CHAPTER VIII.
ON THE INJURIES CAUSED BY INSECTS.
Root-crops are especially liable to injury from the depredations of insects. Thus the turnip may have its seed more or less destroyed by weevils. Immediately the seed appears above the ground, commences the attack by the turnip flea-beetles. The bulb is pierced by beetles, ending in those excrescences called “turnip-warbles;” and there is reason to think that even the root-fibrils are in some soils made the depositories of the eggs of insects, which give rise to extraordinary malformations.
Carrots and parsnips are liable to have the best-grown root made useless by its being pierced and eaten by the larvæ or grubs of a small fly, known as the Psila rosæ.
Even the mangel-wurzel, which has been so strenuously recommended as a substitute for the turnip on account of its freedom from insect attacks, and connected with which Curtis only describes a single insect, a leaf-miner, called Anthomyia Betæ, upon which he remarks that “these insects will seldom cause any loss to the mangel-wurzel crops should they ever abound to any extent.” In spite, however, of this, we find that the increased growth of this crop has caused a corresponding increase in the insect, to such an extent that, during the last two seasons, many crops have entirely failed from its depredations; as witness the following communication to the Agricultural Gazette for August 23rd, 1862:—
My mangel crop was drilled the 17th May, and came up most favourably. On Monday, the 2nd June, I asked my bailiff what was the matter with it; he said, “Oh, it was a sharp frost last night;” but on examination I found that instead of frost the leaves had within them a maggot, which had caused the plant to brown and die off. The late rains and growing weather have enabled the plant somewhat to revive, and also fresh plants to come up (for I had drilled 7 lb. per acre), but found to-day several leaves with maggots in them. My man told me “a quantity had eaten themselves out of the leaf and dropped;” and that he saw “a vast number of sparrows picking up those maggots.” I send you herewith some plants I brought up from the farm. My idea is that the seed was damp and bred the maggots, or that the leaves had been “struck with a fly,” and then the maggot followed. You will please let me have your ideas upon these points.—S. S.
The maggot, or larvæ, here described is that of a fly called the Anthomyia (Pegomyia) betæ, mangel-wurzel fly. An allied species will sometimes be found on the common dock-leaves, mining their galleries between the dermal cells of the leaves.
We have for some time observed the increase of this pest, and we are prepared to state that now we seldom see a crop that is not greatly injured by its attacks. Mr. Curtis thinks that the best method to destroy them is to employ boys to crush the leaves between the thumb and fingers at the part where the larvæ can be seen; and with this we fear we must for the present be content, unless we could devise some means to take the fly before its eggs are laid in the leaves.
We need not here dwell at length upon the natural history of those pests of the turnip—the Haltica nemorum (striped flea-beetle), and H. concinna (black flea-beetle), as the nature of their ravages are tolerably well known. Thus much, however, may be said; namely:—
a. These insects are called fleas because they have the power of hopping on being disturbed, much after the manner of a flea.
b. They have some five or six broods each year; the earlier ones probably being bred on charlocks and other weeds of the same natural order as the turnip; and hence, then, charlocks are pests, not only as being weeds, but as breeding-places for one of our most mischievous insects.
c. They migrate from their weed-haunts to the first crop of turnips, where much of their mischief may be prevented by simply dusting over the young plants with any fine powder, road-dirt answering the purpose as well as anything else.
Various devices have been employed for keeping away and killing these little creatures. We have used a contrivance for catching them, which may be described as follows:—
Some thin board (or boards), making a surface of about 4 feet long by 2 feet wide, is furnished at one end with a pair of light wheels of just sufficient diameter to lift the board about 2 or 3 inches above the plants. To the other end may be attached two crooked handles in such a manner that the machine can be wheeled flatly over the plants, or if four wheels be employed, one at each corner, a single handle can be used either to push or pull the implement. When used, it should have its underside painted over with tar or any handy viscid substance.
This should be used on bright days, the operator pushing it over the rows of turnips, so as, if possible, not to throw his shadow before. The middle of the day will be best, not only for this reason, but also because these creatures feed more actively at that time.
Now, our experience in the use of this simple contrivance on small experimental plots convinces us that a small boy could easily keep under the enemy in a good-sized field.
But now comes a very important question for consideration. Cannot we do more than kill a few of these creatures? cannot we adopt such plans as will render our crops tolerably safe from their depredations? We think so, and to this end advise the following method of proceeding:—
Let each turnip-grower prepare for the enemy by sowing from the eighth to a quarter of an acre of turnips in a sunny part of the farm as early as the first week in April. These patches would quickly attract all the turnip flea-beetles from the wild cruciferæ on which the first broods seem to depend, and in this small compass they can be killed in detail with the simple contrivance just described, so that when the real crop comes up there will be none, or at least only a few, beetles to emigrate to it; whereas, as we now manage, by the time the crop of turnips is sown, enough of the creatures are too often bred to render it necessary to sow two or three times before we can secure a crop.
Anbury is an affection to which only the different sorts of turnips are liable, in which case it differs from finger-and-toe, with which it has been very much confounded, as this latter occurs in all kinds of roots; namely, turnips, carrots, mangel-wurzel, &c., as well as both the common and Swedish turnips.
As a sample of an extreme case of finger-and-toe—digitate root,—we repeat the following [figure] of a Belgian carrot, in which it will be seen that the forks gradually taper to the extremities; in fact, the whole, instead of being a succulent fleshy tap or fusiform root, in which case it could readily be stored, is divided in fingers-and-toes, which are liable to break off, and this renders the product next to useless. Now, this affection may occur in any soil, as it is the result of a degeneracy in the stock of the plant; but in the affection now to be described the case is wholly different, as here the bulk of the swede ([fig. 12]) is affected with rough, cancerous knobs, whilst the rootlets support irregular knobs of a like kind, which have more the aspect of suspended rows of ginger than fingers-and-toes.
Fig. 11 (Fig. 8 repeated).
Finger-and-toe Carrot. Half natural size.
Roots so affected soon rot, and have a fœtid odour, so that they are not only useless themselves, but communicate canker and decay to the whole store. In the putrid mass will be found maggots and flies and beetles of different kinds; but as yet naturalists are not agreed as to whether the nodules of disease are caused by insects, or whether these creatures are merely attracted by the fœtid matter. We are, however, inclined to the belief that some insects are connected with the diseased appearance in the first place, whilst others afterwards step in to fatten upon the decaying matter, induced by the first lot; but still it must be confessed that the subject requires much more attention than it has yet received, in order to settle these important questions.
Still it may be observed that one point has been universally admitted; namely, that anbury only occurs to any extent in sandy soils, where there is an absence of lime, a good dressing of which mineral is the best safeguard against this affection. Still, in soils that are liable to anbury, we should not recommend the continuance of turnip-growing, or at least not so frequently in the rotation as has hitherto been the case, and more especially as the soils which produce anbury to the greatest extent are just those best adapted for parsnips and carrots, which, if not wholly, may occasionally be very profitably grown in the place of the turnip.
Fig. 12. Swede affected with Anbury. Nat. size.
Having given a few notes on the more prominent forms of insect attacks to which root crops are liable, we would now close this chapter, as details of all the insect pests would occupy more space than we can here allot to the subject; but to those who would inquire further upon this fertile theme, we would advise the perusal of “Farm Insects,” by J. Curtis, Esq., F.L.S., &c.