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A. Masked Weevils or Small Bark-Beetles (Scolytide).—These include some very injurious insects in our woodlands. They do far more injury to broad-leaved trees than to conifers. Attacks on the former occur far more often in avenues, and on ornamental trees in parks and gardens, than in woodlands. The broad-leaved trees most liable to attack are the Elm, Ash, and Birch, then Oak and Beech; but on broad-leaved trees most bark-beetles are polyphagous, and often attack them more or less indiscriminately. Both in broad-leaved trees and conifers, however, the attacks are usually confined to middle-aged and old trees in a sickly condition.
Seedlings and saplings, as well as poles and young trees badly damaged, soon die off from the injuries received; but in the woods the larvæ may
often bore for years before trees show signs of decay. Broad-leaved trees have (owing to their larger nutrient reserves) a far greater recuperative power than conifers in recovering from injuries; and many of the bark-beetles infesting the former class of trees live more in the sapwood than in the cambium, and therefore interfere less with the process of nutrition.
Beetles, small and cylindrical, with round head sunk deep in the convex thorax, and short antennæ ; abdomen with five segments (the first and second generally indistinct); legs short, with spinous teeth on outer edge of tibia, and four-jointed tarsi (the third sometimes lobed). Larvæ cylindrical, ventrally incurved, and covered with tubercles and strong hairs. Pupe short, thick, and sprinkled with spines and hairs. Generation mostly
ual, but sometimes extending over one and a half and two years.
The Scolytidæ are small insects, more or less cylindrical in shape, and of quiet indefinite colour. Except during the short time the beetles swarm and reproduce themselves, they pass their life inside woody fibrous plants, within which the eggs are laid. They mostly hibernate as beetles, but some reappear early in spring during warm days in March, while others only come out again in April or May.
For breeding-places they first of all select the most suitable species of trees, preferring neither dead nor very dry wood, nor sound healthy trees from which a strong flow of resin would kill the beetles and larvæ of most of the species. Windfalls, damaged and sickly trees, those broken or injured by wind, snow, or ice, falls of timber and their stumps left, and backward crops, are the most favourable breeding-places. The beetles seek out the fissures of thick-barked trees of this sort, and bore quickly into the sapwood. For such species as only pair inside the stem, a fairly large pairing-chamber (camera copulatrix) is hollowed out under the bark. From this central point begin the mother-galleries, of equal breadth, characteristic of the Scolytide (the breadth of course depending on the size of the beetle). Along these the female lays her eggs in small holes notched alternately right and left. Ovideposition in clusters is less frequent.
The Primary, Main-, or Mother-galleries are formed partly in the cambium, sometimes being more in the bark and sometimes more in the wood (bark-galleries), and partly in the sapwood (wood-galleries). The beetles which breed in the bark or in the wood may be distinguished by the direction in which they bore. Thus, vertical galleries may be formed, running longitudinally along the stem, or horizontal galleries, running round the circumference of the stem, or star-shaped galleries, when several radiate from the pairing-chamber as a central point.
Vertical and horizontal galleries are forked, and have either one or two branches, according as they are bored into on one or both sides of the entrance-hole.
The eggs, often numbering up to 100 or 120, are laid within three to four weeks, and in about fourteen days the feetless grubs, curled in at the ends towards the belly, dirty-white in colour and with a brown head, begin to bore the larval or secondary galleries at about right angles from the main gallery. The borings, small at first, gradually enlarge as the grubs grow in size, and finally they end in a pupal chamber formed for the chrysalis. As they broaden, these larval galleries diverge from each other, and from their direction at right angles to the main gallery, so that sometimes the longest are finally almost parallel to the latter. Each grub usually bores its own gallery and keeps clear of the neighbouring galleries.
In the few species which lay their eggs in clusters, the larvæ form galleries in common (family galleries). The timber-boring beetles do not form true larval galleries ; the larvæ live in the main-galleries, and only hollow out short pupal chambers on each side of the main-gallery (ladder-like galleries). These main and larval galleries generally assume such characteristic shapes that the species of barkbeetle committing the damage can usually be determined by this alone. This is the more convenient, as it would otherwise not be possible to determine the species by means of the larvæ only.
