SECTION XII.

On clearing Lands of Grubs—Detail of a Set of Experiments, by which is ascertained an effectual Mode of clearing Lands of those destructive Insects.

THE suggestions of an anonymous writer for clearing land of grubs, which appeared in the Register for February last, have induced me to try the effect of what is recommended, by a set of experiments ; and as the result has been completely successful, the following account will, I conceive, be gratifying to every agriculturist.

The objects of my experiments were to imitate a clean summer fallow; and to contrast it with land newly broken up, and having upon it young crops, for the sustenance and nourishment of that destructive insect.

With this view, I prepared four, large boxes, with ledges of wood overlapping the insides of the upper edges, in such a manner that the grubs could not escape. These boxes, placed in the open air, were nearly filled with soil, taken from a field lately broken up, where those insects abounded, and had totally destroyed a fine crop of oats after it had grown to the height of 6 to 8 inches.

The soil was carefully sifted, and every grub taken out, before it was put in the boxes. Two dozen grubs, of different sizes (from about an inch and a half in length, to half an inch) were then allotted to each box. Those in No. 1, were fed daily with fresh leaves of mangel wurzel ; No. 2, with potatoes and potatoe haulm ; No. 3 and 4, represented a clean fallow, without a particle of vegetable substance remaining ; the only difference was, that No. 4 was occasionally watered.

It was on the 10th of September last, that twenty-four grubs were put into each of the boxes ; on the 14th, I examined them as follows :

No. 1.—Grubs fat and lively.

No. 2.—Ditto ditto

No. 3.—Some grubs dead, others of a blackish colour ; thin and sickly.

On the 17th, I again examined them.

No. 1.—Some as healthy and vigorous as at the time they were put in the box.

No. 2.—Some healthy ; but in general fallen off.

No. 3.—Some dead ; the rest miserably thin and shrivelled and of a darker colour.

No. 4.—Some dead ; but in general better than No. 3.

On the 23d of September, observing in No. 1 a ball of clay, about the size and shape of a small walnut, and not knowing what it was, I broke it, and accidentally destroyed a chrysalis, the inside of which was filled with a liquid resembling cream. I afterwards found that it is in this manner the grubs surround themselves with soil when they begin to perceive the approaches of transformation.

On the 1st of October, the grubs fed in boxes, No. 1, and 2, were still fat and thriving ; but in the starvation boxes very few remained alive.

On the 13th of October, a chrysalis had been formed in box No 2 : there might have been more ; for I did not, until the 23d, empty the boxes, in order to ascertain the state of the grubs. On that day it was as follows :

No 1, contained 18 lively chrysalids, and 12 healthy grubs these are six more than were put in the box. This increase I conceive to have been occasioned, either from some extremely small grubs in the soil having escaped notice, or from eggs having been hatched after the experiments were begun.

No. 2, contained 16 chrysalids and 6 strong fat grubs : two must have died.

No. 3 contained a single chrysalis.—This possibly proceeded from one of the grubs, at the time it was put in, being in readiness for transformation ; and consequently it escaped the fate of every other that was put in that box.

No. 4. In this box not a grub or chrysalis was to be found, all had perished ; not even their remains were to be discerned. I think it therefore probable that the wood-lice (oniscus armadilla), which were found in great numbers, particularly in No. 3 and 4, had devoured the bodies of the dead grubs.

After the examination on the 23d of October, the grubs and chrysalids belonging to No. 1 and 2 were replaced in their respective boxes, and four young mangel wurzel plants were set in each, and watered. On the morning of the following day the plants in No. 1 and 2 were all destroyed : whilst those in No. 3 and 4 are still in a thriving condition.

On the 30th of October, one of the chrysalids taken from box No. 1, and put into a glass, covered with thin gauze, produced a grey moth.

I have been induced to give this minute detail of the experiments, as I think it may convey some useful information upon a subject that is interesting to agriculturists ; for I remark, in a late publication, wherein several modes for the extirpation of the grub are pointed out, that the author concludes with this observation.

“But notwithstanding these judicious remarks, much still remains to be done, in order to perfect our knowledge of the nature and modes of destroying these very destructive insects.”Farmer's Dictionary, see Grub.

