Dendroctonus micans (Kugelann, 1794)
Great Spruce Bark Beetle
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POLYPHAGA Emery, 1886
CURCULIONOIDEA Latreille, 1802
HYLURGINI Gistel, 1848
Dendroctonus Erichson, 1836
Originally native throughout Asia east to China and Japan this invasive bark beetle began to invade Europe alongside the expanding commercial cultivation of various conifer trees in the 19th century and is now widespread and still expanding its range; it was first recorded from the UK in 1982, although attacks have now been identified dating back to 1973, and now occurs locally across Wales and south and central England and more recently has been recorded from the Lake District and Scotland. Host trees include all species of spruce, Picea Mill, but susceptibility varies between species; those most vulnerable to attack include Norway spruce, Canadian spruce, Omorika spruce and Oriental spruce, other species are less susceptible but nonetheless regularly attacked by the beetle. Unlike most bark beetles which tend to colonize a tree in numbers, a single female can initiate an infection which may rapidly increase; older trees tend to be ignored or only suffer local damage, especially when stressed or damaged by other means, but younger trees are readily attacked; beetles move between trees by crawling but they can also disperse by flight and have been recorded up to 7km from established infestations. The life-cycle is unusually long among scolytids, 12-18 months, and so there is often an overlap of generations with all stages being present in a single tree, and although they tend to overwinter mostly as adults, damage may therefore be continuous and progressive until the tree dies, furthermore the adults are large (the largest European species) and able to withstand resin produced by the tree as they bore through the bark. Adults mate mostly in the spring and disperse by flight from May to August when the temperature reaches 22°C, they attack hosts within a few metres of the ground, chewing through the bark and leaving characteristic runs of resin and wood dust on the surface, and bore irregular brood galleries up to 10 cm in length in which they oviposit at regular intervals on one side of the tunnel, each female producing up to 300 eggs. Larvae develop slowly, passing through five instars; they feed in tight groups among
the inner bark and phloem leaving wood debris and detritus in tight packed masses in the gallery as they expand it so producing characteristic galleries mottled with debris. Pupation occurs in the feeding galleries, often among the piles of detritus, and often close together so that numbers of adults may build up before they start to bore through the bark to emerge. Newly emerged adults are pale in colour and tend to remain in the galleries, maturation-feeding before they emerge although the onset of cold weather may cause them to remain in place until the spring; larvae also overwinter but this is mostly spent in the adult stage. In northern latitudes there is usually a single generation each year although generations may overlap. Infestations often occur when a single vulnerable tree is infected, this may be stressed by drought or infection or damaged by wind, lightening or feeding woodpeckers etc. and populations tend to build up rapidly so that, while the species is generally of local occurrence throughout Europe, it is potentially very damaging to developing conifer stands. A frequently used and very effective countermeasure against large population increases in Europe, including the UK, has been to introduce the native Asian monotomid, Rhizophagus grandis, a specialist predator of all Dendroctonus eggs and larvae.
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Among the three Palaearctic members of the genus the present species is the only one to occur in Europe, the others being restricted to the east. Identification is straightforward as it is much larger, at 7-9mm, than our other scolytids. The entire is insect dark brown to black when mature, sometimes brown with the head darker, with quite long but very fine pale pubescence. Head obvious from above; transverse with flat, broadly transverse eyes, convex vertex and flattened clypeus, strongly punctured anteriorly but much more finely so towards the base. Antennae inserted laterally in front of the eyes, scape long, curved and strongly broadened towards the apex, funiculus 5-segmented and the club broadly rounded. Pronotum broadest in front of the base and narrowed to distinct anterior angles, posterior margin widely bisinuate, punctation variable; usually strong by irregular. Elytra elongate and as wide as the pronotal base; weakly broadened behind the middle and continuously rounded apically, completely covering the abdomen. Elytral base separately and strongly rounded but only weakly, if at all, raised, striae regular, strongly punctured and distinct to the apex, interstices convex and finely punctured. Legs short and robust, all tibiae strongly broadened towards the apex and toothed externally. Tarsi 5-segmented but appearing 4-segmented as the tiny fourth segment may be hidden within the folds of the strongly bilobed third segment.