THE TURNIP FLY.

Waikato Times, Volume XXI, Issue 1778, 27 November 1883, Page 3

THE TURNIP FLY.

■ ■■ '■■ ♦ ■ Thkrb are very few insects which attack farm crops with effects more disastrous than the turnip fly. The insect is not a fly in the true sense of the term : it belongs to the beetle family, ami is ically a minute beetle with comparatively large transparent wings. As it is seen fitting about over young turnip plants, it appears with his large wings and small body to the naked eye very much like a small fly. From its size and general appearance when its wings are closed, it is in some places termed the turnip flea, — 01 flea beetle. It is known to men of science as JPhyllotreta nemorum, or at times Haltica nemorum. There is more than one sort of turnip fly, just as theie are different sorts of tin nips ; but as regards the flies they appear all the same to the naked eye, and alike in their general customs. In its habit of flying it resembles, the cockchafer and other membeis of the beetle family familiar to every agriculturist. We propose to describe in the clearest possible manner what the turnip fly is, how it attacks the turnips, and, most important of all, how it can be destroyed, or, at any rate, how the severity of its attacks can be very greatly lessened. The flies live through the winter, generally in a semi-torpid state, slightly beneath the surface soil of fields, under stones, within the hollow stem of old straw and partly-decayed herbaceous plants, in heaps of dry field rubbish, and in any place that is likely to afford some protection against the cold and wet of winter. With the first sunshine of early spring these little flies emerge from their hiding-places and lun over the surface of the ground. Being so small, they are virtually invisible to tho naked eye ; hut but when they spread their transparent wings and flutter in the light, especially when they are present iv large number?, they become as visible as the ordinal y green -fly, but not so clearly seen as gnats. If some of the specimens aie secured and magnified with a lens aboutfive diameters they will be more clearly seen. The colour of the fly's body is black, with two stripes of yellow from snout to tail. When these creeping and fluttering insects emeige from their winter retreat they are on the look-out for young turnip plants, for it i* their natuie to prey upon cultivated turnips, just in the same way as the corn mildew attacks wheat, and the. potato fungus the potato. They like the very youngest and sweetest leaves ; but if no turnip seeds have been sown, or if the seeds have not yet germinated, these flies make their attack, on plants the nearest allied to the turnip that they can find. Charlock and wild mustard are the commonest and tenderest allies of the turnip, and the beetles seek out these or other closely-allied weeds, make a temporary attack on them, and quietly •wait till turnip seeds grow. The beetles commonly attack the turnip plant as soon as the two fiist smooth green seed-lea\cs show themselves above the ground. As the function of these two leaves is to assimilate food for the plant, it follows as a consequence that if they are oaten off 1 , or if they get seriously damaged, the young turnip plant must perish. That they do very often get eaten off, and the enlu<» first sowing of seed thus wholly lost, is only too well known to every fanner. But at this very eaily point of the attack we sco the necessity ot sowing the best seeds procurable. If the seeds are bad and only one-half come up, tho Hies ha\ c it all their own May, ami pel haps destroy every infant plant. On the other band, if all the seeds come up and theie i 9 a heavy crop, there is more material than the flics can attack, and .sufficient plants arc left for a good turnip crop. Again, if the seeds aic of the best quality the first leaves will be so large that they will withstand a consideiable amount of eating away without fatal injury ; on the other hand, if the seeds are inferior the leaves will certainly be very small, and a slight attack only from the fly will utteily destioy the plants. Whilst the weak and drawn-up seedling raised from poor seed is seriously and perhaps fatally damaged, the much more sturdy seedling laiscd from the best selected seed is not likely to be materially affected by the attack. It is impossible to overrate the importance of seeming the best seed so that robust young seedlings may be obtained. At the time of, and soon after germination, the whole existence of the future turnip crop depends upon the first two little green leaves which emerge from the seed. The function of these two leaves i? most important for the infant turnip plant : their office is to prepare, from dead inorganic materials, the living life-substance of the young plant. The seedling absorbs water by the roots and carbonic acid by these little leaves ; when these leaves spread themselves out to the sun and light, the carbonic acid and water combined form starch ; at night the starch is dissolved and distributed through the infant plant to build up its growing substance. We have no intention of going deeply into chemical or physiological matters here, for we solely \i isli to point out the extraordinary importance to the infant plant of large, healthy, and uninjured seed-leaves. The habit of the turnip fly is to eat these little leaves into holes and rags, and the more they arc eaten away the less chance there is of a good crop of turnips. We therefore strongly recommend not only the purchase of the best seeds, but advise thick and liberal planting, so that the seedlings may be too numerous, as well as too robust, for fatal attacks from the turnip fly. It must be remembered that the chief injury is done to the turnip plant in its infant state, therefore every effort should be made by the best cultivation to hurry the seedlings on to produce a few of the ordinary rough leaves. When a few of these ordinary rough leaves are produced the young turnip is almost proof against attacks of the fly ; at any rate the plants can withstand 01 throw off attacks with ease. We must now describe another phase in the attack of fly upon turnips. In October [in New Zealand] the flies begin to lay egsjs, and when a turnip field is selected for this act, the eggs are laid on the under sides of the rough turnip leaves, the seed-leaves having by this time all decayed. The eggs are not much larger than an ordinary pin's head, and being laid on the under side of the leaves they are virtually invisible. Being underneath, they are protected from the heat of the sun and from storms of rain ; the eggs are soon hatched, and do not give rise to a fly but to a very small grub ; as soon as hatched the grub eats it way into the substance of the leaf and lives between the upper and lower skin, feeding on th green vital material withid the leaf. The grubs continue eating the inner substance of the leaf for about six days, when they drop out of the leaf onto the ground close to the turnip plant and bury themselves au inch or two beneath the surface. In this position each grub changes to a chrysalis, and in about a fortnight a perfect turnip fly emerges from each chrysalis. The first new brood of flies generally appears in November or December, and as less than a month is sufficient for eggs to be laid, the grubs to be hatched and fed, the chrysalis condition reached, and another brood of flies hatched, it follows that there are generally five or six broods of turnip flies produced every season, and always," of course with their attendant grubs. From the above remarks it will be seen what an enormous power of increase the turnip fly possesses in being capable of producing five or nix generations in one season, • In considering the nature of the means to be taken to avoid attacks of the turnip fly, it must be remembered that wet weather is very distasteful to the insect ; as it can neither leap nor open out its fine gauzy wings in rainy weather, thin wet weather, on the other hanfl, is the very condition young turnips mostrequire. Hot, - dry,- •unshinjr weather ia , mpst suitable to the fly, and on the other hand (Vii equally unactable to fae tyrair*. Jt

