25/04/2026
Violet Oil Beetle in the sentry. They love sunny days. They have an extraordinary life cycle. here's some facts from Buglife.
Size: Adults can be up to 30mm in length. Beetle larvae are around 2mm in length.
Life span: From egg to adult, approximately 1 year
Diet: Adult oil beetles feed on the leaves and petals of flowering plants and grasses; favourites include Lesser Celandine (Ranunculus ficaria), soft grasses (Poaceae), Dandelion (Taraxacum officinale) and Cleavers (Galium aparine). Larvae (known as triungulins) feed on stores in solitary bee nests, including the pollen and the bee egg and/or larvae. After leaving the nest, triungulins are often found on Lesser Celandine and Dandelion, but will use other flowers.
Reproduction: Oil beetles have one of the most extraordinary life cycles of any British insect – they are nest parasites of solitary mining bees. Females dig a short burrow into which they lay up to a thousand eggs. For Violet Oil Beetles, these eggs develop and hatch quickly, emerging later in the same season. Once these hatch the larvae (triungulins) climb to the top of the nearest flower and wait for a bee to arrive. They then grab onto the bee and hitch a lift back to its nest; the triungulin will then consume the pollen stores and either the egg or larvae of the bee. The triungulin will then stay in the bee’s nest, where it will pupate and emerge as an adult oil beetle when conditions are right the next spring.
When to see: Adults can be found from March – June in woodlands, meadows and coastal grasslands throughout Britain. They are most active on warm, sunny days. Triungulins can be found from April to June.
Population Trend: Declining.
Threats: Oil beetles are reliant on solitary bees to compete their life-cycles. The health of oil beetle populations is therefore dependent on the health and diversity of wild bees. The wildflower-rich habitats that oil beetles rely upon have declined in quantity and quality due to intensive management of the countryside. Oil beetles are sensitive to changes in land management and are a good indicator of the health of our countryside.
Fun Fact: The triungulins take 1 year to hatch from their eggs. This means that the triungulins found alongside the adults in spring are the larvae of the adults from the previous year!
Devon Wildlife Trust