Oil Beetles

Oil beetle Life Cycle

The Oil beetle has one of the most extraordinary life cycles of any British insect. It is parasitic on various species of ground nesting Solitary bee and most usually the large adults will be encountered in early spring wandering around on paths in heathland, grassland and in open woodland near where they emerged.

The adults can live for up to 5 months. Female Oil beetles are considerably bigger than males and become much larger when gravid with eggs. The females will excavate a small burrow in sandy soil near the bee colonies in late spring and lay up to 1000 eggs. The eggs may remain in the burrow before hatching the following year to coincide with the emergence of the bee hosts.

The tiny larvae are known as tringulins and are extremely active, and will rapidly climb onto vegetation in search of flowers. On the flowerheads many larvae will congregate, waiting for a bee to come and pollinate the flower. Once the bee has landed the triungulins will rapidly attach themselves with specially adapted hook-like forelegs and if the bee is female and a solitary ground nesting species, then the larvae will be carried back to the bee’s nest burrow. The female Oil beetle must therefore lay lots of eggs to ensure that at least some larvae will find a bee nest.

In the burrow

Once inside the bee’s nest, the triungulin changes into an entirely different larval form, which more closely resembles a maggot. This change is necessary as it allows the larvae to feed on the bee’s egg and pollen store without becoming entrapped within the pollen. The larvae grows rapidly on such a sugar-rich source of food and after undergoing 3 more moults it pupates and overwinters in the bee burrow before emerging the following year as an adult Oil beetle.

The adults feed on the leaves of various herbs such as buttercup and lesser celandine.

Oil beetle (Meloe violaceus) © Tiro Piraanenx