Stag Beetles, Lucanus cervus, and How to Help Them.

by Judith John.

Male Stag Beetle.
Photo by Judith John.

Adult stag beetles emerge from soil beneath dead wood from late May through July and can be seen flying erratically at dusk or sometimes grounded on pavements. They mate and the females lay eggs in soil adjacent to dead wood. Adults are occasionally seen apparently feeding on sap runs or soft fruit, but they are very short lived and die shortly after mating.

Larvae hatch and burrow into dead wood which they eat, breaking it down and returning nutrients to the soil for 5-7 years until large enough to pupate. They then exit the dead wood, usually in April, and form a cocoon in which a pupa develops. This may be up to half metre below the soil surface. A beetle is finally formed after about 4 months. The young beetle remains in the soil over winter emerging to reproduce in late spring.

Stag Beetles are nationally scarce: their range has decreased since the 1970s so they are now mainly restricted to the south and south-east of the UK. South and south-west London have important populations. They are declining in Europe and globally threatened.

Female Stag Beetle.
Photo by Bill Welch.

43 records of stag beetles in the London Borough of Bromley were submitted to the Bromley Biodiversity Partnership Habitats and Species Sub-Group in 2017. They show a fairly wide distribution throughout the urban parts of the borough. The rural parts of the borough were less well represented but this may be due to several factors including lower numbers of recorders and the difficulties associated with seeing beetles flying in the dark in the countryside. In addition, quite large areas in the southern part of the borough have shallow, chalky soil which stag beetles generally avoid, possibly because in the cocoon stage and before leaving the soil as adults they tend to be found in quite deep soil.

London Wildlife Trust (LWT) have also been recording stag beetles so our records have gone to them and Greenspace Information for Greater London (GiGL).

It remains important to look after Bromley's stag beetle population however and the following measures can help them maintain and hopefully improve numbers:

1. Keep dead and decaying wood, including old fenceposts in situ where safe to do so and encourage others, including schools and sports grounds to do the same.

2. When dead trees are to be felled, leave as much of the trunk uncut as safety issues allow.

3. Avoid stump grinding, burning or chipping dead wood.

Stag Beetle larva.
Photo by Judith John.

4. Protect the area around dead wood from disturbance to prevent harm to newly laid eggs and pupae.

5. Construct a stag beetle loggery using hardwood (not conifers) in an area which is partially shaded. See this site: Help Stag Beetles and click on 'build a log pile for stag beetles instruction sheet'. Please note that log piles best for stag beetles involve burying logs vertically in soil as shown in the instruction sheet.

6. Cover water-butts during the adult flying season (May to July).

7. Rescue stag beetles from water butts (they can't swim) or the pavement where they may get trampled but put them close to where they were found and remember that females can nip so be careful and use gloves.

8. Avoid using polythene sheets to control weeds because newly emerging stag beetles can get trapped beneath them and die.

9. Avoid the use of insecticides.