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Oil Beetle

Meloe proscarabaeus

Not yet photographed by the community

Flightless, large, blue-black; fascinating parasitic life cycle.

Species Profile

Amber List (UK Biodiversity Action Plan priority species); declining due to loss of wildflower-rich grasslands and solitary bee populations.

Lifespan: 2–3 years in the wild
Size & Weight: 10–35 mm in length; metallic blue-black or occasionally reddish colouration
Habitat: Rough grassland, woodland edges, and sunny banks where solitary bees are present, typically on calcareous or chalk soils.
UK Distribution: Found throughout southern England and Wales, becoming rarer northwards; present in scattered populations in the Midlands and southern Scotland. Resident year-round, with adults active in spring.
Diet: Larvae are parasitoids of solitary bee larvae (principally mining bees and mason bees); adults feed on vegetation including buttercups, dandelions, and other wildflowers.
Prey: Solitary bee larvae (Andrena and Osmia species)
Predators: Ground beetles, robber flies, and occasionally birds; relatively few natural predators due to toxic haemolymph containing cantharidin.
Mating Season: April to June
Breeding: Females lay 4,000–6,000 eggs in batches in soil near bee nests. Larvae aggregate in large numbers ('triungulin' stage) and attach to foraging bees; development within host larvae takes several weeks. Single generation per year.
Behaviour: Oil beetles are slow-moving, diurnal insects that often cluster on vegetation in spring. They are gregarious during the larval stage, forming distinctive aggregations. When threatened, adults exude droplets of cantharidin-rich fluid from leg joints as a defence, which can cause blistering.

Did You Know?

•The species is named for the oily, toxic secretion (cantharidin) it produces, which has been used historically in traditional medicine and blister beetle preparations.
•Larvae use an extraordinary strategy: they aggregate on flowers and climb onto passing solitary bees, then travel to the bee's nest where they parasitise the host larva.
•Oil beetles are one of the few insects that produce and store the chemical cantharidin, a potent vesicant (blister-causing compound) as a defence mechanism.
•The species exhibits hypermetamorphosis, with distinct larval forms including the highly mobile triungulin stage adapted for bee-seeking behaviour.
•Meloe proscarabaeus populations have declined significantly in the UK over the past 50 years, correlating with the loss of flower-rich grasslands and declines in solitary bee populations.

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