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I might describe other applications of the principle to the preservation of iron, steel, tin, brass, and various useful metals; but I shall reserve part of the subject for another communication to the Royal Society.
ARt. XXVIII.--Abstract of a Report on the State of the Elm Trees in St. James's and Hyde Parks. Drawn up at the request of Lord Sidney, the Ranger, for the Treasury, by W. S. MacLEAY, Esq. A. M. F. L. S. &c. &c. [Edin. Philos. Jour.]
fThe following remarks compose the substance of a Report on the State of the Elm Trees in St James's and Hyde Parks, which was lately drawn up at the request of Lord Sidney, the Ranger, in order to be presented to the Treasury. As it is possible that some of your readers may have trees affected in a similar manner, perhaps a communication on the subject may not be deemed unworthy of a place in the pages of the Edinburgh Philosophical Journal. I have the honour to be, &c.
To Professor JAMEson. W. S. Macle Ay.]
So little attention has hitherto been paid to the causes of disease in trees, that few persons ever think of attributing it to any other origin than one entirely vegetable, or, in other words, to the constitution of the plant itself. Yet, in every case, perhaps, where the disease is infectious, and particularly where it is confined in a plantation or forest to the individuals of one species of tree, we may reckon with certainty on its proceeding from the attacks of some insect. Every tree, nay, indeed, every plant seems to have one or more species of insect destined by Nature to feed on it; and when, from the combination of various causes (such, for instance, as the weakness of vegetation in a particular air or soil, inattention to the evil at the proper time for effectually checking it, &c.), the number of insects which attack trees becomes increased beyond its due limit, we must either apply the axe without scruple to the seat of the disease, or make up our minds to submit to the utter destruction of our plantations.
Almost all timber-eating insects are comprised in three orders, viz. Coleoptera, or beetles; Lepidoptera, or moths, butterflies, &c.; and Hymenoptera, or bees, wasps, &c. . All these in their youngest state are worms or larvae, and it is while in this state of their life that they commit the direct injury to the trees, either by gnawing off the bark, or devouring the wood. The communication of the disease to other trees is
periodical; for when the above mentioned worms or larvae arrive at their perfect and winged state, the mischief committed by them directly is comparatively trifling, and, in fact, generally results, not so much from their voracity, as from their attempts to extricate themselves, and to arrive at the external air, or from their endeavours to commit their eggs to a proper nidus. But as they are now winged, and capable of depositing myriads of eggs, the germs of as many devouring larvae, the disease is thus dispersed throughout the neighbourhood of the tree originally infected. If, however, it be, in this their perfect state, that the insects are most formidable, having attained the power of propagating the disease, it is also from an accurate knowledge of them while in this state, that we can alone derive any hope of being able to counteract their mischief. The first thing, indeed, to be done in all such cases is, to ascertain the species of perfect insect which occasions the disease. The experienced naturalist is able from this examination of the worm or larvae which he finds devouring the wood, not only to ascertain the order and family, but often the genus, and even the species of winged insect which has . it; and having determined the genus or species, it becomes an easy matter to know the season of its appearance. None of these timber-eating insects remain in their perfect or winged state throughout the year, and rarely for more than eight weeks. We may therefore easily ascertain the proper time for cutting down those trees which are so much infected with larvae, as to afford no hope of saving them; for it would obviously be the height of imprudence not to seize the only opportunity of preventing the annual dispersion of the disease, by destroying the brood of larvae while yet in the tree. As soon as metamorphosis has taken place, as soon as the winged insect has made its appearance, the mischief for the ensuing year is done. Not only is time thus lost, and more trees inevitably destroyed, but the future eradication of the disease is rendered much more difficult. We may also derive another advantage from ascertaining the habits of that particular insect, which causes the disease, and the season of its appearance, for it is often thus possible, by timely measures, even in places where the perfect insect * prevalent, to prevent any deposition of eggs in such wood as may have remained sound. At all events, the devastation committed by these animals is at times so great, that it is WOL. II. No. 3. 35
clearly worth while to make experiments to obviate it; although it is difficult to conceive how such experiments can ever be made philosophically by persons who do not in the first instance make themselves acquainted with the natural history of that particular species of destructive insect which t may have occasioned the mischief.
