Prionus coriarius (Linnaeus, 1758)
Tanner Beetle
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POLYPHAGA Emery, 1886
CHRYSOMELOIDEA Latreille, 1802
PRIONINAE Latreille, 1802
PRIONUS Geoffroy, 1762
This impressive longhorn, the only member of the Prioninae to occur in the UK, remains locally common throughout Europe and across North Africa, east to Asia Minor and Russia and north to southern Scandinavia, and is present on many Mediterranean islands. It occurs from lowlands to about 1000m and, despite a general decline due to habitat abuse over recent decades, is not classed as threatened or the subject of conservation concern. In the UK it is also thought to have declined and there may have been a reduction of its range, especially in the midlands, but it remains locally common across Wales and the south of England and, given that the species is nocturnal, it may be more frequent than supposed. Habitats include deciduous and mixed woodland and wooded parkland with a good supply of decaying and fallen timber, adults are typically crepuscular or nocturnal and are attracted to light; on warm summer nights they may aggregate or swarm to light in large numbers as occurred in our local woods in 2005 and 2006 when dozens of specimens were seen, but they are also active on hot and sunny days when they take flight and may occur kilometres away from suitable habitat e.g. we found a female specimen crawling on the pavement in Watford town centre, and a male alighted on the author’s shirt on a hot July day in our local woodland. Adults occur from July until August or September and will often be found in numbers; their emergence is synchronised and we have seen a series of adults emerging from the ground beneath a pine tree while others were emerging from other trees nearby, at this time they tend to be rather active and may be detected by the noise they make among foliage or leaf-litter, and emergence may also be a dangerous time as fragments of elytra etc. are sometimes found around the base of trunks, no doubt attacked by rodents. Host trees include a range of broad leaved trees e.g. Quercus, Fagus, Alnus, Castanea, Malus, Salix, Fraxinus, Betula, Carpinus and Corylus and also, but less frequently, various conifers e.g. Abies, Picea and Pinus; the preferred host is often quoted as Quercus but in our local park and woodland, where the beetle is common and there is a wide variety of trees, Betula is by far the preferred host and fallen trunks and logs seem to be preferred over standing
trees. Mating occurs low down on trunks or on the ground during the summer and females oviposit in wood crevices or under bark near the ground or beneath the soil among the roots, we have seem gravid females walking around pine and birch stumps in our local woods during July, and they will often choose logs or fallen trunks in contact with the ground. The larvae develop in lower parts of trunks and stumps, often within the roots or among the damp wood near the soil, they grow slowly and usually take 3 years to become fully grown. Larvae bore out of the wood and enter the soil to pupate in early summer, at this time they may be 75mm long and they are very broad so the emergence holes are readily seen; turning logs in our local park sometimes reveals them along with the round holes they excavate into the soil, a pupal cell is constructed from soil and wood debris some 10-15cm below the surface and adults eclose after a month or so.
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​This large and distinctive species will not be confused with any other in the UK. The form of the pronotum; transverse with 3 large lateral teeth, is unique among our fauna but in any case the large size, 18-45mm, and dark body with leathery-brown elytra should be instantly recognized. The sexes are easily separated by the form of the antennae, in the male they are very stout, strongly serrate and 12-segmented, in the female they are 11-segmented, relatively slender and only weakly serrate. Overall females are much broader, have more deeply coloured elytra and a shinier and less densely punctured pronotum.