« PreviousContinue »
segments bearing two rows of spines directed backwards. Before the larva becomes a pupa, it spins a strong web, intermixed with particles of wood, which constitutes its cocoon (b); in some instances the larva changes to a pupa under ground. In fig. 1233., e, f, g, h, and i are representations magnified of the spines upon certain of the abdominal segments: e represents the 4th abdominal segment seen laterally ;f, three of the basal row of spines; g, three of the hinder row of spines; and h, three of the basal row of spines of the 9th abdominal segment. Fig. 1234. represents the jaws, or mandibles, of the larva, with which it cuts its way through the wood in this figure, a is the mandible; b is the labrum, or upper lip; and c shows the clypeus. These mandibles are formidable-looking instruments, each having the appearance of a sort of chisel, with a toothed edge. The perfect insect (d in fig. 1233.) has dark grey wings, clouded with dark brown, and streaked with black. The imago belongs to the class of insects that fly by night, and it appears about the end of June. The female lays but one course of eggs, but these generally amount to 1000 in number, and are always deposited at the base of the trees; whence the caterpillars penetrate the bark, wherever they can find the easiest entrance. The eggs are small, in proportion to the size of the imago; and the caterpillar, which grows to a large size, is said to remain in the larva state three years. The large size of the larva, Samouelle observes, compared with the smallness of the egg, strengthens this idea, and prepares us to expect that it would be likely to consume a great quantity of wood in the progress of its growth. The smell of the larva is so strong, as to be easily perceived by persons passing near trees infested with it. (Samouelle.) The green woodpecker preys upon these caterpillars, and its stomach, on dissection, has an intolerable stench. The principal kinds of tree which the cossus feeds on are, the elm, the alder, the oak, the ash, the walnut, the beech, the lime, and some kinds of willow and poplar. The larvæ devour the liber, or inner bark, making long galleries in the wood, like the insects that attack the pear tree (see p. 886.), and finally destroying the tree. Many remedies have been proposed; but that of Latreille appears to be most approved of in France. This consists in surrounding the base of the tree, where it has been observed that the females always deposit their eggs, with a thick coating of a mixture of clay and cow-dung, which the insects cannot penetrate. For further inform
ation respecting this insect, see Gard. Mag., vol. xii. p. 464. The fourth enemy of the elm tree is the scolytus. The S. destructor Oliv. is generally considered by far the most injurious; but it is assisted in its ravages by another species, the S. armàtus.
Scolytus destructor. The female insect (fig. 1235., in which a is the natural size, and d the insect magnified), about July, bores through the bark, until she has reached the point between the soft wood and the inner bark; she then forms in the latter a vertical channel, usually upwards, of about 2 in. in length, on each side of which she deposits her eggs as she advances, to the number of from 20 to 50 in all. It appears probable that, after doing this, she dies, without making her way out again, as she may be often found dead at the end of the channel. About September, the larvæ are hatched; and they commence feeding upon the matter of the inner bark (c), at the edge of the channel (b), and, in a very
slight degree, on that of the soft wood opposite; advancing, as they feed, in a
course at about right angles from the primary channel, on each side of it. (See fig. 1236.) The true food of the insect is the inner bark; and the erosion of the soft wood is so slight, as to be, perhaps, nearly accidental. The course of each individual larva, on each side of the primary channel, is about parallel to that of the larva next to it; and each forms a channel by its feeding that is enlarged as the larva increases in size. When each larva has finished its course of feeding it stops in its progress, turns to a pupa, and then to a beetle; after which it gnaws a straight hole through the bark, and comes out. The beetles begin to come out in about the latter end of May of the year following that in which the eggs were deposited. The sexes afterwards pair, and the females, bearing eggs, bore
through the bark, as before detailed; and so on from generation to generation, and year to year.