Of the evil which is mentioned above in general terms, St James's and Hyde Parks afford us at present too many exo amples. The elm trees in both, and particularly in St James's Park, are rapidly disappearing, and unless decisive measures be soon taken to resist the progress of the contagion, we must not only expect every tree of this species to be destroyed in the Parks, but may have to regret the dissemination of the | evil throughout the vicinity of London. In St James's Park, which has more particularly been subject to my examination, there are several species of beetle to be found attacking the elms. That species, however, which occasions all the ño. which we have now to lament in the o Mall and Bird-Cage Walk, is the Hylesinus destructor of Fabricius, or Scolytus destructor of Latreille, an insect of which the history is briefly as follows. A small beetle, with its head rather covered with hair, having a polished black thorax, and brown wing-cases, may be seen in numbers running over the trunks of the elms from the end of March to the first days of July, but principally about the end of May or commencement of June. It may then be seen entering into holes, with which the bark appears perforated as ‘though with a gimlet. It insinuates itself into these holes, or into the crevices of the bark, for the purpose . of depositing its eggs. On stripping off a piece of o loose bark, we may easily at any season understand how the barking of trees is affected by these minute animals, for the surface of the wood thus exposed presents to the view innumerable impressions, which may be compared to impressions or casts of large and broad scolopendrae. The middle or body of this singular impression” marks the path of the perfect female insect, while employed in laying her eggs, which is to her, as to most other winged insects, the immediate forerunner of death. From this tubular path, however, in which she deposits her eggs, the larvae, which are hatched from these eggs, in the shape of little white apod worms, proceed nearly at right angles, eating their way in parallel smaller tubes, which, lying close to each other, effectually serve to separate the bark from the tree. The larvá remain feeding in the tree, generally between the bark and the wood, throughout the winter season. About the commencement of spring they assume the pupa or nymph state, and, before the end of this season, the bark of an infected tree begins to appear as if all its crevices were full of a very fine saw-dust. The last change of the insect takes place; and being now winged, it tries to arrive at the external air, for the purpose of propagating its species, and laying its eggs in other trees. Each hole, which now appears as if made with a gimlet, marks the exit of a perfect insect. In the first instance, the voracity of the larvae, and, in the second, the endeavours of the perfect insects to liberate themselves from the wood, particularly when such attempts are made by almost infinite numbers, soon occasion the bark to fall in large pieces. The consequence is, that the new leaves only make their appearance to wither, and the tree perishes. The early entomologists, little acquainted with metamorphosis, on finding the perfect Hylesinus destructor (le Scolyte of Geoffroy), on dead or dying trees, erroneously considered their disease to be the cause, and not to be the effect of the insect's appearance. Hence the habitat of this species, in many of the older entomological works is said to be rotten wood. The absurdity of this notion will, however, be obvious, on the slightest investigation of the evil, in its earlier stages. It is, for instance, allowed on all hands, that the tree perishes by being barked; and the mere inspection of any of the trees so barked will sufficiently shew that the mischief is effected in the manner above stated. In order to prove that experienced naturalists are now aware of the true cause of such vegetable diseases, it may be sufficient for me to cite, from the third yolume of M. Cuvier's Regne Animal, the prefatory remarks of M. Latreille on the whole family of insects to which the Hylesinus destructor belongs: “Les Xylophages viwent presque tous dans le bois, les larves percent ou y creusent des sillons en divers seus; et lorsqu’elles sont très abondantes dans une forêt, particulerement dans celles de pins et de Sapins, elles font périr en peu d’années, une grande quan
* Impressions of a similar nature, when made on the wood by insects of a neighbouring genus, have given rise to such trivial names, as typographus, micrographus, &c. with which the entomologist is so familiar in the genus Bostrichus, .
On a review of the above remarks, it seems, in the first place, advisable for those persons who have elm trees in the state of those in the Parks, to inspect them twice every year, first, in summer, when the perfect insect is on wing; and, secondly, in winter, when those trees which are much infected ought to be cut down. Such trees ought, if possible, to be burned with the larvae in them, or if this be not convenient, they should immediately be subjected to such heat or fumigations
as may destroy the larvae, which at this season are near the
surface, and therefore not so difficult to kill. To rest content with having cut down the trees, without destroying the larvae, or even removing the trunks from the vicinity of the sound wood, is, in reality, to do no good at all. It may also be recommended, that no more elm trees be, during the continuation of the disease, planted in places where it is prevalent. The Hylesinus destructor is peculiar to the elm, and was in all probability introduced into the Parks, with some of the young elms which have lately been planted in different parts of them. The other species of trees, in these places of public resort, seem, on the whole, to be very free from disease; but in planting, and particularly in ornamental planting, it may be well to bear in mind, that indigenous trees are much more subject to the attacks of our indigenous insects, than those which are not natives of this country. With respect to those trees which are in an unsound state, it is very difficult to point out a cure for them. When the insects attack the branches, these ought obviously to be examined, and if infected, ought, as soon as possible, to be lopped off, and burnt. I scarcely know, however, what to propose, for the preservation of those trees, of which the trunks are infected. Perhaps it may be of use to cover over, in the month of March, with a mixture of tar and train oil, to a certain height from the ground, all such trees as it may be thought proper to attempt to save. I venture to recommend this coating of tar, not only by way of experiment, as protecting the trunks from the access of the perfect insects, but for the purpose of filling those little of holes, which, it is easy in summer for an accurate observer to perceive, afford peculiar facilities for the communication of the disease. To those persons who, being unacquainted with natural history, may therefore be disposed to neglect the power of these insects, because they are individually minute, and who totally overlook the consequences of their being almost infinite in number, I have only to remark, that they may judge from