Turnip Flea
or TURNIP FLY. Names commonly given to two or three species of Phyllotreta (which see). This genus of small Beetles belongs to the large group formerly included under the generic name Haltica, but now, for convenience, subdivided, by minute characters, into numerous genera. All the insects in this group have the thighs of the hinder pair of legs thickened, whereby they are enabled to leap several inches. This power and their small size have gained for them the popular name of Fleas, though widely different from true Fleas in many respects. They all feed on plants, several being attached to the Cruciferae. Turnips are often ravaged by P. nemorum; and P. concinna is also a dangerous foe to these plants in some localities. P. consobrina and P. obscurella are mentioned by Curtis, in "Farm Insects," as not uncommon on Turnips; and P. flexuosa and P. Lepidii have been recorded as dangerous in Germany. However, the general appearance and the habits of all are much alike; as are also the remedies that should be employed against their attacks. The general form of the species in the genus is well shown in Fig. 133; but the size varies from 1/12in. to 1/8in. in length. All have the ground-colour very dark olive-green or metallic-green. P. nemorum has a broad ochre-yellow streak down each wing-case (see Fig. 133); and the legs are rusty-ochre, with pitchy thighs. P. concinna is more oval and convex than the former species, and is of a uniform greenish-black, with a shining coppery or brassy lustre--the legs being black, except the rusty-red base of the tibiae. P. consobrina, P. Lepidii, and P. obscurella, are much like P. concinna, the four species differing in the pitting of the surface, in the peculiar shade of the metallic lustre, in the colour of the base of the antennae, in the proportions of the fourth and fifth joints in these organs, and in size of body, to a slight degree. P. flexuosa is more like P. nemorum, being black, with a pale clay-yellow line down each wing-cover; but the pale line is narrow and wavy, and the knees are pitchy-black, not reddish; and the fourth and fifth joints of the antennae are shorter than in P. nemorum. An account of the life-history of the very common species P. nemorum may be regarded as fairly representative of the habits of all that attack Turnips; so that the following remarks, though referring primarily to that species, will also, in great measure, be applicable to the others. The Beetles usually pass the winter concealed under rubbish on the soil, e.g., stubble or other dead herbage, clods, &c., though an occasional warm day brings them out of their shelters. In spring, they begin to feed on the cruciferous weeds (Shepherd's Purse, &c.), which are only too plentiful everywhere, alike in cultivated ground and in waste spots. When the cotyledons and first leaves of the seedling Turnips appear above the soil, they form the favourite food of the Beetles, which gnaw away the tissues till little more than the skeletons of the leaves remain, and the plants die in vast numbers. The crops are occasionally ruined; and even second and third sowings are sometimes destroyed in the same way. After the young plants have produced the rough leaves, the danger of serious injury from this cause is comparatively slight; though large holes are gnawed by the insects in the leaves. The female beetles each lay a few white eggs, here and there, on the lower surface of the leaves. In about ten days, the larvae are hatched. They are pale yellowish maggots, with three pairs of legs in front, and a sucker at the end of the body. They gnaw through the skin of the leaf; and, for about a week burrow in the tissues below it. They then are full-fed, drop to the ground, and burrow from 1in. to 2in. into the soil, to become pupae. In about a fortnight, the beetles emerge. There may thus be, and usually there are, several broods in a season.
Prevention and Remedies. It is most desirable to clear away all cover that can protect the insects. The ground should be cleared of all cruciferous weeds, such as supply the insects with food before the appearance of the Turnip crop. Digging and ploughing the soil in winter destroys many of the Beetles, by burying them too deeply to allow of their return to the surface. It should be kept in mind that the dangerous period to the crop is white the seedlings are quite young, and that the danger is very much less when the plants have formed the rough leaves; consequently, whatever strengthens the plants, and shortens the duration of the dangerous period, is a valuable means of preventing harm. It is, therefore, of great importance to have the soil thoroughly prepared before the seed is sown, as well as to give a good supply of manures, such as phosphates and nitrogenous manures, and also to water the plants in dry weather. It has been suggested that thick sowing (up to 10lb. of seed per acre) will give a better prospect of escape from complete loss by the Turnip Flea; but this is not advisable, as overcrowding is apt to render the plants weak, and to prolong the period of danger from the Beetles, as well as to do harm in other ways.
A method sometimes employed to protect Swedish Turnips, is to mix the seed with one-third of its weight of the seed of common Turnips, the young plants of which are preferred by the insects.
Should the crops be attacked, the injury may be lessened, even though a cure should not be effected, by one or other of the following methods: A light, shallow tray may be tarred inside, and dragged over the rows of Turnips, having something at such a distance in front as to disturb the insects, so as to cause them to leap into the tray when roused. Many can thus be caught, the tar holding them fast; or a large hand-net, such as entomologists use in "sweeping" for insects, may be made use of, the plants being swept with it. Rolling the ground in the early morning has been found beneficial, as has been also the treading of a flock of sheep, kept in constant motion in the field, for an hour or two, in the early morning. The latter operations should be carried out only while the dew is still on the herbage; and the same rule should be observed in the application of dressings of all kinds, as these are found to be far more efficacious if put on while the plants are wet. Soot may be sprinkled along the drills of Turnips; but the dressing that has given the best results is said to be made up as follows: One bushel of fresh gas-lime, one of fresh lime, ten pounds of soot, and six pounds of flowers of sulphur, mixed and powdered thoroughly, and applied broadcast or along the rows while the dew is still on. The above amount is enough for two acres. Another mixture also strongly recommended is two bushels of road-scrapings, one of fresh lime, and fourteen pounds of sulphur per acre, applied as above.