Polygraphus Erichson, 1836
This is a mostly Holarctic genus of about 100 species; they mainly occur in cooler northern regions although a few are known from tropical Africa and South East Asia. The greatest diversity is in Asia, and the Nearctic fauna includes only 3 species, all of which are endemic and more-or-less confined to northern spruce forests. Six species and subspecies are known from Europe, and with the exception of P. punctifrons Thomson, C.G., 1886 which occurs across the Palaearctic region and is also known from the Canary Islands, (although ssp. punctifrons Thomson, C.G., 1886 occurs only in Europe), and P. poligraphus (Linnaeus, 1758), which has been recorded from Siberia, all are confined to the Western Palaearctic region. Of the remaining European species, P. grandiclava (Thomson, C.G., 1886) and P. opacus Thomson, C.G., 1871 are widespread, while P. griseus Eggers, 1923 occurs only in Sweden and adjacent parts of Russia. So far as is known all species are associated with conifers, especially spruce (Picea Mill.), firs (Abies Mill.) and pines (Pinus L.), although there are rare exceptions e.g. P. grandiclava is also known to breed in various species of Prunus (Rosaceae). Adults generally occur year-round, overwintering under bark or in old galleries and emerging in the spring to swarm, disperse to new host trees or fallen branches where males may produce pheromones to attract females. Reproduction occurs in galleries within the bark; both sexes bore into the bark to produce a mating chamber, and mated females then either nibble into the bark and lay batches of eggs, or they construct a separate chamber in which to do so. Larvae then produce galleries leading away from the oviposition site, the form of these galleries can be diagnostic for some bark beetles, but not for Polygraphus, where they tend to meander as they radiate away from a short oviposition chamber. Most of the vascular cambium is destroyed as the larvae develop, and in dense infestations entire trees may be repeatedly girdled, but the xylem remains largely unaffected and so tree canopies may appear healthy for weeks or months after the tree’s fate is sealed. While many species are no more than a nuisance, the Small spruce bark beetle (P. poligraphus) can be a very serious pest of Norway spruce and Scots pine in northern Europe. Another notable pest is P. major Stebbing, 1903, which is among the most serious pests of commercially grown Bhutan pine (P. wallichiana Jacks.) in Kashmir and the Himalayas. Flight-interception and pheromone trapping are used to assess populations in the spring, and insecticides or lures may be used to destroy the beetles, although entomopathogenic fungi have proven useful against many scolytid beetles, including P. major.
1.8-3.5 mm. Elongate (generally 2.0-2.4X longer than wide) and cylindrical, usually with the forebody tapering, vestiture often consists of numerous close-fitting oblong scales as well as semi-erect short pubescence, colour varies from black to pale brown, in some with the forebody darker, with brown appendages. Head substantially visible from above, strongly convex and almost spherical, with eyes divided into two almost equal halves, and long and diverging temples, surface finely and densely punctured and variously microsculptured, especially towards the base. In males there is usually a transverse raised line from the anterior margin of the eyes that has two projecting median tubercles. In females the head is broader and flatter. Frontoclypeal suture absent, apical clypeal margin truncate or slightly curved, and often raised. Antennal scape long, longer than the 4-6 segmented funiculus, and gradually broadened from the base, club flattened and without sutures, oval and asymmetric, usually larger in males. Pronotum transverse, broadest behind the middle and narrowed to obtuse or rounded (from above) anterior angles and near perpendicular posterior angles, often with a weak subapical constriction, apical margin curved, sometimes with a median indentation, basal margin simply curved or weakly bisinuate. Pronotal surface convex, usually with the midline distinctly raised, and moderately densely punctured throughout. Front coxae contiguous. Scutellum absent or only minutely visible. Elytra parallel-sided from rounded shoulders to a continuous apical margin, apical declivity smoothly curved, basal margin bisinuate and finely crenulate, surface finely and closely granulate and punctured, without distinct striae although there are usually a few impressed longitudinal lines near the base, suture and apex. Tibiae gradually broadened from the base, and with at least some external teeth, sometimes only towards the apex. Tarsi with 5 simple segments, the basal segments short and the terminal segment long and curved. Claws very finely serrate.
Our species may be separated as follows:
1 Antennal club as long as or only slightly longer than the scape. Funiculus 5-segmented. Median keel of the pronotum only poorly developed, sometimes obsolete towards the base and apex. Elytral scales sometimes dark from the base, otherwise substantially pale. 2.2-3.0 mm.
Antennal club much longer than the scape. Funiculus 6-segmented. Median pronotal keel well-developed and always distinct. Elytral scales each with a longitudinal dark mark from the base. 2.5-3.5 mm.
Polygraphus grandiclava. This widespread European species was discovered in a domestic garden in Detling in Kent in 2006 (Chuter, 2010), under bark on logs from a moribund White pine (Pinus strobes L.) Several specimens were found and it is likely the species was breeding, but despite being found on a range of pines on the continent, this remains the only British record.
Polygraphus poligraphus, otherwise known as the Small spruce bark beetle, is also widespread in Europe, extending from the Mediterranean to above the Arctic Circle in Fennoscandia. In the UK it occurs very locally in south east and central England, and Wales. In northern Europe the species is sometimes a serious pest of commercially grown Norway spruce, Scots pine, White pine, fir and larch, but in the UK, despite it sometimes occurring in large numbers, it is rarely more than a pest. Both adults and larvae overwinter under bark or in galleries, and this generation of adults will disperse by evening flight during late April or May. Males seek out new host trees and signal for females to join them; the species is polygamous and each male will attract up to six females after preparing a subcortical breeding chamber. Mated females excavate brood galleries up to 6 cm in length and about 1.2 mm in diameter, and lay small batches of eggs in niches placed regularly along the length. Larvae emerge after a few days and create irregular curved galleries. Larvae develop rapidly and are fully grown by June; those developing in smaller branches or twigs will construct a pupal chamber in the xylem, whereas those developing in larger branches and trunks will pupate within the bark. Fresh adults emerge after 8-12 days, during July or August, and will remain in situ, maturation feeding in or about vacated larval galleries. Occasionally there may be a sister generation, and here all developmental stages may occur at a single site, and fresh adults will continue to appear into late summer. On the continent the species sometimes occurs alongside the Six-toothed spruce bark beetle (Pityogenes chalcographus (Linnaeus, 1761) and the Larger eight-toothed European spruce bark beetle (Ips typographus (Linnaeus, 1758), and damage is then often severe. Younger and weakened trees up to 30 cm in diameter are the usual hosts; entire stems and larger branches of intact trees are attacked, while the upper parts of cut stumps are favoured.