Oil beetles are kleptoparasitic (steals its food from another animal) and they need solitary mining bees for their life cycle and survival.  The loss and fragmentation of flower-rich habitats, and the decline of host bee species have contributed to the decrease in oil beetle population numbers.  In Somerset we have three of the five oil beetle species: the Black oil beetle (Meloe proscarabaeus), Violet oil beetle (Meloe violaceus) and the Rugged oil beetle (Meloe rugosus). 

Oil beetles get their name from the bitter tasting oil that they release from their leg joints when they are threatened.  The oil contains a toxic chemical called Cantharidin.  Some other insects such as midges and the cardinal beetle will bite the beetles to either feed on the oil or use it to enhance their own chemical defences.  

Oil beetles have fascinating life cycles.  Their larvae are known as triungulins as they have 3 hooks on each foot.  When they emerge from their eggs in the spring, they climb up the vegetation and sit on top of flowers.  There they wait for a solitary bee to attach themselves to, although they are known to often attach themselves to the wrong insect!

Once attached to a solitary bee, they hitch a ride back to the bee’s nest.  In the nest they disembark and then get to work feeding on the bee’s supply of eggs and pollen.  There it pupates and will emerge the following year as an adult.  You will see the Black and Violet oil beetles during the spring and summer, but not the Rugged oil beetle whose adults are active during the autumn and winter and are nocturnal.

The Rugged oil beetle is classed as nationally scarce.  Whilst populations have declined, they may also be under-recorded because of the time of day and season that they are active.  Two years ago I started a project to survey sites as there were just a few isolated records in Somerset.  With a willing band of volunteers we headed out on a few evenings just as it was getting dark.  Wrapped up warm, we walked the calcareous slopes of the Mendips and Poldens with our headtorches.

We found two newly recorded populations, one at Natural England’s Rodney Stoke and a rather healthy population at Somerset Wildlife Trust’s Green Down reserve.  The Green Down find was especially exciting as it was the first known records for the Polden hills!

During the spring the following year we visited the two sites to look for the triungulins of the Rugged oil beetle.  They are yellow with a black head and measure only 0.5mm in length, so a hand lens was required.  It was useful to map the locations of the larva and compare to the areas that the adults were found.

Hopefully this autumn the weather will be a lot better, and we can survey new sites on the Mendip and Polden hills.  Knowing where populations are will assist in the management of the sites and also indicates that there is a healthy solitary bee population.   Any oil beetle sightings, whatever the species, can be sent to the Oil beetle recording scheme. 

If you want to know more about the Rugged oil beetle or get involved with future surveys contact becky.walters@somersetwildlife.org or somersetruggedoilbeetleproject@outlook.com.