Left: Dorcus parallelipipedus Lesser stag beetle adult male (18-32mm) Right: Larva of stag beetle Lucanus cervus
These beetles lay their eggs on suitable dead or dying trees, mostly during spring-early summer. Female bark beetles create short tunnels under the bark and deposit eggs along the length of this maternal tunnel. On hatching, the larvae create individual tunnels that radiate out from the maternal tunnel. The pattern of these tunnels is often distinctive for a particular bark beetle species. Females of ambrosia-type beetles will seed the maternal tunnel with the fungi and yeasts that will provide sustenance for the larvae. When fully fed, bark beetle larvae pupate at the end of their tunnels. Most bark beetles have one generation a year and overwinter as pupa, with adults emerging in early summer.
Beetle larvae that feed on wood as they tunnel through the branches, trunk or roots of a tree have a diet that is very low in nutrients, so it can take several years before they are ready to pupate. The stag beetle can spend up to five years as a larva. Most wood-boring beetle larvae pupate within their tunnels but stag beetle larvae move out into the soil. They construct an egg-shaped chamber made of soil and wood fragments in which they pupate.
Role of dead wood beetles in gardens
Wood-boring beetles, along with fungal decay, help to break down and recycle dead wood. None of the native wood-boring beetles is a threat to garden plants but some exotic species, such as the Asian longhorn and the citrus longhorn, would be a problem if they become established here, as they have larvae that feed in a wide range of live trees.
Bark beetles associated with ash are likely to do well in the short term as the fungal disease known as ash dieback
spreads through Britain and provides breeding sites. This disease is spread by air-borne spores, unlike Dutch elm disease where spores are carried on the adult beetles’ bodies.
The larvae and pupae of bark beetles and wood-boring beetles provide food for woodpeckers. Beetle tunnels that have been vacated after the adult insects have emerged can be taken over by some solitary bees and wasps as nest sites. Even tunnels with a small diameter, such as those made by the common woodworm beetle, can be reused in this way.
Other sources of information
Mark Telfer's web page
on longhorn beetles
Hicken, N. E. (1963) The insect factor in wood decay. Hutchinson