the basal margin is sometimes raised and crenulate and in many there are distinct and strongly punctured striae, otherwise the punctation tends to be fine and random, the vestiture may consist of fine short, long or compound pubescence and in some there are scales while many are virtually glabrous. The elytral apices may be simply convex and rounded or variously excavate and toothed, sometimes elaborately so, adaptations to provide purchase when boring into hard substrates or for removing wood debris from burrows. Similarly the legs may be variously adapted to assist in boring, especially the tibiae which may be simply expanded apically or armed with teeth, spines or apical spurs, the tarsi are 5-segmented with the third variously expanded and the terminal segment often long and slender. Most species are drab coloured, dull and rather nondescript but the group is distinct and soon becomes familiar. Sexual dimorphism is present in many species and may be extreme e.g. in species of the pan-tropical genus Coccotrypes Eichoff, 1878 males are small and blind and mating occurs before the female emerges from the host substrate; most develop in large seeds, but generally females may be larger than males or secondary sexual characters may be present on the front of the head or the elytral declivity. Among other groups of small beetles with clubbed antennae members of the Ciidae are most likely to be confused with the present subfamily, more especially so in the field as they are also saproxylic, but they are distinct in lacking an antennal scape and in having 4-segmented tarsi which lack the bilobed third segment seen in many Scolytinae; in the UK at least both groups will quickly become familiar.

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Scolytine larvae are small, C-shaped and without legs, they are usually creamy white in colour with a darker and well-sclerotized head which usually lacks ocelli, the maxillary lobes are broadly rounded internally and the palpi 2-segmented. The abdominal segments lack ampullae but bear 3 tergal folds. They are not distinguishable from those of other weevil subfamilies using morphology. Most develop in conifers and broadleaf trees and, in temperate regions, only a few in seeds and herbaceous plants.

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Introduction

Bark beetles develop in living and dead phloem and cambium tissue of a wide range of trees and are among the most important pests of commercial grown timber and other products in many areas of the world e.g. the coffee borer Beetle, Hypothenemus hampei (Ferrari, 1867), native to Africa but now widespread throughout tropical areas, is among the most damaging of coffee harvest pests and is thought to affect the economy of 20 million families worldwide. In temperate areas the damage is generally most severe in commercially-grown timber but a notable exception is Dutch Elm Disease, so named because it was identified in the Netherlands, which destroyed mature elm and other trees throughout Europe, North America and New Zealand; this is caused by various fungi endemic to the eastern Palaearctic which reached other areas during the twentieth Century and spread by various scolytids; in the UK our mature elms were wiped out by Scolytus scolytus (Fabricius, 1775) during the 1970’s but elms continue to survive as small trees as their small girth will not allow the larval galleries to form but as soon as they become large enough they become infested and destroyed. In general most tree species have associated Scolytine pests and where they are grown under artificial conditions as monocultures these pests may thrive and become severely damaging e.g. spruce, fir and hemlock are known to have more than 20 associated and serious Scolytine pests in commercial stands. In North America where bark beetles can cause very high levels of tree mortality, the Department of Agriculture estimates that massive outbreaks of mountain pine beetles have destroyed millions of acres of forest since 2005. Most bark beetles are polyphagous on either conifer or broadleaf trees while a few will affect both types and some are monophagous.

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Bark beetles generally attack trees already weakened by damage or disease or stressed by drought or overcrowding etc. they are attracted to host material by volatiles, generally ethanol produced by fermenting sap in phloem vessels, and will communicate by pheromones, attracting others from a wide local area, in the case of Scolytus scolytus about a 2-mile radius. Very generally the lifestyle is as follows. After choosing a suitable host and pairing one or both members of the pair will bore an entrance into the bark, generally as far as the xylem but some species bore much deeper, and tunnel along the surface before excavating a mating chamber, after mating they will bore 2 or more tunnels away from the mating chamber in which eggs will be deposited. Eggs are either laid in a group at the end of each tunnel or at intervals along each one and the resulting larvae will then tunnel into the host away from the oviposition sites. Patterns of larval tunnels are often characteristic for a genus or species and can demonstrate the presence of a species etc. long after the beetles have left the wood. Fully grown larvae generally construct a pupal gallery at the end of their tunnel near the surface of the host and so adults need only bore through the bark or wood surface to emerge.  Adults may thus be detected by ejected wood dust below entrance or exit holes. Both adults and larvae of many species can digest starch and other sugars etc. present in the host material but many have formed a symbiotic relationship with certain fungi, the so-called ambrosia beetles, which adults carry in modified body structures called mycangia and will use to infect new galleries, the fungi will then attack and develop on the host material and the larvae will then graze upon mycelia exposed on the gallery walls or on sporodochia (groups of fungal spores), ambrosia beetles are thus able to feed indirectly on a wider range of timber in a wider state of decay than other wood boring species. Newly eclosed adults collect masses of spores and store them in sporangia before leaving the host to disperse. In turn the fungi are thought to be dependent on transport and inoculation by ambrosia beetles for their dispersal as the various species do not occur in other habitats. Each species of beetle will thus have an associated fungus or small group of fungi and some will find dead or decaying timber more attractive than living timber which may produce saps or resins, which may include insecticidal and fungicidal chemicals, in response to attack. When large monocultures of hosts are maintained the populations of bark beetles tend to increase over a few years until suddenly an outbreak occurs which will overwhelm the hosts chemical defences and damage the trees; when adults and larvae begin to tunnel they tend to bore across long series of phloem vessels which transport sugars etc. around the host, and severe infestation will soon ‘collar’ a stem and so cut off part of a branch or trunk from nutrition supplied from the leaves. Mycangia are also seen in beetles of other families e.g. Attelabidae and Lymexylidae as well as other insects such as wood boring wasps, while some other beetles are known to enter scolytid galleries and take samples of fungi.  

 

Undoubtedly the most notorious species, and one that remains in the public memory, is Scolytus scolytus which is responsible for removing our common place and beautiful elm trees but many others are equally damaging and the following list includes a few of the more significant bark beetle pests of northern temperate regions.

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• Xyleborus ferrugineus (Fabricius, 1801) is one of the most widespread, common and commercially important of all ambrosia beetles, most prolific in warmer areas where it is a pest, especially of coconut palms, it is widely polyphagous and occurs throughout temperate regions; it has occasionally been recorded from the UK but has not established.

• Ips typographus (Linnaeus,1758).  European spruce bark beetle. Native to Europe and Asia Minor; transmits fungal pathogens over long distances, attacks and kills healthy as well as weakened trees and causes wood stain by introducing blue-stain fungus. Populations are detected by concentrations of wood dust on trunks or, in bad infestations, by areas of red foliage.

• Tomicus piniperda (Linnaeus, 1758). Common pine-shoot beetle. Native to Europe and north-western Africa this is one of the most destructive shoot-feeders in northern Europe, the primary host is Scots pine but other pines as well as larch and spruce are also attacked. It was first found in North America in 1992 and is now widely established as a serious pest of various pine species.

• Trypodendron lineatum (Olivier, 1795) Striped ambrosia beetle. Holarctic and widespread in Europe, including the UK, considered to be one of the most damaging ambrosia beetles across the western United States, attacks mostly conifers but occasionally broadleaf trees as well.

• Pityogenes chalcographus Linnaeus, 1761 Six-toothed spruce bark beetle. Transpalaearctic and widely considered a serious pest of conifer plantations, it is also quick to establish in warmer areas outside the native range.