The preceding detail undoubtedly affords a striking proof of a clean fallow being one of the most effectual modes of clearing lands of the grub. It was, indeed, reasonable to suppose it would be so ; because, if no vegetable substance remain in the soil, for the sustenance of so voracious a creature-whether he be newly hatched by the summer heats, or arrived at full growth, it seemed, at least probable, that he must infallibly perish. Every one can judge whether or not this fact has not now been clearly established.

In the first stage of these experiments I was, for a short time, apprehensive that the plan recommended by the anonymous writer might not succeed ; because I observed that some of the grubs, that were put up in paper, had discharged a considerable quantity of earthy substance. It seems therefore probable, that dike worms) the grubs, in some degree, subsist upon earth ; but the results have shewn that they cannot possibly exist, even for a few weeks, without vegetable food.

A prolonged, or a repeated fallow would, no doubt, be more efficacious than the short period of my experiments. This should be given in warm and dry weather ; for it is then the eggs, deposited by moths, bring forth the young grubs.—Upon these too, in their tender state, the effect would be more speedy than upon grubs that are arrived at full growth. In this stage they are extremely tenacious of life. I have seen them survive, for half a minute, upon ashes so hot that my fingers were burnt in a few seconds. I have kept some in a strong solution of tobacco—others in a solution of alkali, and found them alive, after being twelve hours in these steeps. The common roller has no effect upon them ; yet I think the spiky roller would be a means of destroying them.

The grubs, on which my experiments were made, are, when full grown, about an inch and half long, and a quarter of an inch in diameter.—They are of a bluish colour, and of the. moth tribe. This was ascertained by laying their chrysalids upon soil, in a glass covered with thin gause. I have already stated, that on the 30th of October, the second and final transformation took place : the grey moth that was produced measured seven-eighths of an inch in length.—It had two large dim eyes, very near to its month, and a feathered like top on its head, resembling an owl. One of these months lived in the glass seven days without food.

It is said, that “all caterpillars of the phalænæ tribe alter having several times cast their slough, spin their cod, in which they are transformed into chrysalids :” but the cod of the St. Helena grub is substituted by a crust formed of soil, and the excretions of the insect. Heat, or cold, evidently contributes to forward, or retard, the final metamorphosis. It has been shewn that the two changes were effected in seven weeks—that is, between the 10th of September and 30th of October. In colder climates, however, they are said to require a much longer time ; and that the greater part do not come forth until the ensuing year.

The phalænæ, or insects, sprung from those chrysalids, do not possess the brilliance of butterflies ; but what I imagine to be the male moth is more brilliant and active than the female. There is, indeed, a great difference in their form and colour. The wings of the one are wrapped up or folded round the body ; and those of the other are more spread, and in shape resemble a delta. Some authors have given the moth the name of “night butterflies;”—and there seems to be a sort of analogy similar to that between birds of day and of night. Moths are known to be fond of light, and bet into rooms when attracted by it. I have observed many of the species that produces the grub, fluttering around candles, and destroying themselves. It may, therefore, be inferred that great numbers of the parent of grubs might be annihilated, by placing fires of dry furze, or straw, or torches, in the fields during the night, at the times they are observed to have undergone this final transformation.

There seems to be also another method of destroying the grub, and preventing its propagation. It has been usually remarked that, at certain times, and more especially at night, it remains upon, or near the surface. I therefore conceive that, by means of a red-hot roller, or a perforated hollow cylinder filled with burning charcoal, and moved slowly along a field, many of them might be destroyed.

Mr. Hale, in page 478, vol. iii. observes, that “the grub is the worm produced by the egg of the beetle . There is one particular kind more destructive than the rest, and when the grub is mentioned, without any distinction, this kind is meant. It is a thick, short, whitish worm, with a hard red head, and six short legs. It is found among the roots of corn, and does prodigious mischief; it feeds on the sweet matter of corn, which is, at that time, a kind of pap, like cream.”