The result of the erosions of the female parent, and of the larva, in the inner bark and soft wood, is that of cutting off the vital connexion between these two parts; and, when the erosions effected in a tree have become numerous, of occasioning its death, by preventing the ascent and descent of the sap. It has been said that the scolytus never attacks a tree in a perfectly healthy state; and, also, that trees suffering under carcinoma (see p. 1385.) are particularly liable to it. In the year 1825, an avenue of elm trees in Camberwell Grove were attacked by this disease, which was supposed to be brought on by the gas which escaped from the pipes laid down along the road being absorbed by the roots; and which gave rise to a suit in Chancery between the inhabitants and the proprietors of the gas- works. Various persons, considered as competent judges, were employed to ascertain the cause of the decay of the elms; and their general conclusion was, that the carcinoma had been brought on by old age, excavations for building in an exceedingly dry soil, and an extraordinarily dry summer, and that the gas had had no influence in producing the decay of the trees. The trunks of the trees, when examined in 1826, were found infested with an immense number of larvæ feeding on the soft inner bark. An interesting account of the Camberwell elms will be found in the Gardener's Magazine, vol. i. p. 378. In relation to the capability of the scolytus to effect injury on elm trees, it is stated that 80,000 have been found in a single tree. It has also been remarked that the scolyti seldom destroy the trees they attack the first year that they commence their ravages; and that they prefer a tree that they have already begun to devour, to a young and vigorous tree. (See the observations of Mr. Spence in p. 1389.) It is easy to ascertain the infested trees, as the bark will be found perforated by small holes, as if made by shot or a brad-awl, in various parts; and little particles of a substance like fine sawdust will be found on the rough surface of the bark, and at the foot of the tree. The scolyti, as Mr. Denson, sen., has observed, never attack dead trees. The Scólytus destructor, as an enemy to elm trees, appears first to have attracted the attention of entomologists in England about the year 1824, by M'Leay's Report to the Treasury upon the state of the elms in St. James's and Hyde Parks. (See this Report in Edin. Phil. Journ., No. xxxi. art. 12.; and see Tilloch's Phil. Mag., Oct. 1823, art. 51.) In the year 1828, a controversy was carried on in a Cambridge newspaper, between Mr. John Denson, sen., the author of A Peasant's Voice to Landowners, &c., and Mr. J. Deck of Cambridge, respecting the cause of
the death of certain elms in the public walks in that city. Mr. Deck's opinion was, that the trees were destroyed by the insects; and Mr. Denson's, that the trees were only attacked by the insects after they had become injured or diseased. To prove this, Mr. Denson selected in his own garden, in the spring of 1828, a healthy young elm, about 18 ft. high, and 1 ft. in diameter at the surface of the ground. At about 30 in. up the stem, that is, at b, fig.1237, he says, "I cut out completely round the stem a band, or ring, of bark, about 4 in. broad, expecting by this act to intercept the passage of the sap to c d, and thence to have ed in a duly diseased and paralysed state, to be perforated by the scolytus in June or July; while, by retaining a alive, and in a growing state, I should be able to witness whether the insect would attack the live part also, or not. Quite contrary to my expectation, c d (the tree had been deprived of its head when I adopted it for my experiment) emitted side shoots, and grew as freely through the season of growth, both of 1828 and 1829, as a
itself; evincing, indeed, no difference, either from a, or other elms standing near it, except that the leaves turned yellow somewhat earlier, and fell somewhat sooner. Too impatient to wait longer, early in 1830, from c d I cut off d, a piece about 9 ft. long, and placed it near the remainder of the tree; and, to my great gratification, in June, d was visited by scolyti, perforated in many places, and, from the eggs then deposited, now (Sept. 9. 1830) teems with larvæ; while a b c did not receive a single perforation, and now does not contain a single larva. This result satisfies my mind that the Scólytus destructor is altogether guiltless of causing the death of healthy growing trees."
