How Long Do Stag Beetles Live? Years Underground as Larvae

The stag beetle life cycle at a glance

The European stag beetle goes through complete metamorphosis, like all beetles: egg, larva, pupa and adult. The stages are very unequal in length.

Years underground as larvae

After mating in early summer, a female stag beetle burrows into the soil beside old stumps, roots or buried logs and lays a few dozen eggs. The larvae hatch within weeks and start eating the rotting wood.

Stag beetle larvae are soft, creamy white and curled into a C shape, with an orange head, strong brown jaws and six orange legs. They molt twice, passing through three larval stages, and a fully grown larva can be up to about 10 cm (4 in) long. They can even squeak to each other by rubbing a ridged leg against a comb on another leg.

The larval stage usually lasts three to seven years. The main reason is the food. Rotting wood is poor in nutrients, and the larvae rely on fungi and bacteria to break it down. Larvae grow faster in warm soils and in wood that is well decayed and rich, and more slowly in cool climates or in poor wood.

Because the larvae live hidden underground, nobody can easily follow a single wild larva from egg to adult. The commonly quoted range of three to seven years comes from rearing larvae and from long-term observation of breeding sites, which is why sources give a range rather than one exact figure.

Pupa and the hidden winter

When a larva is fully grown, it leaves its food and builds a cocoon in the soil from earth and chewed wood, roughly the size of an orange. It pupates there in late summer or autumn. The pupa already shows the shape of the adult, including the long jaws of a male.

The adult beetle forms within the cocoon in autumn, but it does not dig out straight away. It stays underground through the winter, its body hardening and darkening, and only emerges in late May or June of the following year, when the soil warms up.

A short life above ground

Once above ground, adult stag beetles have just one job: to reproduce. Males emerge a little earlier than females and fly at dusk on warm evenings looking for mates, often fighting rivals with their antler-like jaws, explained in why stag beetles have big jaws.

Adults hardly feed. They drink tree sap and the juice of overripe fruit, but live mainly on fat reserves from their larval years. Most males die by the end of July. Females survive a little longer, until they have laid their eggs, and a few are still seen in August. In total, an adult lives only a few weeks.

Many adults die even sooner. Magpies, crows, foxes, badgers and cats catch them, and many are run over on roads or paths on warm evenings.

How does this compare with other insects?

Long, hidden juvenile stages followed by a short adult life are common among insects. Mayflies live for a year or more as larvae in water, but adults of some species live only a day or less. North American periodical cicadas spend 13 or 17 years underground as nymphs before emerging for a few weeks.

Beetles that feed on dead wood, called saproxylic beetles, often develop slowly because their food is so poor. The stag beetle is one of the best-known examples in Europe. Tropical giants such as the Hercules beetle grow faster in warm, rich rotting wood, but still spend most of their lives as larvae.

Why dead wood matters

Because a stag beetle needs the same buried, rotting wood for several years, it is very sensitive to tidying. Removing stumps and old roots, clearing dead trees from woods and parks, and burning garden logs can wipe out several generations of larvae at once. A stump that stays in the ground for decades can be used again and again.

This is one reason the stag beetle has declined in northern and central Europe and is listed as Near Threatened in the IUCN European assessment. Leaving stumps in place, letting fallen wood rot where it lies and burying logs upright in the soil all give larvae the years of food they need.