THE PEAR-TREE SLUG.

New Zealand Herald, Volume XII, Issue 4297, 21 August 1875, Page 1 (Supplement)

THE PEAR-TREE SLUG.

We slightly condense a very luwircauxi* paper, by Edward Newman, on the natura] history of the above-named pest, published in a number of The Field,-to hand by lasl mail. The slug-like caterpillar, which he describes as infesting the orchards of the old country, is a near relation of the one which has ravaged onr fruit trees of late years; it is, indeed, even more dispisting; as to the repulsive appearance, which it haa in common with our species, it adds tho unpleasant accompaniment of an offensive odour. The chief value of the paper to our fruit-growers is, that an effectual remedy is named. There aro three kinds of hellebore kept by druggists,—tho dried root of the black hellebore, the dried leaves of the foetid

hellebore (or bear's foot), and the powdered root of the white hellebore. The last-named is evidently the kind used by the Rev. Charles Bethune. Care should bo taken in handling the powder, as if any of its dust gets into the nostrils it excites distressingly violent sneezing. The earliest, best, most complete, and most accurate account of this most objectionable insect was written by Professor Peck, and was printed at Boston, U.S., at the very end of last century, by order of the Massachusetts Agricultural Society. This body awarded 50 guineas and a 'gold medal for the memoir, which it is now difficult or impossible to procure. Dr. Harris, however, one of the most eminent of American entomologists, has given us the substance of this essay at p. 418 of his "Treatise on Inseets Injurious to Vegetation." This second account, with some abbreviations and modifications, has been adopted by all subsequent writers, and its chief points are incorporated in the ...resent memoir, not, however, unadvisedly, or without a careful study of the insect in a state of natui e. In May the mother fly emerges from the earth in which she had voluntarily buried I herself. Her winged life is at the utmost of I three weeks' duration. Her first thought, like that of our own female relatives, is matrimony. Her second thought, or instinct, or duty, is preparing for a family. _ A word as to her personal appearance : she is always in mourning. Even before matrimony she wears the sable garment of widowhood ; her head, antennas, body and legs are clothed in black ; her wings, otherwise colourless, wear j a blackish band or veil across their middle. I could here enter into many other particulars, but I fear to inflict on the readers of The Field a technical description of this | lugubrious inßect Suffice it to say that she is about the size of a grain of wheat, but of | course of a very different colour. By j shaking the leafy twigs of a fruit tree i over a sheet of white paper or a white | cloth, you will be sure to see some of these ; black, grain-like, and seemingly lifeless crea- | tures fall on the cloth ; and should it be held ]

in a slanting direction they will roll over and over until they tumble off the edge, and so deprive you of the chance of minute examination. A great number of insects have this habit of feogoiug death, evidently with the object of rendering their appearance unattractive or unrecognisable to those other insccts or animals of any kind which make living insects their customary food. As though purposely to aid in this life-preserv-ing and therefore very excusable deception, their bodies are bo fashioned that by bending their beads downwards beneath their breasts, pressing their antenna?, lees, and wings closely against the body, and resolutely aontaining from all movement, the whole appearance becomes so perfectly inorganic that oven the sharp prying eye of a bird would bo deceived, ana the delicate, discriminating touch of a spider would fail to detect life under this mask of death, so perfect is the deception. Then she may be seen in the act of oviposition, and a serious matter she makes of it; so serious and so intent is she in the performance of this maternal duty, that you may sometimes take her off the leaf between your finger and thumb. She will evince no disposition to fly, make no effort to run, but only resort to the expedient •of feigning death—an expedient that facilitates her capture rather than otherwise, especially if you hold one hand beneath the leaf on which she is operating. I need scarcely say that this insect is a member of the great family of sawflies, a family that has long attracted the attention and admiration of the entomologist; nor need I again describe the saw with which all of them seem to abrade the cuticle of the leaf, leaf-stalk, or twig on which they deposit their eggs. Suffice it to say that the abrasion made by the insect whose history I am relating is of ■curved or crescentic form, and the egg is laid in this abraded portion; the denuded parenchyma of the leaf thus comes into immediate contact with the under side of the egg, which is of an oblong shape, and is covered with a leathery shell, capable of considerable expansion as the inclosed larva increases in size. Thus the egg is seen very obviously to grow—a fact perfectly familiar to entomologists. This faculty of growth in the egg state was known to Linnscus, and li--m been repeated by all subsequent writers on ; .this tribe of insects. To criticise or contra- I diet observers so careful as Professor j Peck .and Dr. Harris is out of the <iuestionbut there iB ouc point in wnicli I-differ from these most observant and accurate entomologists. Both Peck and Harris I either state, or lead ub to infer that the egg ' is laid and the larva feeds on the under sine '