The period of development from egg to beetle lasts on the average only from eight to ten weeks. It varies, however, not only for different species, but also depends greatly on climate, weather, and temperature of the breeding-place. It is longer in mountainous tracts than on warm plains. In the pupæ all the different segments of the beetle are already distinguishable, the main differences consisting in their lighter colour and softer structure. After about eight days of pupal rest the fully-developed beetle emerges. It usually, especially during cold raw weather, spends a few days feeding on the soft cambium round the pupal chamber, and this obliterates to some extent the characteristic shape of the larval galleries. But in fine weather the beetle bores a small round exit-hole in the bark, and at once begins to fly, pair, and reproduce itself. This new brood develops completely in the same season, but does not generally emerge until the following spring, the beetles hibernating under the bark-fissures and cracks, or in roots, tree-stumps, &c. Most bark-beetles, if not all, have such a double generation, but local and climatic conditions always more or less affect the period of development, so that any one given species may sometimes have a double generation under favourable conditions, and only a single or annual generation under less favourable circumstances.
When stems lying on the ground are attacked by bark-beetles, bore-dust can be seen at the entrance-hole ; while both the entrance-holes and the air-holes opened here and there along the main-gallery are easily seen unless the bark is very rough and scaly. The presence of beetles in standing timber is shown by the outflow of white drops of resin, while the bore-dust falling down from above gets caught on rough bark-scales and collects near the foot of the tree. When numerous exit-holes are to be seen scattered irregularly over the stem, this shows that the beetles have already emerged, and that it is then too late to try and destroy the larvæ and pupæ.
Prevention. As bark-beetles usually attack sickly and backward crops, damage is least likely to occur if the woods are well thinned and cleared of unhealthy material, of windfalls, and of broken or damaged trees. Recent falls of timber should be removed as soon as possible ; the stumps of such trees should be grubbed up, and their roots and branches made use of; and if that cannot be done, then the felled stems should be barked. These are the only good and practical measures for preventing the attacks of bark-beetles on a large scale. When suitable breeding. places are not available, the beetles also attack sound trees, and are mostly killed by the ensuing outflow of resin ; but when thousands of such small-bore wounds have been punctured, the tree sickens and then becomes a favourable breeding-place for the remaining beetles. This is always the case when bark-beetles swarm in very large numbers. All sylvicultural operations tending to obviate injuries of any sort (from wind, ice, snow, game, &c.) are also useful in preventing serious attacks of bark beetles. The chief protection against these, as against other injurious insects, consists in forming mixed woods in place of pure forests ; because in mixed woods the conditions are not only less favourable for the breeding of noxious insects, but insectivorous birds are also usually more numerous than in pure woods.
Annihilation.- Laying down decoy-stems as beetle-traps here and there (see p. 63) and peeling and burning the bark from these after all the eggs of a new brood have been laid, and before any of the beetles have had time to emerge, 1 is the best measure to adopt. It is best to burn the bark ; but if this is inconvenient, it should be exposed, soft side upwards, to the sun if the brood has been trapped in the larval stage. But if the bark be piled in large heaps when bark-beetle attacks are on any big scale, burning the bark is necessary to destroy the brood. It is also necessary to follow up these first results with another supply of decoy-stems prepared against the second brood in summer. Fresh decoy-stems should as a measure of safety be laid down every four to five weeks during summer and well on into autumn, wherever bark-beetles are likely to breed in large numbers.
What has above been said applies especially to the bark-boring beetles. The much less numerous wood-boring beetles in conifers and broad-leaved trees (e.g., Oak) are technically injurious insects. The trees they attack, and especially the broad-leaved trees, often long survive the damage done, although it is best to fell and use them as soon as convenient, because they gradually lose in value. In these latter cases decoy-stems are of no use as traps. Winter fellings and thinnings and early removal of felled trees, or barking of those that have to remain longer on the fall, are here the only practicable measures.
The bark - beetles are divisible into three groups of importance to the forester :(1) Sapwood-Beetles (Scolytini), with abdomen obliquely shortened. They mostly
attack broad-leaved trees, especially Elm. (2) Cambial-Beetles (Hylesinini), with wing-cases extending beyond the abdomen,
and not flattened or toothed at lower end. They mostly attack conifers by boring in the bark or slightly into the sapwood, but do not bore deep into the
wood of the tree. (3) True Bark-Beetles (Bostrichini, syn. Tomicini), with wing.cases often contracted
or flattened and toothed towards the end. They mostly attack conifers rather than broad-leaved trees, and bore partly under the bark and partly in the wood.
They do not breed in tree-stumps and roots like many Hylesinini. (1) Sapwood-Beetles (Scolytini).-This group consists of only one genus (Scolytus), formed by the large and the small Elm-bark beetles, which mainly infest and greatly damage the Elm (and chiefly the English Elm) by the mass of larval-galleries hollowed out in the cambium and slightly entering the sapwood. The vertical main-galleries go deeper into the sapwood Avenue and park trees are very liable to attacks from this insect, and the Elm-trees in the London parks have suffered much during recent years.