However correct this may be in the United Kingdoms , it does not apply to St. Helena. “This creature,” says Mr. Hale, “is the produce of the cockchaffer:”—but certainly the most destructive grub here is what is called “the black grub,” which is the same as I have described ; and its parent is undoubtedly a moth.

Mr. Hale's description corresponds, however, in some degree, to a white maggot (“hog-worm,” as it is here named, from hogs being extremely fond of it) which is found in great numbers in old grass lands, when newly broken up. These have not hitherto been injurious to potatoes, or crops of corn : yet they are very destructive to pasture lands. I knew not, until lately, the cause of the barren appearance I had observed in many parts of the pastures. I had been told it proceeded from shallowness of soil, or from barren clays under the soil:—but, upon breaking, up some old lays, it was discovered that, under these apparently barren spots, this “large white grub, with a red head, six short legs, and nine breathing holes in each side, and measuring, from an inch to an inch and a half in length,” had been at work, and had absolutely separated the sward, for all inch or more, from the sub-soil. I have examined many spots, where the verdure had disappeared, and invariably have found this destructive maggot. I have seen some taken out at 12 or 15 inches under the surface ; and, at other times, have caught them destructively employed within a few inches of the grass, feeding on its roots, and occasioning, the mischief. It seems to me, unless the lands, on which they have taken up their abode, are dug up, or ploughed, that the most serious consequences may be apprehended. Mr. Thomas Greentree, the other day, informed me that, on his lands, this species of insect did not appear until last year.—Already it has spread over several acres ; and unless it is checked, it is impossible to foresee to what extent those depredations may be committed. It is not merely at Mr. Greentree's, but on Church-ground, at Mrs. Harper's, and to the southward of Mr. Defountain's, and many other places, that the ravages of this insect may be discovered. In the year 1751 an insect of the same description, but seemingly of a different species (for at St. Helena I believe the maggot becomes a brown cricket,) made its appearance in the county of Norfolk . So much did it multiply, that not only the verdure of the pastures was destroyed, but the roots of all sorts of vegetables were attacked—and their devastations were so great, that they had nearly ruined many of the farmers in one of the most fertile counties in England.

It was the want of potatoe seed that led to the experiment of oats, upon newly broken up land. I was aware of the hazard to which the crop might be exposed : but as the adjoining field, treated in the same manner, and planted with potatoes, has yielded a good crop, it proves clearly that there is little or no risk in beginning, newly broken up land, with potatoes. The node of planting here, is also favourable—for by using the small whole potatoe as seed, there are many shoots: wherefore if even a few escape the grub, the crop will succeed : and this is the reason why potatoe crops are seldom seriously affected by the depredations of that insect. Repeated cropping and stirring the land, I have also found an effectual remedy ; for in some that was at first greatly infested, there is not now a grub to be seen.

“Grubs feed on the sweet matter of corn, which is a kind of pap. like cream.” This property it evidently possesses at the time the plants are very young and tender—but, when they attain size and strength the juices lose that sweetness, and are therefore, much less adapted to the nourishment of the young grubs : which are, indeed the most to be apprehended.

The crop of oats that was destroyed, was sown, perhaps, too early—that is, on the 4th of June ; and consequently about four weeks before the expected rains.—These, however, this year, in a great degree, failed—and the few showers that did fall, were immediately evaporated by the son's heat. It was during this dry weather that the eggs of the grub were hatched ; and that the young grubs came forth just in time to feed on the tender corn, that had been considerably retarded by the dry weather.

To guard, as much as possible, against such formidable attacks, it seems advisable to sow corn towards the end of June or December ; at which times, the rainy seasons may be supposed to be near at hand—for, whilst the rains continue, the grub cannot be hatched—and if accidentally they should come forth when the corn has been well advanced in growth (a circumstance also unfavourable to their generation, owing to the natural moisture in the corn, and the shade it affords the land) the young grubs would be in a manner deprived of their natural food ; for, if the corn have attained strength, and have lost that sweetness it possesses when young, numbers would die for want of nourishment ; and the corn, by setting forth numerous shoots, would be placed beyond the reach of the feeble attempts of young insects, and be less liable to injury from those that are full grown.


10th December, 1811.



 Section XIII