Chapter XXI Insect Enemies of Trees
Whether viewed from a commercial or sylvicultural point of view, the widespread damage caused to timber from insect attacks can scarcely be overrated. The depredations in various parts of our own country, particularly in young plantations, are bad enough, but when compared with those of Europe and America they appear insignificant. In France and Germany whole woods have been wiped out by insect pests, while the Government of Bavaria were mulcted in something like £100,000 by the destruction of its spruce forests. The United States fares no better, for we find that over a period of ten years the amount of timber killed and reduced in value was calculated at fully £10,000,000. The coffee plantations of Ceylon suffered much from the attacks of a fungus, and we could go on multiplying cases. In our own country the ravages of the pine beetle and of the larch disease have caused incalculable damage; indeed, in the latter case there is hardly a plantation of larch where the presence of the fell disease cannot be distinctly traced, while the pine beetle has ruined whole plantations both in England and Scotland. Though the adult bark and wood-boring beetles do a great amount of damage, yet that inflicted by the caterpillar or grub from the egg is greater still, and in the case of fungi we have a typical example of their destructive properties in the case of the well-known larch disease.
The Pine Beetle (Myelophilus {Hylurgus} piniperda) is a dreaded enemy to not a few species of Pinus, but particularly P. sylvestris, P. laricio, P. austriaca and P. Strobus. The injury done by this beetle consists in its destruction of the leading shoots of the tree it attacks. It enters by boring a hole into the side of the shoot until it reaches the pith, after which its course is directed upwards, and an exit made at the terminal bud. This tunnelling of the shoot so weakens it that frequently during stormy weather it is broken across at the point where the beetle entered. Not only are unhealthy trees attacked by the pine beetle, but young and robust-growing specimens frequently fall a prey to its insidious depredations.
June, July and August are the months when it is most commonly found.
The only remedy is to collect and burn the affected shoots—work that requires to be done with the utmost care to prevent the escape of the wary insect. Burning all brushwood in plantations is a great preventive.
The Pine Weevil (Curculio {Hylobius} Abietis) is another destructive insect, which differs from the former in waging its attacks against the buds of the leaders and branches, as also by eating patches of the bark here and there on the stems and branches. The various species of Abies suffer most, but the pines are occasionally attacked as well. It is always most destructive in young plantations growing on the margins of old woods, and equally bad amongst trees that have been planted on the site of a former pine plantation.
The beetle is about half an inch long, and nearly black. One remedy, probably the best, is to place fresh pieces of pine bark on the ground, beneath the infested trees. By shaking the trees and examining the traps the following morning, many may be destroyed.
Bostrichus typographus is another pest of our woodlands, and may frequently be seen, like fine white wool, spreading over the stem and branches of the silver and other firs. It spreads with terrible rapidity, first appearing in small patches here and there on the bole, and particularly on the under sides of the branches. The tree infested soon becomes unhealthy, and frequently dies off prematurely. Trees growing in low-lying, heavy ground would seem to fall a first prey to this insect.
Bostrichus laricis is nearly allied to the former, but its devastations, which are, however, not very deadly, are principally confined to the larch. It is usually known as the “larch blight.”
The Pine Shoot Moths (Retinia buoliana and R. turionella) would seem to be more numerous in this country than is generally supposed. Quite lately I visited a large plantation of young Scotch fir, the terminal buds of which were greatly injured by the caterpillars of this elaborately-coloured moth. The moth lays its eggs at the base of the buds, and into these the caterpillars enter by hollowing out the centre, thus destroying their vitality and causing them to take on a withered appearance and to feel soft and empty to the touch. Trees infested by this insect resemble greatly in their stunted shoots and exudation of resin such as have become a prey to the Pine beetle (Myelophilus piniperda), only in the latter case it is the fresh young shoot and not the bud that is attacked. The Retinia would seem, from all my notes and observations, to be most abundant in what might be termed neglected fir plantations, that is, where the trees have suffered from overcrowding, or from unfavourable conditions as to soil, etc., and particularly when the wood is composed entirely of one species. There is no method of dealing with large infested areas, for the attacked trees have repeatedly been cut over and removed without any seeming diminution in the numbers of the insect. One experiment with a small infested corner has been rewarded with good results, viz. the lighting of a fire to windward, and causing the smoke of coal tar to pass over the infected area. This might be worth trying in the case of fruit trees infested by particular insects.
The Larch Miner (Coleophora laricella).—Few, other than those specially interested in tree diseases, have the remotest idea that the yellow, withered appearance of many of our English larch plantations is due to the larvæ of the above tiny moth. It usually attacks young trees, say, from five to twenty years old, and although it may not kill them out, yet the repeated onslaughts year after year tend to keep the trees in an unhealthy condition, and so render them liable to other and more deadly diseases.
