Oil beetles belong to the blister beetle family; however, their small, flexible wing stubs give them a very different appearance compared to typical beetles.
These large, slow-moving insects should be ideal prey for birds and small mammals, but they contain a surprise. When disturbed, they release an oily substance in a response called ‘reflex bleeding.’ Their blood contains cantharidin, a protective chemical that irritates skin and may cause blisters.
Kentucky’s common blister beetles are active around the middle of July when they can move into hay fields to feed on flowering plants. In contrast, oil beetles are on the move from late fall through early spring. They cannot fly, but they crawl around while feeding on winter weeds. If abundant, they do pose a threat to grazing horses. Usually, several oil beetles occur in an area. Larvae develop in nests of ground-nesting bees where they feed on developing bee larvae.
By Lee Townsend, Extension Entomologist