Home › T › Turnip Sawfly

Turnip Sawfly

(Athalia spinarum). One of the most dangerous enemies to Turnips in many years, since the larvae (known as "Niggers," or "Black Palmers," because of their dark colour) frequently appear in myriads in the fields, and devour the leaves of the young plants. Often a second sowing has been rendered necessary; and even a third or a fourth has been required at times. The perfect insects are Sawflies (see Tenthredinidae), about 1/4in. or 1/3in. long, and rather heavy in form. Their ground-colour is clay-yellow, covered with dense, whitish, short hairs. The antennae, head (except white mouth), spots on thorax, and tips of shanks and of joints of the feet, are black. The wings are transparent, with nerves (except at base) dark. The female lays about 250 eggs, often from ten to twenty on a single leaf, along the margins. The eggs hatch in from six to twelve days. The larvae at first are white, with two black dots on the head; but when nearly full-fed they are black above, paler below, with slate-coloured and black spots on the sides. There are usually several folds of skin, but no hairs, on the body. Often many occur on each leaf, and the leaves are gnawed away to the ribs by them. In about three weeks they reach their full size, and then burrow underground, and form oval cocoons. In these the summer broods lie about three weeks, when they emerge as Sawflies. There are usually three broods of Sawflies each year-in May, July and August, and September. They are partial to flowers, but also sit underneath the leaves of Turnips in dull weather, and may be caught by sweeping the plants with a net. They have been observed to proceed from district to district, beginning at one side of a field, and crossing it to the other, in regular succession. When the larvae are very numerous, the Turnips are almost wholly eaten down, and suffer so much that the crop is rendered nearly worthless.

Besides the well-known A. spinarum, another Sawfly does almost as much harm to Turnips in the larval stage. This is A. ancilla (also called A. glabricollis). The insects are much like A. spinarum; but their ground-colour is reddish or clay-yellow, and they are not hairy. The larvae of the two species are very much alike, and it is probable that those of A. ancilla are often wrongly ascribed to A. spinarum. The remedies recommended below are beneficial against both species.

Remedies. Disturbing the larvae when about to pupate, prevents this latter change, and the insect dies. It is also well to disturb them when changing their skins, as this is seriously hurtful to them, and destroys many. With this object, the plants may be brushed over with a rope, or with such branches as those of Firs, &c., and the larvae thrown to the ground may be crushed; or quicklime or gas-lime may be sprinkled on the soil around the plants. Anything that will promote the rapid growth of the Turnip-plants is valuable, as diminishing the risk of injury; hence, watering or irrigating the plants with liquid manure, or even with pure water alone, is advisable, and has the further advantage of being directly injurious to the larvae. Thick sowing is recommended, as part of the crop usually escapes, unless the larvae are excessively numerous; but it brings with it serious drawbacks. Ducks have been employed with advantage to reduce the numbers of larvae, and have, at times, saved the crops; but they are apt to suffer from eating the larvae in quantities, becoming affected with diarrhoea, and getting very thin.