•of the leaf. My own experience is exactly the reverse of this, and agrees with that of the iU;v. Charles Uethuue, as given at page .51 of his "Annual lteport of the Entomological Society of Ontario. The eggs continue to grow during thirteen days, at first slowly, towards the end of that period more rapidly. Gn the fourteenth day, according to Professor Peck, the young grub emergen from the .egg. I have no doubt this utateaacut is -correct as .regards the United States, but _ 1 cannot say that I have verified it in England. On emergence they *re white or colourless, but in a veiy short time they- are covered with a black, brown, or oli.ve-coloured jelly, of offensive scent and disgusting appearance. Although Peck, Say, Harris, Bcthune and others la America hare written on this grub, De <Jeer, Reaumur, Boucfee, Hartig, and many othecs-on the continent <of Europe, and Mr. West wood in England, and although I have read tkeir observations with the attention they merit, I cannot say that I thoroughly understand the mode in which this jelly or mucilage is produced. It accumulates on the auxface of the wW™ until the creature becomes a dark mass, without apparent life, or e«en organisation. The grab glides with extreme slowness over the surface of the leaf, aDd apparently by means of clampers, a pair of which are attached to the under-side of every segment except the first, fourth, and thirteenth. Theseckspers

seem to possess littleof that tenacity which is so striking a character in the claapera of the caterpillars of wothß and butterflies. Legs there appears to be cone; but, like the onisciform larva; of certain Lepidoptera, the creature moves by the alternate dilatation and contraction of the ventral surface. The head is entirely withdrawn into the second segment, and concealed from view. The body is somewhat small at the anterior extremity, gradually but slightly attenuated to the posterior extremity. It seems destitute of any rambling disposition, its food,

which is the upper cuticle and parenchyma of the leaf, being always within rcach. It consumes these iu a very methodical manner, leaving the lower cuticle entire; this very soon dies, withers, and turns brown, making the whole tree look as though eovered with dead leaves. The hinder segments are generally raised slightly from the surface of the leaf, a very common character in thia tribe of insect The slug sometimes destroy* the foilagc of the pear so entirely, that the tree appears to be dead ; but there is still vitality within, and new leaveß and now bIo8Bom»--those intended for another year—areput foith out of time and out of eeaiKia. Thus the entire nature of the tree

