Oil beetles are one Britain’s most fascinating insects but unfortunately are under threat. There were once eight species of Oil beetle native to the UK but three have become extinct. The five UK species remaining are: Black oil beetle (Meloe proscarabaeus), Violet oil beetle (Meloe violaceus), Rugged oil beetle (Meloe rugosus), Short-necked oil beetle (Meloe brevicollis) and Mediterranean oil beetle (Meloe mediterraneus). The beetle in the images below are Black Oil beetles, which can easily be confused with the Violet Oil beetle due to them being very similar in appearance. The distinguishing feature on this Black Oil beetle is the almost straight lower edge to the thorax, which is flat and has a small tooth at the base of the thorax compared to the Violet Oil beetle, which has an indented lower edge to the thorax and has a large tooth at the base.
The reason the Oil beetle is such a fascinating creature is its life cycle. The beetle solely rely on mining bees nesting areas. The beetle will locate these underground nests and dig two to three burrows where the female can lay up to 1000 eggs. The images below show this burrowing/egg laying behaviour. The video is a short clip showing the Oil beetle burrowing.
The females are generally larger than the males and become much larger when their abdomens become swollen with eggs. Once the eggs hatch, the larvae (known as triungulins) will climb on to nearby flowers and wait for other solitary bees to collect nectar and pollen. The beetle larvae will then climb on to the bee and unwittingly be taken back to the bees nest where it will find a plentiful supply of eggs, nectar and pollen to feed on. The larva develops in the bees nest and will emerge as an adult oil beetle to start the cycle again.
Female Black Oil beetle excavating burrow
Female Black Oil beetle laying eggs