In this controversy, we are informed by William Spence, Esq., F.R.S., who has recently attended to this subject, that both parties, like the knights who quarrelled about the shield with one side of gold and the other of silver, are both right and both wrong. It is quite true, as Mr. Denson maintains, that the female scolyti never deposit their eggs in trees perfectly healthy; but it is equally true, that both they and the males pierce young and healthy trees for the sake of eating the inner bark, which constitutes their food; and that the numerous holes which they thus cause, partly from the loss of sap which exudes from them, and partly from the effect of the rain which lodges in them, in a few years bring the trees in which they occur into that incipient state of ill health in which the female selects them for laying her eggs, just as in trees beginning to decay naturally; and thus healthy trees are effectually destroyed by the combined operations, first and last, of the scolyti of both sexes, though not in consequence of the sole deposition of the eggs of the female. That this explanation of the subject, so happily reconciling former apparently contradictory facts, for which those who are interested in the preservation of the elm are indebted to the distinguished naturalist, M. Audouin, professor of entomology at the Muséum d'Histoire Naturelle at Paris, who has recently closely studied the habits of these insects, is correct, Mr. Spence, to whom he communicated it this spring, informs us he has had numerous opportunities of proving in the most satisfactory manner; having, both at
Brussels (where, in consequence of his suggestions to the local authorities, it was found necessary to cut down from 20 to 30 large trees attacked by Scólytus destructor in the Park, and from 50 to 60 younger ones in the boulevards), and also during a tour in the north of France this summer (where he found the promenades of elms equally ravaged by the scolyti at Dunkirk, Calais, Boulogne sur Mer, Montreuil, Rouen, Havre de Grace, Caen, St. Lo, Granville, &c.), seen hundreds of young trees in that incipient state of decay indicated by M. Audouin as arising from the attacks of the scolyti simply for food; and great numbers of these in which the females, having found them sufficiently debilitated, had deposited their eggs, and given birth to numerous broods of larvæ, which had caused them to be either dead or fast dying.
It is scarcely possible to overvalue, in an economical point of view, the importance of M. Audouin's discovery, which, if it had been formerly known and acted upon, might have saved the greater part of the fine elms in the promenades in many of the principal cities in the north of Europe, which have fallen victims to the ravages of Scólytus destructor, as well as 50,000 young oaks in the Bois de Vincennes, near Paris, which it has been recently necessary to cut down in consequence of the attacks of another insect of the same tribe, S. pygmæ us. The practical directions to which it leads, in all cases where there is reason to suspect the presence of scolyti, are very simple, and may be briefly expressed as follows:
1. The first thing to be done is, to pare away the exterior rough bark with a cooper's spokeshave, or other convenient tool: this admits of a distinct inspection of the actual state of the trees, which, if there is no trace in the inner bark either of small holes in old trees, or of those superficial furrows which the scolyti make for food in young trees (and which may be distinguished from the natural crevices in the bark by their dark-coloured and dead margins), may be pronounced to be in a sound and healthy state, and requiring no further attention.
2. If the inner bark exhibits either of the appearances just mentioned, the next thing to be ascertained is, whether the female has already deposited her eggs in it, and if it contain the larvae of the scolyti: to know which, it is necessary to cut away portions here and there of the bark down to the actual wood, and examine them; and, if the existence of larvæ be proved, the trees should be cut down, and their bark peeled off, and every fragment of it carefully burnt.
3. Those trees which, though pierced with exterior superficial holes or furrows, have no larvæ in them, are such as have been attacked by the scolyti for food only; and, if they be carefully brushed over with coal tar, the smell of which is highly offensive to the perfect scolyti, there is every probability that they will be secure from the future attacks of the females; and that the repetition of the same process in the spring, for a year or two, would enable them to resume their vigour, and to become healthy trees; for the future fate of which, if, at the same time, the entire removal of all the trees actually diseased has been attended to, there would be no need for apprehension. It is in this way, as we are informed by Mr. Spence, that a great number of the young elm trees in the boulevards at Brussels, brought into an incipient stage of debility by the attacks of the scolyti for food, but not yet attacked by the females, were treated in the spring of 1836 with every prospect of a successful result; though, of course, some years must elapse before any absolute deductions can be drawn from the experiment. The above most important information was communicated to us by Mr. Spence in December, 1836.
Recorded Elms. Evelyn, to prove that the elm attains "a prodigious growth in less than a person's age," mentions a tree which he had seen," planted by the hand of a countess, living not long since, which was near 12 ft. in compass, and of a height proportionable." He mentions elms, "now standing in good numbers which will bear almost 3 ft. square for more than 40 ft. in height."