follows, therefore, if the turnips can be hu tried on in wet weather well through the seed-leaf stage, they will grow at their best v hilst the fly is at its woist. It is in the highest device desirable that the surface ot the gtound on which turnips are to be sown should be fine and oven, without, if possible, even moderate-sized clods ; for if the sin-face is not finely picpaied the young seedlings cannot effectually pierce it with their roots, ami if the seedlings are in any difficulty (however sliglic) it invaiiably pioves advantageous to the turnip fly. As the turnip flies wake up from their winter's rest in October, and as seedling tin nips are commonly not above ground till the end of November, it follows that it the flies aie deprived of food for this six weeks or two months they will piobably be greatly weakened, diminished in numbers, or even killed. As the tin nip fly can only live on the turnip or some of its allies (chiefly charlock), it is manifestly advisable to remove as far as possible all weeds of the turnip and cabbage class t.o frequently seen on ill-kept farms and in hedge-sides. All weeds of the charlock class, with yellowish or whitish flow ers of four leaves (petals), like the cabbage and turnip, are harbours of refuge for the turnip fly. The general opinion of practical farmers is that late sowing is best — the broods ot fly are known to be weakest in January : — we advise thick sowing of from three to four pounds of seed per acie of the best vai ieties — the sow ing to take place at a time when there is a piospect of a week or fortnight's wet weather. This w ill be found to be the better plan on the whole, as it will probably avoid the expense, trouble, and delay of re-sow-ing. We confess that we cannot advise the steeping of seed in turpentine, Kerosene, paiafiin, caibolie acid, petroleum, &c. These materials, if properly diluted with water, arc no doubt distasteful to the fly and do not injure the seed, as the outer skin of every seed is already a dead substance, the vital mateiial being within. Now the turnip fly does not want the dead outer husk ot the tin nip seed, but the living, juicy, gieen seed-leaves fiom within ,is they arc spread out in the sun ; these leaves aie not likely to be hugely tainted by paraffin, and upon these leaves the flies rest, or leap fiom one to the other and eat. Some persons have stated in print that the veiy odour of p.uafh'n in the soil is distasteful to the fly. No doubt tins is the case, but it must be remembeied that the fit st shower of rain will wash the paiafiin or other substance, with its odour, entirely away. In reference to manure, our opinion is that without unduly disparaging farmyard manute, which has its undoubted value, yet artificial manure is best. The foi mer not only harbours the fly of the turnip, but its scent attiacts it. Fai 111yard manuie invariably contains the lising gems of innumeiablefouns of plant disease. We advise the application of Webbs' special turnip manuie at the tale of foui 01 five cwt, per acte. The summary of our ad\iee is : l"-t, keep the fields us clear fiom weeds and rubbish as possible ; 2nd, piepaic the g_iound with a fine e\cn sui face ; Old, sow the best selected seed ; 4th. sow thn Idy ; oth, sow dining weather which is likely continue wet for a shoit time ; (ith, manure diseiectly, tor too mucli manuie h as bad as too little ; 7th, w lieu convenient give tlicpiolciemio to aitificia! lather than to f aim-yard manuie. —Webb and Nona' Ciicular.

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