Scolytidae
An extensive group of small Beetles, often known as "Bark-beetles," because the larvae of by far the greater number of species feed between the wood and the bark of trees, especially when the trees are dying. The insects, when mature, come out through holes in the bark. They are not, however, the only Beetles that feed in bark; but probably in no other family do so many species possess this habit. They are distinguished by their small size and cylindrical form fitting them for their mode of life. They have four joints in each foot, the first joint being decidedly shorter than the next three together. The head is usually narrow, and occasionally it is lengthened into a beak, as in Weevils. The antennae are clubbed. The colour is almost always some shade of dark grey or brown. The Beetles, after pairing, burrow through the bark, and form below it passages, which are usually straight or nearly so. The females lay eggs along the sides of the passages formed by each pair; and the larvae, on hatching out from the eggs, burrow away from the main passages, taking the directions least likely to bring them into the burrows of one another. As they grow, they increase the width of their burrows to correspond with their own size, and when full-fed they become pupae in the ends of the tubes; and from these they emerge as Beetles in the following spring. A considerable number of species have been found in Britain. Of these, some are confined to one kind of tree, while others feed in several different kinds. Most trees are liable to the attacks of several species of these Beetles; and Coniferae, especially Scotch Firs, are peculiarly infested by them. Each species makes tunnels so definite and characteristic in form that a practised entomologist can generally recognise, from the marks in the bark and the wood, the species of Beetle that has made them. The family Scolytidae has been split up into a number of genera, minor characters being employed for this purpose; but upon these it is unnecessary now to enter, since their habits, the injuries done by them to trees, and the means of preventing or combating their attacks, are much alike in all the species. Fortunately, they seldom appear in quite healthy trees; but if trees have been weakened by any cause (e.g., unfavourable soil, or gases in the soil or in the atmosphere), their destruction is often completed by these insects in a short time. Examples of this are frequently seen in the destruction by them of Elms and of other trees in public promenades in large towns (e.g., in Paris). Remedies. The beetles prefer dying or dead wood in which to breed; hence, all such trees should be removed without delay, or, if the trunks are too large to be removed, the bark ought to be torn off and burned. It is well to leave some dead trunks as traps, since the beetles resort to them in preference to living trees, and the logs can be burned with the insects in them. These traps should be renewed about every month during the breeding season, in summer. Where trees show holes the bark should be removed from the infested parts about the end of June. In a short time the larvae will be dead, and most of them will have been removed by birds. Choice trees may be protected by saturating the outer bark with tobacco solution, or other applications distasteful to the beetles, about the breeding season. The more destructive species in Britain are as follows: On Elms, in avenues, &c., Scolytus Geoffroyi (also called S. destructor S. pygmaeus, and Hylesinus vittatus; on Ash, Hylesinus crenatus and H. Fraxini; on Birch, Scolytus Ratzeburgi and (in the wood) Bostrichus domesticus; in Beech-wood, B. domesticus; on Scotch Firs and other Conifers, Hylesinus poligraphus, Bostrichus chalcographus, B. typographus, B. Laricis, B. suturalis, B. bidentatus, B. micrographus, B. lineatus, B. cinereus, Hylesinus angustatus, H. palliatus, H. atra, H. piniperda. Scolytus Pruni, on the Continent, occasionally proves hurtful to fruit-trees, e.g., Apple and Pear.