Tetropium castaneum (Linnaeus, 1758)
POLYPHAGA Emery, 1886
CHRYSOMELOIDEA Latreille, 1802
SPONDYLIDINAE Audinet-Serville, 1832
ASEMINI Thomson, J., 1861
TETROPIUM Kirby, 1837
This is a common and often abundant longhorn occurring throughout Europe generally, but especially in the north, and east throughout Asia to China, Japan and Korea. Seemingly absent, so far, from the New World but it is probably only a matter of time; it has been reported from imported timber in Greenland. In Europe it is regarded as a pest; serious infestations may cause between 33 and 44% reduction in timber along with the associated economic loss. In the U.K. it occurs sporadically and infrequently; there are records scattered throughout England and Scotland to the Highlands.
The preferred host in Europe is Norway Spruce, Picea abies, whole in Siberia it is Pine, throughout the range various Firs, Abies Spp., and Larches, Larix Spp., are also infested. Freshly felled trees and stumps are usually chosen as well as those weakened by drought or insects and fungi but healthy trees will also be attacked. Depending on latitude, adults are active between May and September, generally peaking in June and July. The life cycle takes between one and two years. White oval eggs between 1-1.2mm in length are laid either singly or in groups of up to ten low down in bark crevices or under the scaly bark of host trees. Eggs hatch within two weeks and the young larvae excavate wide, irregular galleries among the inner bark which become packed with granular brown frass. Mature larvae bore horizontally through the sapwood for about 5 cm then turn and bore parallel to the grain for another 3 or 4 cm so forming L-shaped galleries at the end of which a pupal gallery is excavated. In stumps these cells are excavated at the base or in exposed roots. When the bark is intact and thick the gallery may be formed within. The pupa lies vertically with the head towards a frass plug which seals the chamber. In Europe adults begin to eclose in late May and June, boring through the bark and emerging through exit holes 5mm or so in diameter. Adults do not feed; they live for around 3 weeks and, depending on latitude, emerge over a long season.
Adults vary widely in size from 8 to 19mm. They are flattened and parallel, typically black with brown elytra and brown or red appendages. Occasionally the elytra are black or black with pale humeral marks and epipleura. Among the British fauna the species is distinctive by the combination of the grooved vertex between the eyes and the pronotum which is shiny and sparsely punctured, the distance between the punctures being generally at least as wide as the puncture width. The head and pronotum are sparsely pubescent. Head transverse and finely punctured; generally as the pronotum or a little finer. Surface lustrous and with a weak median groove between the antennae. Eyes divided across the middle. Antennae robust and about half the body length. Pronotum quadrate or a little transverse and rounded laterally, narrower across the base than across the apex and with a deep transverse furrow along the basal margin. Elytra with fine silvery pubescence and each with two weakly developed longitudinal carinae.