Unfortunately the attacks of the larch miner are by no means confined, as is usually supposed, to trees growing under unfavourable conditions, for I have this season noticed in an unusually healthy, fast-growing plantation in Sussex that almost every tree was more or less affected. Certainly in another large extent of larch in Gloucestershire which I examined lately, where nine-tenths of the trees were being ruined by the Peziza, the larch miner was very abundant; but, I think, that young trees, whatever may be their state of health, suffer alike, although where hard-wooded trees form a portion of the crop the larch certainly suffers less than when grown in pure woods. The moth lays its eggs at the end of June on the needles of the larch; the caterpillar mining into and feeding upon the interior of the needle causes it to turn faded and yellow. It lives in the tube thus formed during the winter, changing to a pupa, and ultimately to a moth. It is a most difficult matter in the case of this insect, as, indeed, of all others that are fairly abundant, to suggest a remedy, and I have looked over and examined larch plantations that are differently situated in many respects to find out under what condition the attacks are most persistent, but with little or no success—healthy and unhealthy, native or Tyrolese, faring alike when grown as a pure crop.
Where the larches are intermixed with hard-wooded trees—sycamore, oak and beech—the attacks are certainly less frequent, as I have noticed in a number of cases. Trees growing at high altitudes do not seem to suffer less than those only a few feet above sea-level, and this point I have paid particular attention to.
Whether the wounds caused by this insect will serve as a nidus for the spores of Peziza Willkommii has yet to be determined, but special importance should be attached to all larch-feeding insects, and their depredations minimized to as great an extent as possible.
The Pine Sawfly (Lophyrus Pini).—Fortunately, this insect is not abundant in the British Isles, though on the Continent the damage it does in the pine forests is by no means inconsiderable. The insect may readily be recognized by its wide, flattish body, and usually dark appearance. Having attained to full size in the trees, they form cocoons among the foliage or on the stems, and remain in this condition until the following spring, when, in April or May, the perfect insects make their appearance. The male is considerably smaller than the female, while the full-grown caterpillar, which is of a greenish-yellow colour, with a row of black spots on either side, is about an inch long. The remedial measures are not at all easy, especially when a large number of trees are attacked, but single specimens may be entirely cleared by shaking the caterpillars into a sheet placed beneath the tree.
(a) WILLOW BEETLE
(Phyllodecta vulgatissima),
AND (b) ITS LARVA.
The Larch Aphis (Adelgis laricis) and Giant Sirex (Sirex gigas) are both, more or less, harmful to the larch. The latter is a formidable and splendid insect, which is, however, not very abundant in this country.
Generally felled trees, or such as are somewhat sickly, are chosen by the female in which to lay her eggs. These are deposited beneath the bark by means of the powerful ovipositor, and in course of time the whitish cylindrical maggots make an appearance, and with their strong jaws form large borings in the affected tree.
Cutting down and burning infested trees is the only practical remedy.
The Willow Beetle (Phyllodecta vulgatissima) causes considerable damage to osier plantations, and would appear to be greatly on the increase of late years. Much damage has been done to osier holts in various parts of the country, and in northern Ireland the ravages of this beetle were particularly noticeable during the past five years. The insect, which is metallic green or blue in colour, passes the winter in the adult state, at which time it may be found amongst refuse of the osier beds, such as the heaps of bark, and also at the base of old stools and beneath stones or other shelter. The larvæ have a tough yellowish cuticle with conspicuous brown bristles, the head and prothorax being black and hard. The eggs are laid on the undersides of the leaves in spring, and when the larvæ are hatched they feed on the leaves, eating holes quite through to the upper surface. Burning all rubbish in the osier beds is to be recommended, and spraying with Paris green or lead arsenate has been found useful.
The Larch Sawfly (Nematus Erichsonii).—This is a species of sawfly the larvæ of which bear considerable resemblance to those of the caterpillar of the pine sawfly, and also to that of the better known gooseberry caterpillar. The larvæ are about three-quarters of an inch long, and possess twenty feet. From July to August they feed on the leaves of the larch, and a plantation that has been attacked presents a partially leafless condition with quantities of the brown cylindrical cocoon cases lying amongst the grass beneath the trees. There have been several notable instances in which larch plantations have suffered severely from the attacks of this insect, and in northern England, particularly Cumberland, whole areas of plantation have been attacked. Burning all brushwood and grass beneath the trees in infected plantations is probably the best means of lessening the numbers of this dread insect in our larch plantations.
The Spruce Gall Aphis (Chermes abietis).—This is a common insect, and one that renders many fine young spruce trees very unsightly by reason of the cone-like excrescences that are formed by the action of the insect on the shoots of the infested specimen. The formation of this excrescence is brought about by the female aphis piercing with her beak, or sucker, one of the buds, and drawing off the sap, the consequence being an unusual growth at that part.
When the young larvæ appear, they also, by piercing the gall, extract the juices, and the gall enlarging soon causes the larvæ to become embedded at the bases of the leaves, which, by this time, have become curiously malformed. The insects are scarcely one-tenth of an inch long.