Korynetes caeruleus (De Geer, 1775)
This is a widespread native Palaearctic species occurring throughout Europe and Mediterranean North Africa to the south of Fennoscandia and extending east through Siberia to China and Japan; in the wild it is saproxylic but it is also a pest of various stored products and so through transportation has been recorded under artificial conditions more or less worldwide. In Europe it is a mostly lowland species which varies widely in abundance; it is generally common in the south but rather patchy further north e.g. it is common in Denmark but generally rare on Poland and in the UK is a local insect of southern and eastern England and, apart from the Severn Valley and a few coastal records from Wales, is absent from the west. Adults appear in the wild from April and may persist until September, the typical habitat is dry deciduous or coniferous woodland, wooded borders and parkland and they are usually associated with decaying timber infested with scolytids or ptinids etc. although early in the season they may also occur on various flowers during warm sunny spells. Mating occurs in spring and early summer on trunks and branches, often on areas devoid of bark and close to the entrance burrows of other beetles, and soon after the female will visit several burrows and lay a few eggs either on the surface or just inside, and then she will die. Larvae develop within these burrows feeding upon the early stages of other species and eventually pupating within a chamber near the surface. Adults may eclose in the autumn and remain in place until the spring or, in cooler northern latitudes, eclose in the spring after the larvae have continued feeding into the autumn and pupated later in the year. The presence of Korynetes usually indicates a severe infestation of wood-boring beetles and so adults may be attracted to infested timbers in buildings during the summer and oviposit indoors, they may also be attracted to various commodities and under these circumstances they may become continuously breeding, with adults and larvae present through the year. Species of beetle recorded as prey under artificial conditions include Dermestes frischii Kugelann, 1792, Xestobium rufovillosum (DeGeer, 1774) and Niptus hololeucus (Faldermann, 1836), and adults have been reared from larvae developing on old bones, grain, stored meat and wheat, both in domestic situations and commercial bone-meal works, glue-factories, grain warehouses and timber-storage facilities.
The size, 4-7mm, general habitus is very distinctive among our UK fauna and might only be confused with the superficially-similar member of the same family, Necrobia violacea (Linnaeus, 1758) but as well as obvious differences in the antennae and dorsal punctation, in Korynetes the head, including the eyes, is as at least as wide as the anterior margin of the pronotum whereas in Necrobia it is narrower. Without experience various chrysomelids, e.g. Lema and various Oulema, might also be tempting but here the antennae are never clubbed. Entire body bright metallic blue, appendages black or very dark, often with various median antennal segments and tarsi lighter. Dorsal surface with dark and quite long erect setae, appendages with dense recumbent pale pubescence. Head broadly transverse from above, with a wide, finely punctured and almost flat vertex and strongly convex, prominent and asymmetric eyes, mandibles very robust; almost triangular, with several large teeth internally towards the apex. Antennae long and slender, the club gradually developed and elongate (cf. Necrobia). Pronotum quadrate with lateral margins strongly sinuate before the anterior and posterior angles, surface evenly and weakly convex, with large punctures mostly separated by about their own diameter, anterior margin curved to very obtuse angles, basal margin strongly bisinuate to obtuse and sharp posterior angles. Elytra much broader than the pronotum, with broad shoulders and continuously rounded apical margin, usually gradually but distinctly broadened from behind the shoulders to the apical curves but this is variable and may be stronger in females, surface finely wrinkled and punctured and with large punctures arranged for the most part in longitudinal rows but they may become confused on the disc. Femora unarmed, tibiae slender, only weakly broadened towards the apices and with tiny apical spurs. Tarsi 5-segmented; the third segment strongly bilobed.
Antennal club abrupt and much broader.
Head proportionally narrower.
Pronotal punctation finer and denser.
Serial elytral punctures stronger.