"Mine own hands,” he adds, “ measured a table more than once, of about 5 ft. in breadth, 93 ft. in length, and 6 in. thick, all entire and clear. This, cut out of a tree felled by my father's order, was made a pastry board.... The incomparable walks at the royal palaces in the neighbourhood of Madrid were planted," he continues, "with this majestic tree." These are said to have been the first elms that were planted in Spain; and Baron Dillon tells us that, when he saw them, about the end of the last century, they were 6 ft. in diameter, and in a healthy state. The plants were taken from England by Philip II., who had married Mary Tudor, daughter of Henry VIII., and Queen of England. Henry IV. of France planted an elm in the gardens of the Luxembourg, in Paris, which stood till it was destroyed during the first French revolution. An elm in Switzerland, near Morges, at the time it was blown down, had a trunk 17 ft. 7 in. in diameter, and was estimated to be 335 years old. Queen Elizabeth is said to have planted an elm at Chelsea, which was cut down in 1745, and sold for a guinea by the lord of the manor, Sir Hans Sloane. It was supposed to have become a nuisance to the public road, close to which it stood, from its great size and age. It was 13 ft. in circumference at the ground, and half as much at the height of 44 ft. Before the hard frost în 1739-40 had injured its top, it was 110 ft. high. The Crawley Elm, which has been figured by Strutt, stands on the high road from London to Brighton. It is 70 ft. high, and the trunk is 61 ft. in circumference at the ground. Its trunk is perforated to the very top; and it measures 35 ft. round the inside at 2 ft. from the base. There is a regular door to the cavity in this tree, the key of which is kept by the lord of the manor; but it is opened on particular occasions, when the neighbours meet to regale themselves within the cavity, which is capable of containing a party of more than a dozen. The floor is paved with bricks. Madame de Genlis says a poor woman gave birth to an infant in the hollow of this tree, where she afterwards resided for a long time A hollow elm stood formerly at Hampstead, but in what spot is uncertain. It was engraved by the celebrated Hollar, in 1653; and fig. 1238. is a copy of it from Parke's Hampstead, reduced to the scale of 1 in. to 12 ft. "The Great Hollow Elm Tree of Hampstead," as it is called in the engraving, was upwards of 42 ft. high. It was hollow from the ground to the summit, from which the trunk appears to have been abruptly broken off; and in the hollow a wooden stair, or ladder, was formed, which conducted to a turret on the top, containing seats on which six persons might sit. The following quaint description is given on the margin of the engraving:-"1. The bottom above ground, in compass, is 28 foote. 2. The breadth of the doore is 2 foote. 3. The compass of the turret on the top is 34 foote. 4. The doore in height to goe in is 6 foote 2 inches. 8. The height of the turret is 33 foote. 11. The lights into the tree is 16. 18. The stepps to goe up is 42. 19. The seat above the stepps six may sitt on, and round about roome for foureteene moore. All the way you goe up within the hollow tree.” (Parke's Hampstead, p. 34.) About the time that the engraving was published, a number of rhymes were printed on the subject of this tree, some of them by Robert Codrington; and others were printed by E. Cotes, and were " to be given or sold in the Hollow Tree at Hampstead." Hollar's engraving appears also to have been sold at the tree. Nine elm trees, standing on Hampstead Heath in 1805, were celebrated in a poem by Edward Coxe, Esq., published in that year. (Ibid., p. 40.) In a manuscript lent to Professor Martyn by Craven Ord, Esq., of Purser's Cross, and probably written by Oldys (the translator of Camden's Britannia, who died in 1761), mention is made of several remarkable elms. One at Charlton, in Kent, about which it is said Horn Fair was kept, spread 8 yards on every side; the height was about 10 yards, but the trunk not above 1 ft. in diameter. One of Sir Francis Bacon's elms, in Gray's Inn walks, planted in 1600, was felled, upon a suspected decay, in 1720 or 1726, and was 12 ft. round; its head contained 45 ft. of timber. In 1750, not above eight trees of his planting were left. They were planted in 1600. At Fulham are, or were, some elms planted in the time of King Edward VI.; and one at Richmond, said to be planted by