Oil Beetle

Cantharidin blisters

Oil beetles produce cantharidin — a blistering chemical agent — in their haemolymph (blood). When threatened, they exude this substance from joints in their legs in a process called reflexive bleeding. Cantharidin causes severe blisters on skin and mucous membranes. Historically, cantharides (Spanish fly, from a related beetle) were used as an aphrodisiac and in folk medicine; in large doses, it is acutely toxic. Handling oil beetles carelessly can cause blistering.

The triungulin strategy

The oil beetle life cycle involves one of the most extraordinary strategies in insects. Females lay thousands of tiny eggs in the soil. The first-stage larvae — called triungulins, for their three-clawed feet — climb onto flowers and wait for mining bees (Andrena species) to visit. The triungulin grabs the bee and is carried back to the bee’s burrow. There, it changes form to a parasitic grub and feeds on the bee’s pollen stores (and sometimes the bee larva itself). It passes through multiple larval stages before pupating. The entire development takes place hidden underground.

Flightless and distinctive

Oil beetles are unmistakable: slow-moving, flightless, with a shining metallic blue-black or oil-green body and a dramatically swollen abdomen that extends well beyond the vestigial wing cases. They walk clumsily over open ground in spring (February–May), particularly along coastal paths and chalk grassland — the best time to find them.

Declining

Oil beetles have declined severely in Britain due to loss of their host mining bees and the short turf habitats they both require. Coastal grassland management, rabbit grazing, and agri-environment schemes that maintain suitable habitat are essential for their conservation.