Hylobius abietis (Linnaeus, 1758)
Large Pine Weevil
Despite the common name this species is associated with a wide range of coniferous trees and adults will also occasionally attack deciduous species such as beech, oak, lime and ash, the main hosts are pine and spruce but they are widely polyphagous and have been recorded feeding on birch trees growing among conifer plantations. The distribution is mainly western Palaearctic; there are widely scattered records from eastern Russia, China, Japan and even New Zealand but it has not become established beyond its native western range; it occurs throughout Europe south to Armenia and Turkey and north to Fennoscandia and the UK where it is locally common to the Far north of Scotland. Adults live for up to three years and may remain active through mild winters, they mate multiple times throughout their lives and oviposition begins in the spring about two weeks after the females emerge from their overwintering quarters under bark etc. and continues into the summer. Females chew small cuts into any wood in contact with the soil and eggs are deposited singly or in small groups, they may also oviposit directly into soil near to roots to a depth of 30cm, they usually choose old conifer stumps but when the beetles are swarming in abundance they will also use fallen timber or the roots of living trees; each female will lay up to 70 eggs and larvae will emerge after a week or two. Larval development is very variable, depending on temperature and the nutritional quality of the host material, it can last up to five years but under good conditions they pass through five instars and pupate within a year or two, they are cold-tolerant and can enter into a winter diapause lasting up to six months, hence development can be protracted at higher latitudes, they feed under bark producing long irregular tunnels in the cambium and gradually move into the upper regions of the roots where they will excavate oval chambers in which to pupate. When fully-grown the larvae reach 14-16mm, they are soft-bodied, pale-cream coloured with a brown head, curved and legless. Pupation occurs in the summer and adults may eclose late in the year and pass the winter within the pupal chamber or the pupae may overwinter to produce adults in the spring. This is generally the most destructive pest of conifer plantations throughout most of Europe, including the UK, the larvae are probably beneficial in removing dead wood
Hylobius abietis 1
Hylobius abietis 2
but it is the adults that are destructive; they feed on the tender bark of seedlings, stunting their growth and introducing harmful pathogens, but they often ring the bark so that the seedlings rapidly die-off. Adults feed throughout their lives but the new generation emerging in the spring need to feed in order to become sexually mature and at this time the destruction can be severe and within a year or two they may destroy entire plantations of seedlings or young trees. Among natural conifer forests the damage is usually much less severe and the weevils tend to occur in moderate numbers with occasional years of abundance, but in plantations the numbers tend to be much higher and swarms much more frequent; destruction of young trees depends on many ecological factors and it may be that some cultivars are more resistant than others, trees already weakened by insect attack or drought are more susceptible, and adults are known to transmit a wide range of fungal pathogens between trees and the feeding damage they leave exposes trees to further infection by airborne spores etc. Various parasitic wasps and nematodes have been used as biocontrol agents but with only limited success and when the weevils occur in huge numbers they may be lured in large numbers to pheromone or alcohol traps to reduce the population or insecticides may be employed; in commercial conifer plantations the seedlings are usually treated before they are planted. Wild areas used for cyclic planting and clear-felling of conifers will usually yield adults in numbers from early spring until late in the year, they tend to be common through the year and peak in abundance in the spring and autumn, but they may occur in any moderately open coniferous or mixed woodland, they are diurnal and nocturnal and their presence may be detected by small areas of missing bark (feeding scars) which produces deformed, stunted or dead seedlings or young trees, they may be swept from foliage, particularly from smaller trees, or searched for among bark crevices.
Adults vary in size from 8-14mm, they are elongate with prominent rounded shoulders and distinctively patterned elytra and might only be confused with species of Pissodes Germar, 1817, but are easily distinguished by the antennal placement, in Pissodes they are inserted near the middle of the rostrum while in the present species the insertions are near the rostral apex. Entirely dark grey to brown, dorsal surface densely punctured and with scattered fine yellow pubescence, elytra with irregular transverse bands of dense yellow scale-like pubescence. Head small with weakly convex eyes and widely diverging temples, rostrum weakly curved, about as long as the pronotum and dilated towards the apex, the scrobes narrowly visible from above. Antennal scape long and gradually widened in the apical third, funiculus 6-segmented and club compact and elongate. Pronotum transverse, broadest near the middle and narrowed to a rounded apical margin and obtuse posterior angles, surface strongly and confluently punctured except for a variable longitudinal median line. Scutellum small, triangular and sparsely and very finely pubescent. Elytra much broader across the base than the width of the pronotum, with rounded shoulders and a distinct constriction before a continuously-rounded apical margin, the surface densely punctured and wrinkled but usually with distinct striae consisting of rows of much larger punctures. Femora with a strong ventral tooth , tibiae with a sharp incurved apical process, and tarsi pseudotetramerous, claws smooth and without a basal tooth. Males may be distinguished by their impressed abdominal sternites, in the female they are smoothly convex, but they are also more slender and elongate when compared to the females and with a little experience may be recognized in the field.