The Violet Oil Beetle

The Violet Oil Beetle, Meloe violaceous, is an unusual looking beetle, similar, but not to be confused, with the Violet Ground Beetle and quite large, at about 3cm long and as its name suggests, a rather striking violet colour.

It is an insect of strange proportions, its front end being almost ant-like, with a rounded head and twitching antennae, while its vastly enlarged rear extends well beyond the vestigial wing cases. This species also has a squarish thorax and kinked antennae and can vary in colour from being an almost metallic blue, through purple, to almost entirely black.

Habitat

It is quite rare in the British isles and its range has shrank in the past few decades due to various reasons, thought mainly to be the loss of its meadow habitats and a corresponding decline in numbers of the solitary bees upon which its breeding cycle depends.

It can be found in a few locations in the Northwest, two of which are in the north of the Bowland area, I’ve personally seen them in Roeburndale at the north end of the Salter Way. Usually it is seen at these locations in warm sunny weather among long grass or on flowers, especially Dandelions and Celandine, where they will often gorge themselves on the foliage, which is their preferred diet along with various soft grasses and cleavers.

In bright sunshine they often remain stationary, sunning themselves on pathways and grass stems and waiting for alighting bees, upon which it relies for its fascinating, and somewhat bizarre lifecycle.

Bizarre life cycle

In late spring the female, seen here gorging on Celandine, will lay up to 1000 eggs in a burrow close to a colony of ground-nesting solitary bees. When the larvae, called triangulins hatch 2 to 3 weeks later they will frantically run up and down nearby plants, appearing as if they are on Cocaine, and seemingly without purpose, until they gradually calm down and accumulate on the heads of nearby flowers and await for the arrival of a passing pollinator.

Once clustered on the flower heads the triungulins will start giving off waves of special pheromones, these have been evolved to attract male Solitary bees by resembling the pheromones that the female Solitary bee emits. The male bee will try to mate with the sexy smelling cluster of triungulins and they will attach themselves to the visitor and get a free ride back to the host’s nest.

Hyper-metamorphosis

Once back at the nest the triungulins will detach from the male and make their way to a female bee, she will then unwittingly carry the intruders to her underground nest, where they will first feed on her supplies of pollen, which she has stocked up for her own children. Here they will eat all of the pollen reserves until they are gone, and then they will start devouring the developing bees.

While this is happening, the morphology of the triungulin changes very dramatically, they change from being agile little things into sedentary, fat grubs, and eventually, after having consumed enough pollen and bee larvae, they pupate, emerging after a while as adults.

This type of life cycle, with the extra, highly mobile, larval stage, is known as ‘hyper-metamorphosis’, and is typical of many parasitic insects, especially those like the Oil beetle, where the chances of finding the right host are extremely low so are increased by having thousands of larvae which can move about rapidly when needed. Of course many of the triungulins will attach themselves to the wrong host or simply fall off on the way, so high numbers increase the odds.

Why are they called ‘Oil’ beetles?

The famous natural historian and author Gerald Durrell described the noxious nature of the Oil beetle in Chapter 7 of ‘My Family and Other Animals’ when recounting a story about checking under Swallow’s nests for insect specimens;

‘Gerry checks under the nests twice per day for new specimens and, one day, finds a very large and strange-looking beetle. The beetle leaves an oily and smelly residue on Gerry’s fingers and the scent makes Roger sneeze. The following Thursday, Theodore comes for tea and identifies the insect as a flightless female oil beetle. He happily explains that the beetles’ larvae are parasitic to a certain species of bee’

‘At this, Theodore launches into a story about a white horse he had the misfortune of riding in a Greek parade during the First World War. The horse behaved itself until someone threw eau-de-Cologne at it—which is customary—but the scent splashed into the poor animal’s eye. The horse was so upset, Theodore had to withdraw from the parade and ride the horse along back streets, stinking of the scent’

Cantharidin

Like all species in their family Meloidae, the Violet Oil beetle is capable of synthesizing a chemical compound called ‘Cantharidin’, which is one of the most poisonous natural compounds known to man. In tiny doses cantharidin was used as an aphrodisiac, indeed one of the meloid beetles, Lytta vesicatoria, is well known as the ‘Spanish fly’ and up until recently ‘Spanish Fly’ was the name used for a type of pill which contained cantharidin extracted from the insect and was a precursor to today’s Viagra!

Cantharidin was not evolved for this purpose however and was found to be ineffective, it is actually intended as a repellent! In fact even a slight overdose can kill a human being, or any other vertebrate for that matter, and for this reason oil beetles have few natural enemies. The compound is present in the blood of the adult beetles and they defend themselves through an action called ‘reflexive bleeding’, where they intentionally bleed from their leg joints as a reflex to feeling under threat.

Its future and how we can help it

Despite having a highly effective defensive system, a unique breeding cycle and hardly any natural predators, the Violet Oil Beetle is still under threat of extinction in most parts of the British isles. As the reasons for this are thought to be loss of the wildflower rich habitats it needs and the bees upon which it parasitises there are some things we can do to help it, these include;

Grazing

Grazing of upland areas and grass banks by Sheep and Cattle or by wild animals such as Deer and Rabbits can help maintain an open wildflower rich habitat for them.

Bare ground

Bare ground at the edges of footpaths and tracks will be used by the host Solitary bees for nesting and the female Oil beetles for laying their eggs, this can be created by grazing livestock and the burrowing actions of Rabbits too.

Encouraging wildflowers

Increasing the abundance of wildflowers in areas of otherwise species poor grassland is beneficial to the Solitary bee hosts of Oil beetles and other pollinators too, extending these areas may also link up separated colonies of Oil beetles and help them recover lost territories .

A B-H