It’s not just people who are drawn outside when, after the rainy and cool April, the sun shines through more often and the temperatures slowly rise. Many insects also crawl out of their hiding places. Most of them are harmless, but there are a few in Germany that you should be wary of. Along with wasps, bees and hornets, which everyone knows can sting, the lesser-known black-blue oil beetle (Meloe proscarabaeus) also belongs in this category.
The beetles, which are up to three centimeters long and shiny black, contain the poison cantharidin. Just a few milligrams of the substance are enough to kill an adult human. In ancient Greece, the animals or their poison were therefore used for executions like the more well-known hemlock cup.
The animals in Germany seem to be on the move in large numbers right now. In North Rhine-Westphalia, parts of a schoolyard recently had to be closed because several oil beetles were spotted there. The good news is that the only way to kill the oil beetle venom is to ingest it, which isn’t something you normally do with an insect several inches tall. If it does happen, you should contact the Poison Control Center as soon as possible.
Skin contact causes redness and blisters
However, one should not touch oil beetles either, as the insects then feel threatened and secrete the poison in the form of an oily substance from their knee joints. The consequences of skin contact are not dramatic, but unpleasant: reddening and blisters occur. The Poison Control Center advises washing your hands thoroughly with soap and cooling the affected area.
The beetles are not aggressive and usually crawl rather sluggishly, for example along garden paths. And of course, they don’t produce their venom to harm humans. The cantharidin protects the insects from being eaten by ants, for example. This also explains the rather unusual location of the poison pores on the knee joints: ants usually attack from below.
So if you leave the beetles alone, you have nothing to fear. On the contrary: “It’s actually a good sign when there are oil beetles in the garden,” says Jennifer Calvi from the German Wildlife Foundation. It means that the garden is ecologically at least reasonably intact: where there are oil beetles, there must also be sand bees, because the beetles need the bees to reproduce. And sand bees only exist in semi-natural gardens; they will not survive in an environment with turf and gladioli monitored by the robotic lawnmower.
The beetles can only reproduce if they meet sand bees
“Oil beetles are parasites of sand bees,” says Jennifer Calvi. The larvae of the beetles climb onto flowers and wait there for unsuspecting bees that want to gather food there. If one comes by, the larva claws and flies into the nest of the sand bee. Once there, she crawls into the brood chamber and first eats the egg of the sand bee and then also the entire supply that the bee has actually created for her offspring. Sated and full, the beetle larva leaves the bee’s nest and can then hibernate in the ground without further food intake. Next spring, after further intermediate stages, the finished beetles hatch, which only live for about a month.
Sometimes the reddish-yellow larvae of the oil beetle don’t climb a flower individually, but “gang together and imitate a flower themselves,” says Calvi. Bees that fall for it are immediately covered with several larvae.
The oil beetle’s propagation strategy is sophisticated, but also risky: Most flowers are visited by various insects and the larvae cling to everything that’s hairy and can fly. If the flight then ends in the nest of a honey bee or a bumblebee, the larva dies. Her life only goes on if she is lucky enough to meet a sand bee by chance.
It is estimated that only one in a thousand larvae will become a mature oil beetle. This is also one of the reasons why oil beetles are relatively rare. In Germany they are on the red list of endangered species and are strictly protected.