1. The large Elm-bark Beetle (Scolytus destructor) is one of the most destructive of the bark-boring beetles in Britain (Fig. 143). It deposits its eggs between the soft wood and the inner bark of the Elm, and occasionally also of the Ash.
Appearance. It varies considerably in length, but it is generally about to
"In a piece of bark measuring 28 in. long by 12 in. broad, which I cut from one such trap-tree, I found thirty mother-tunnels ; and allowing 100 eggs for each, no fewer than 3000 beetles might have escaped from this piece alone.”—(R. S. MacDougall on Pine Beetle (Hylesinus piniperda), in Trans. High, and Agri. Socy. Scot., vol. xiv., 1902, p. 231.)
? A monograph on The Elm-bark Beetle is contained in the Trans. High. and Agri. Socy. Scot., vol. viii., 1896, pp. 258.269.
of an inch long, and black with chestnut-brown wing-covers, and reddish-brown antennæ and legs.
Life-history. The female bores through the bark in June, forming a more or less vertical tunnel 3 or 4 inches in length, along which the eggs are deposited at intervals.
The larvæ, hatched out in autumn, at once begin to feed on the inner bark of the tree, forming secondarygalleries to the main tunnel. They continue this until the following spring or early summer, when they pupate and emerge through the bark as perfect insects either in August or the following spring. After pairing, the female bores into the bark as before, laying her eggs, &c., as the galleries, increasing with the size of the grubs, are formed The Elm-bark Beetle (Scolytus de
structor), magnified five times. partly in the bark and partly in the sapwood. The stem is more or less completely girdled, hence the tree soon becomes diseased and is often killed outright.' Trees from which the beetles have emerged look as if
Elm-bark showing borings of Scolytus destructor (# nat. size). a. Galleries in which
only part of the eggs have hatched out; b. Galleries of S. multistriatus.
the bark had been riddled with fair-sized shot; and very fine sawdust will be found either at the bottom of the tree or upon the bark. They never attack dead trees, but usually select young or old healthy and vigorous stems, although often found upon unhealthy trees. On the Continent there is a double generation (August and May), but, fortunately, in Britain only a few beetles emerge in August.
Prevention and Extermination. If only old moribund trees are attacked, it is best to fell and bark them late in July or early in August, and to burn the bark to prevent the beetles spreading to other Elms. A plan adopted with success in France is to shave off with a spoke-shave the rough outer bark of infested trees in which the bore-holes of the female beetle are noticed, as many of the grubs are killed by their burrows being exposed, and by the flow of sap which takes place. The application of insecticide washes is then often successful.
There are several other species of this genus. These include (Fig. 144) the Small Elmbark Beetle (S. multistriatus), which is only about half the size of S. destructor, but otherwise resembles it in appearance and life-history, and usually accompanies it in its attacks ; S. ligniperda, which bores into some of the conifers; S. intricatus, which attacks the Oak; S. vittatus, on the Lime; and S. fraxini, on the Ash. All of these bore like the S. destructor into the bark of the trees which they attack, and destroy their healthy cambial action.
For monograph on The Biology and Forest Importance of Scolytus (Eccoptogaster) multistriatus, see Progs. of Roy. Socy. Edin., vol. xxiii., 1900.
(2) Cambial Beetles (Hylesinini).
1. The Ash-bark Beetles 1 include three species found in Britain (Hylesinus fraxini, H. crenatus, and H. olei perda). They bore between the bark and the wood, and the first two often do a very considerable amount of damage to ash-trees of all ages, more especially in the warmer tracts of Central and Southern England. Though sickly trees are most liable to attack, these pests often infest round healthy stems and cause their decay.
Appearance.—These three species may be distinguished as follows :
brown todark. five longitud. brown, ning trans. more at right
inal punc: bristly. versely, and angle to main. surface thick. tures
about equal gallery (Fig. ly covered small tuber
cles in front,
and covered with
Black (occa- Marked with
2-armed, run. Longer, more ning trans- irregular, and versely, but often finally one often
running par. much longer allel to main. than the gallery (Fig. other.
H. olei perda . About 1 Black, and coy. Marked with Yellow- 2-armed, broad, Usually soon
ered with yel.
cross, and be
Wood of lar and con. culed inter
fused (Fig. spaces.
1 A monograph on The Bark-Beetles of the Ash will be found in the Trans. High, and Agri. Socy. Scot., vol. xi., 1899, pp. 245-262.