Meloe rugosus Marsham, 1802
Rugged Oil Beetle
POLYPHAGA Emery, 1886
TENEBRIONOIDEA Latreille, 1802
MELOINAE Gyllenhal, 1810
MELOINI Gyllenhal, 1810
Meloe Linnaeus, 1758
As with many members of the genus this species has undergone a general decline over much of its range during recent decades but it remains locally common across warmer parts of the Palaearctic region from Europe to the far east of Russia and e.g. in parts of the Middle East, it is occasionally common enough to be regarded as a pest of a range of crops including onion, beans, peas and wheat. In Europe it is widespread, occurring from Spain to Italy and Greece in the south and north to Germany, Poland and the UK, it is generally common in warmer southern and central regions but otherwise very local and sporadic. Here it is locally common in the west, from West London to the Severn catchment, and there are scattered records from Sussex to South Devon and from South Wales. There has been a general decline through the late 19th and 20th centuries but, maybe due to increased recording effort during the 21st Century, it now seems to be increasing, at least according to sightings. Adults have been recorded throughout the year; they are active from September to March, peaking in abundance during October and November, and are only rarely seen after May. In Northern Europe, including the UK, they are mostly nocturnal while in warmer southern regions they are diurnal, especially when they form breeding swarms, as well as nocturnal. They also tend to occur in numbers in the UK and this may lead to increased survival as they produce the poisonous chemical cantharidin which may be secreted onto vegetation etc. and will deter predators. Adults gorge themselves on a very wide range of herbaceous and woody foliage and it may be that they are able to detect which species will provide a source of cantharidin precursors (e.g. ranunculin from many species of Ranunculaceae), this behaviour soon produces the distended appearance in most specimens and is probably an essential prelude to mating which occurs through the autumn and winter. Males search out females for mating in the evening or early morning and may pass over several before a suitable partner is found, and mating may be brief or pronged, pairs sometimes remaining in cop for an hour or more. Females
Special thanks to Jennifer Gilbert for providing this specimen.
are enormously fecund and soon become even further distended with each producing between 20,000 and 30,000 eggs during its brief lifetime. Gravid females may spend hours searching for a suitable oviposition site, and once satisfied they use their mandibles and front and hind legs to excavate a shallow chamber in the soil into which they deposit a longitudinal egg mass which they then cover with soil. First stage larvae, or triungulins (so named for the extra claw on each tarsus) emerge in the spring, they leave the soil en masse and climb to nearby flower heads where they wait for visiting bees. At this time larval mortality is very high as they will cling on to any visiting insect and most of these will be unsuitable as hosts. Those few that find the correct host, which are various species of mining bee (Andrena Fabricius, 1775), may then make it back to the nest where they will consume the pollen store and maybe attack the bee larva before moulting into the second stage and completing their development by consuming bee larvae, detritus and pollen. Pupation occurs within the nest during late summer and autumn, and adults emerge within a few weeks. Fresh adults are small and inconspicuous but soon expand as they feed.
Mining bees tend to dig their burrows on open soils where the vegetation is short and patchy, they prefer light and well-drained soils which are exposed to the sun and so the best places to find the beetles are meadows and south facing hillsides exposed to the sun. In suitable habitats mining bees tend to form colonies, often with many burrows close together, and so parasitism by the beetles can be high, this explains why the beetles are sometimes common but it might also partly explain why they have declined as such habitats are prone to human interference. Adult beetles do not fly and they tend to disperse only short distances by walking, probably because they need to be close to suitable plants that will maintain their Cantharidin metabolism. Searching by night is the best way to find them but they are difficult to spot in the dark and they are slow and awkward in their movements and so their presence is seldom obvious. When handled they produce oily yellow exudates from the joints which smells foul and may cause blistering or rashes in some people and so care should be taken when handling them.
6.0-18.0 mm. Body and appendages dull black or very dark grey, forebody finely pubescent. Head transverse and convex with small, weakly convex eyes and broadly rounded temples which curve to a wide and often bulging neck, surface shiny between uneven and quite dense punctures and with a variable median longitudinal impression which extends forward to the frontoclypeal suture. Without a longitudinal impression behind each eye. Antennae simply filiform in both sexes. Pronotum convex, transverse and broadest in front of the middle; strongly narrowed to rounded or widely obtuse anterior angles and slightly sinuate before obtusely-rounded posterior angles, surface strongly and densely punctured and with a deep median longitudinal impression. Elytra with variable but usually strong wrinkles and impressions throughout, these often forming distinct longitudinal series, especially laterally. Abdomen variable but usually with more strongly chitinized and wrinkled median areas, and membranous, more shiny or silky lateral parts which have small patches of pubescence in the dimples. Legs long and robust; all femora simple and all tibiae with strong apical spines. Tarsi 5-5-4 in both sexes.