These beetles are named after their ability to produce a bitter oil-like fluid from their knee joints when disturbed – this deters most predators. They have a very unusual life history. The adult beetles mate soon after emergence. In the Violet and Black Oil beetles the males have kinked antennae which are used to grab on to the female’s antennae during courtship. Once mated the pair remain together for over an hour during which time the male is dragged along behind the female wherever she goes. Watch a video of Black Oil Beetle courtship here.
After mating the female digs a short burrow in the soil into which she lays a batch of about 1000 eggs before covering the entrance with soil. In her lifetime (about 2 months) a female may lay up to 40, 000 eggs.
When the eggs emerge the larvae, known as triungulins as they have 3 hooks on each foot, climb up vegetation and sit on flowers. More information on identifying triungulins They lie in wait before attaching themselves to any insects which visit the flower. The few that survive are those taken back to the nests of solitary mining bees. Here the triungulin disembarks and consumes the egg and pollen supply of the host bee. It then pupates and emerges as an adult the following year.
Most oil beetles are active in the spring and early summer and can be found where solitary bees abound in meadows, coastal grasslands and woodland glades. The adults are active on sunny days and feed on the leaves and flowers of a number of plants including celandines, buttercups and dandelions. The Rugged and Mediterranean Oil Beetles differ from the others by being active during the late autumn and winter and are mainly nocturnal.
Oil beetles often attract small midges which feed on the oil produced by the beetle but do it no harm.