There are many large black beetles on the move in spring. But one of the rarest is the oil beetle. In fact, their life-cycle is so weird and chancy, that just the mere existence of an adult is something of a miracle.


Are oil beetles rare in UK?

There are four species of oil beetle in the UK and several are very rare indeed. However, the most widespread are the violet oil beetle, Meloe violaceus, and the very similar black oil beetle, Meloe proscarabaeus. Both are worth looking out for – for their glorious, glistening oleaginous beauty, if nothing else.

What do they look like?

They are both big (females can be about 3cm long) and bold. The bumbling confidence to stride about in full sight of insect-loving birds is down to the noxious compound ‘cantharidin’ contained in their blood. Handle one roughly and they’ll leak this liquid from their leg joints all over your fingers.

Not ‘classic’ beetles in their proportions, they have been described as looking like ‘fat men in tight waistcoats’ – an apt description, as their well-upholstered abdomens seem to bulge out from under their redundant relic wing cases.

Oil beetle lifecycle

The females in particular grow to uncomfortable-looking dimensions as the multitude of eggs, carried within the straining segments of her abdomen, swell and ripen. For this to happen, they need warmth and food. So, like articulated trucks of insect life, they gracelessly go heaving their heavy cargo about, seeking flowery places and suntraps.

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After mating, the female finds herself some warm, friable soil and excavates several shallow burrows into which she deposits her eggs. By next spring, if you use a hand-lens to search inside flowers near the same locations as you notice the adults, you might find tiny, active earwig-like larvae.

Unlike the typical grub-like larvae of many beetles, these are known by the wonderfully descriptive name of ‘triungulins’ – so called because each of their grabby feet ends in a triad of sharp grappling hooks.

Watch them and they seem impatient, jostling for the best and highest anther or petal. Gently tap the flower and their frenetic scrambling reaches even greater levels of urgency. The reason for all of this is that they are doing nothing but waiting. It’s now or never – it’s imperative they latch onto a bee.

Triungulins have been found hitching a lift (a behaviour known as ‘phoresy’) on all manner of flying insects, but in order to be successful they have to grab onto a bee of a particular species. A bumblebee or honeybee is not good enough, it has to be one of the many solitary bees that abound at this time of the year. For the few that get onto the right bee – and it really is a small number (statistically it’s been calculated that fewer than 1 in 2,000 will make it to adulthood) – they are then carried to the bee nest.

Here they jump off, and after the bee has sealed her nest, laid an egg and left it to develop, the little triungulin starts to feast. As it moults into a more grub-like incarnation, it consumes the brood and the painstakingly collected pollen. It’s a life-cycle with a surprising twist that comes full circle next spring when, having wintered as an adult beetle deep in the soil, it pushes up into the warmth and spins the roulette wheel of survival once again.


Main image © Peter David Scott/ The Art Agency


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Nick BakerTV presenter and naturalist