Oil Beetles

Oil Beetles

Oil beetles are large glossy black beetles with a small head and a swollen abdomen. They have short wing cases (so-called Elytra which are modified forewings) and lack hind wings so that they are flightless. There are five species in the UK and all have a similar life cycle. In the summer the adult females seek sandy soil in which to lay their eggs. They lay about 1000. The eggs hatch to produce a strange larva which is called a triungulin larva. It is about the size of an ant and has long legs with gripping claws at the end. It climbs up a flower stalk and then waits for a solitary bee to come to the flower. The triungulin larva leaps onto the bee and gets a lift to the bee’s nest where it turns into another maggot-like larva which feeds on the bee’s eggs and the pollen which the bee has collected. Eventually the larva pupates and the adult oil beetle finally emerges from the pupa. Oil beetles are sometimes called blister beetles because when they feel threatened they will emit a haemolymph from between their segments which is an irritant to the skin.

Four species of oil beetle have already gone extinct from the UK and some of the remaining species are very rare. They depend on the right habitat, which is the increasingly rare flower meadow. We are fortunate in having flower meadows in Highland Perthshire, for example the wonderful Weem Meadow (Site of Special Scientific Interest - SSSI), along the River Tay, and Keltneyburn Scottish Wildlife Reserve. I have found an oil beetle (probably the Black Oil beetle) at Braes of Foss and my friend, Peter Orton found them at the Black Wood of Rannoch. Flower meadows need the type of management that happened with the old fashioned methods of farming whereby they were grazed in late summer but not in spring or autumn. Cattle are ideal for this kind of management as they are non-selective grazers and will not take out all of one type of flower. If the grassland is not grazed the grass becomes rank and the flowers are out competed and swamped. Certain plants which are half parasitic on grasses (hemiparasites) are useful in weakening the grasses to give the flowers a chance. Such hemiparasites include red bartsia and yellow rattle which are parasitic on the roots of grasses but can also photosynthesise themselves.

The conservation organisation, Buglife, is interested in hearing about records of oil beetles and so if you find one let them know.

(23rd December 2019)