Oil beetles are another one of those species that not everyone is comfortable with due to their interesting life cycle which we’ll come to.
They belong to the genus Meloe which is a large group with some 35 species of Oil beetle in
Europe and 15 in France
although I’m not sure how rare some of them may be and they are also somewhat
understudied according to OPIE. Certainly the commonest are the Black Oil
Beetle Meloe proscarabaeus and the
one I find at our place the Violet Oil Beetles, Meloe violaceus, and we sure have a lot of
them, so many that it’s hard not to tread on them when walking among the trees
where the Lesser celandines are flowering at the moment.
Click images to enlarge
Below: Newly emerged female before putting on weight.
Below: Female having put on weight.
Below: Recently emerged male with pronounced kinked antennae.
Below: Another male, again showing pronounced kinked antennae.
They are a flightless beetle without functional wings, and shortened elytra, (modified, hardened front wings), and they have a very interesting life cycle as mentioned. Soon after emergence in March / April the adult beetles mate after first putting on some weight. With both the Violet and Black Oil Beetle the males have kinked antennae which they use to hang on to the females antennae with during courtship. Once coupled they remain attached with the male being dragged around for an hour or more. The female then lays her eggs in a small hollow she digs in the soil and when these hatch the larvae, (called triungulins as they have 3 hooks on each foot), climb up the vegetation and wait on a flower head for a passing bee to settle to which they attach themselves. Very few survive but those that do and manage to hitch a ride are taken back to the solitary bees’ nest where they consume the bees’ eggs and the nutrition that has been put there. They then pupate and emerge the following year.
Below: Violet Oil Beetles coupled.
Below: Violet Oil Beetle eating Celandine.
They are classified as cleptoparasites and not actually parasites.
“”Kleptoparasitism or cleptoparasitism (literally, parasitism by theft) is a form of feeding in which one animal takes prey or other food from another that has caught, collected, or otherwise prepared the food, including stored food (as in the case of cuckoo bees, which lay their eggs on the pollen masses made by other bees). The term is also used to describe the stealing of nest material or other inanimate objects from one animal by another.” SOURCE
Despite this behaviour which some dislike they are a good indicator of the level of solitary bee activity where they are located for without them they can’t exist.
As can be seen in the photo Oil beetles often attract small midges which feed on the oil produced by the beetle but do it no harm.
Rugged Oil Beetle Meloe rugosus, Mediterranean Oil Beetle Meloe mediterraneus and the Short-necked Oil beetle Meloe brevicollis are some other well known but scarcer French Oil Beetles.
If in the UK Buglife are running a survey on Oil Beetles and would appreciate your help.