch at the head. Shortly before hatching, which takes between three to five weeks, the eggs become more opaque. High temperature and humidity levels considerably increase the numbers of eggs laid and reduce the time before they hatch.

The creamy-white larvae covered in golden hairs, are hook-shaped, have six legs, and grow up to 10 mm. Soon after hatching, larvae are active, searching for a suitable crevice, preferably of oak or elm, into which they burrow. It is the wood-eating activities of the larvae, not the adults, that affects wooden structures. Such damage to old timber can be very severe and result in the total failure of load-bearing beams.

The larval stage may vary from one year in ideal conditions to 12 years or more, if conditions are not favourable.They grow up to 6–9 mm.

The mature larvae eventually enlarges a gallery immediately beneath the surface of the wood for a pupal chamber. They pupate in the autumn and the metamorphosis to the adult takes place a few weeks later. The pupae are also creamy white and resemble the adult beetle in shape but the legs and antennae are held down (though not adhered to the body) by the thin, transparent, pupal skin or case.

Beetles remain in the timber until the following spring, when they usually gnaw their way out though characteristic exit-holes of 3–5 mm. Their colour is dark chocolate brown to greyish brown thickly tessellated with yellowish scale-like hairs in small patches. However, the yellow scales rub off a few days after emergence when the colour becomes more reddish. The role of the adult beetle is solely to reproduce.

Once the female has mated she must find an oviposition site for egg laying which has the potential for feeding the new generation. After mating and the laying of eggs adult beetles live through the summer season and tend to die off in the autumn.

It has also been shown that the adults do not necessarily need to emerge, and can mate in cavities within the timber, and further, that adult females, if they have emerged to mate, sometimes re-enter existing flight-holes and lay their eggs deep in the timber, rather than on or near the surface. What is still unknown is whether some adults have always mated and laid their eggs without emerging, or whether this behaviour has evolved to counter surface chemical treatments.