Hylesinus varius (Fabricius, 1775)
Often quoted in the literature by the older name of Leperisinus fraxini (Panzer, 1799), this is a generally abundant species of woodlands, parks and gardens throughout Europe from North Africa to central Scandinavia and east across western Russia, in the UK it is among our most common bark beetles across Wales and the south of England, becoming more scattered and local in the West Country and north to the Scottish borders. It is primarily associated with ash but on the continent is occasionally a serious pest of olive trees and occurs more rarely on a wide range of trees including beech, oak, lilac, apple, pear, hazel, robinia and walnut. Adults occur year-round but are most obvious during April and May when they swarm before dispersing by flight through wooded areas, at this time they may be swept in numbers from vegetation below host trees or may be swept in flight or occur in interception traps. Their presence may be inferred by trails of wood dust on branches and stems or by the cankers that form on living branches, generally above ‘crotches’, as a result of adult maturation feeding; overwintered adults bore into healthy bark and produce short feeding galleries which allow fungal pathogens to enter the wood and these activities cause the galling. Mating occurs shortly after maturation feeding; the species is bigamous and one male will mate with two females after which one female will bore into weakened or even dead bark and both will enter to produce egg-galleries from this common origin; each will bore in the opposite direction horizontally under the sapwood, each gallery being more or less straight and leading off at a slight angle from the point of entry although in smaller branches they become irregular and often wavy. Eggs are deposited at points along these tunnels and the larvae bore parallel tunnels perpendicular to the egg tunnels. These larvae develop through the spring and summer, pupate at the end of the feeding tunnels and produce a generation of adults in August and September that will go on to overwinter in short tunnels bored for the purpose. Meanwhile the original adults will oviposit for a second time later in the summer which will produce overwintering larvae or pupae and fresh adults in the spring, this sister generation occurs throughout Europe and the south of the UK but further north there appears to be only a single generation with adults overwintering and ovipositing in the spring. Although nowadays this species is no more than a nuisance it has in the past been a serious pest of commercially grown ash trees; cankers produced by adult feeding and fungal infection disfigure the branches while larval galleries ruin the appearance of the wood which made much of it unsuitable for the manufacture of walking-sticks etc, the beetles are also capable of feeding on relatively fry wood and have been recorded emerging indoors from furniture.
Hylesinus varius 1
Hylesinus varius 2
Hylesinus varius 3
Adults are recognized by the irregular pattern of grey and pale brown scales on the dorsal surface, a feature also seen in the smaller (1.8-2.3mm) Pteleobius vittatus (Fabricius, 1792), which occurs on elm, but here the elytra have a distinct sub-apical declivity, in Hylesinus the elytra are smoothly and gradually declined towards the apex.
2.5-3.5mm. Elongate and subcylindrical with the head clearly visible from above. Head densely punctured, with pale recumbent pubescence, broadly reniform eyes and relatively long antennae; scape long and curved, funiculus 7-segmented and the club elongate and pointed (in the similar H. wachtli Reitter, 1887 the club is broader, less elongate and more rounded apically). Pronotum transverse, broadest near the base and narrowed to a rounded anterior margin, basal margin widely bisinuate and straight in front of the scutellum. Surface densely punctured, with an irregular pattern of broad, almost round scales and narrow, semi-erect setae. Elytra sub-parallel and continuously rounded apically; declivity smooth, not angled, basal margin separately rounded and roughly sculptured. Surface densely punctured and irregularly scaled, for the most part the punctured striae visible, with semi-erect and rather long scale-like setae that are obvious along the lateral margins. Appendages dark or with the antennae and tarsi variously pale. Legs long and very robust; femora near parallel-sided, all tibiae broadened towards a truncate apex and with several strong teeth externally towards the apex, tarsi 5-segmented with the third strongly bilobed, the fourth tiny and the terminal segment long and broadened apically, claws smooth and without a basal tooth.