The stag beetle Lucanus cervus has eclectic tastes, forming associations with a wide range of broad-leaved trees and shrubs as well as other woody material. A list of tree and shrub species in Suffolk with confirmed records of stag beetle larvae (see photo below) in the decaying wood (see second photo) was published in the Suffolk Naturalists’ Society’s Transactions Vol. 34, p. 43 (Hawes, 1998). Since then, a further 21 tree and shrub species have been added to this county list (See table at bottom).

Final instar larva of Lucanus cervus, almost ready to pupate

Larval habitat is in the rhizosphere, the plant root region below the soil surface. Here, decaying wood retains the moisture that the larvae require. Wood above ground often becomes too dry for stag beetle larvae to survive, though they sometimes feed on the underside of logs when these are in contact with the soil. Larval pabulum varies from the wood of dead tree stumps and roots to logs, the sunken part of wooden fence posts, woodchip, compost, and most strange of all, cat litter. Oak Quercus robur and elm Ulmus procera seem to be the most favoured food species, closely followed by apple Malus sp., sycamore Acer pseudoplatanus and birch Betula sp. Coniferous wood seems rarely to be used as larval pabulum., probably due to its sticky, resinous content. Both native and non-native tree and shrub species can provide food for the larvae.

Larval life, which can last up to six years, is spent partly in the soil and partly in the wood that provides their food. There are five instars. Stag beetle eggs are laid in the soil, sometimes up to 40cm deep, close to suitable dead wood (a female can lay up to 30 eggs). The first instar larvae hatch some three weeks after eggs are laid. The larvae then move towards the wood and tunnel into it using their chisel-tipped mandibles. Excavations are distinctive, so much so that tunnels made by the 4th and 5th instar larvae can be used to identify past occurrence of the species even when larvae are no longer present.

Piece of dead sycamore trunk showing excavations made by stag beetle larvae

When the 5th instar larvae have completed their growth they pupate. Pupation takes place underground, up to 40cm deep, inside a pupal cocoon. Each pre-pupal larva builds its own cocoon from the material available around it, such as soil particles and woody debris. Cocoons are often the size of a goose egg, oval in shape, with walls about 1cm thick, and are quite fragile.

Larvae are sometimes unearthed when gardening in composted soil, or when removing old shrubs or tree stumps. If they are accidentally exposed, they should be reburied as soon as possible with some of the decaying woody material on which they were feeding. However, if for some reason they cannot be reburied either where they were found or at another location in the garden, please place them in a sealed container with some moist (not wet) compost or decaying woody material and contact me, Colin Hawes: telephone 01473 310678 or Email c.hawes AT homecall.co.uk