| is changed, its functions disarranged, and its fruit-bearing intentions, for two years at, | least, are frustrated. Notwithstanding its jelly-like covering, the slag changes its skins' five times before arriving at its full Bize. At the last change it loses its jelly-like surface, and appears in a neat yellow skin without any viscidity. This great change occurs nearly a month aft%- their first: escape from, the egg-shell; the head and segmental divisions are now quite as perceptible as in any other species of sawfly. Henceforward it eats no more, but crawls down the trunk of the tree and buries itself in the earth: at the depth of three or four inches, each forms a neat little oval cell in which to undergo its final changes to a chrysalis ana perfect fly. This cell is formed of earth, but is lined and intermixed with liquid gum secreted in the stomach, and ejected by the mouth of the grub. This gum-glue is obviously nothing more than silk in a liquid state—a preparation with which nearly every moth, butterfly, hymenopter, or coleopter is provided, more or less abundantly, and one which is always applied to the fabrication of a cocoon, cell, or covering of some kind in which to undergo its transformation. When this gum has once hardened, and assumed its final state of silk or leather, it is insoluble in water, and forms a perfect protection from wet. In this cocoon the grub remains for about a fortnight, and then emerges as a fly to found another, an autumnal generation of slugs ; these go through the same cycle of transformations as their progenitors, and at the approach of winter retire into the earth to pass that leafless season underground. A word remains to be said about the supposed remedies. Our Transatlantic cousins, having made themselves thoroughly acquainted with the enemy, have recourse to practical measures, with a view to compass his destruction. Sand, ashes, lime, and i powdered hellebore have been tried with great energy, but the last only has been found reliable. The results of these experi- 1 ments were recorded in the September number of the Canadian Entomologist for 1870. As soon as the slugs were observed at work in spring, they were treated to a plentiful supply of dry sand, thrown up into the higher branches with a shovel, ana over the lower ones through a sieve. Tho sand stuck thickly to the slimy skins of the grubs, completely covering them. Supposing the enemy conquered, no notice was taken of him for some days, when he was found to Lave recovered from tho assault, and to be as vigorous as ever. It was then determined to test tho sand experiment on a smaller scale. Several small branches of pear trees were selected and marked, on each of which were six slugs ; and theso were well powdered over, and completely covered with sand. On examining them, it was found that they had shed their sand-covered skin, and hod crawled out as slimy as before. The Band was applied a sccond and a third tim«, with similar results. Seeing, then, that sand was useless, the slugs were treated to a strong dose of hellebore and water, which soon finished them. Ashes were next tried in the same manner as the sand had been, and were found equally ineffectual. Another experiment was tried with a solution of hellebore, and is thus reported:— ."On the 13th of August, at S a.m>, a branch of a eherry-tree was plucked, on which there were sixty-four Blags. This branch had only nine leaves, so it may be supposed they were thickly inhabited. A dose of hellebore and water was showered on thom about the usual strength, an ounce to the pailful, when they soon manifested symptoms of uneasiness, twisting and jerking about in a curious manner; many died during the day, and only six poor sickly-looking specimens remained alive the following morning, and these soon after died. During the past season (1S70) these slugs have been unusually abundant on our pear-trees, in many cases destroying the foliage so thoroughly that they looked as if they had been scorched by a fire ; every leaf, in some instances, dropping .from tho trees, so that for a time they were as bare as in midwinter. Nearly a thousand trees, in the young pear orchard of the writer suffered Beverely. During the latter part of June and the early days of July, we had no opportunity of inspecting these trees, and when we visited them on the 7th of July, they were so much injured that we thought they could not be much we<bc ; and, as the slugs were then full-grown and fast disappearing, and as the application of a remedy to so many trees was a matter of so much labour, nothing was attempted to remedy the evil then. Then follows a list of the pear-trees injured, 3nd from this it appears that some varieties suffered much more severely than others. In the course of a fortnight after these observations were made, new leaves push out vigorously on the defoliated trees, acid within a month or six weeks all was green again. A " In the meantime, says Mr. Bethune, the mischief-makers were preparing for a second leseeut, and we in our turn were preparing io reoeive them. On the 29th July, when ;cing thrcigh the orchard in the afternoon, ;he new brood of flies were found in the greatest abundance, resting on the young eaves on those portions of green ivhich still remained on the leaves partially »ten by the last brood. They were congregated, however, most thickly on those trees where green leaves were most abundant. On listurbing tliem, they would fall to the jround with the attenme beut under tho »dy, and the head bent downwards. We aught sixty specimens, and might have aken hundreds. They were so thickly ipread, that in many instances there were ;wo or three on a singlo leaf. By tho last reek in August tho sccond brood of slugs hatched. Now those trees which had previously -escaped were all more or less inested. A raisod platform was rigged up in i one-horse cart, in which was placed a »rrel of water in which a pound of powdered lellcbore had been mixed, and from this derated stand this mixture was showered lghtly on the trees from the rose of a water-ng-pot. It was astonishing how quickly tho ;rees w«rc cleared by this method; scarcely i slug could be found on a tree the morning ifter the application had been made, and 10 bs. of hellebore, with five or six days' work >f a man and horse, served to go over tho whole ground." Powdered hellebore has been succcssfiilly iried in England on a small scale, but there s an evident difficulty in raising the water » a sufficient height to be of much service tmong the giant pear trees of Worcestershire tnd . Herefordshire ; still I would by no neans discourage the attempt. In a scien;ific point of view, it would be interesting to iscertain the identity or otherwise of the 'slugs" of Europe and America, and to iscertain also whether the slugs had migrated, either naturally or through the initrumentality of man, from the old to the lew continent, or rice versa.