Turnip Moths
The larvae of several species of Moths feed on Turnip--plants, some preferring one organ, some another; but the insect which is usually denoted by the name of "Turnip Moth" is one of the Noctuae (see Moths and Noctua). Its scientific name is Agrotis segetum. The popular name is rather misleading, as the larvae feed on many other plants, including most root crops and numerous weeds. Moreover, several other Moths are almost as hurtful, in the larval state, to the roots of Turnips as is A. segetum. The genus Agrotis is a large one, and in the same species there may exist great variations in markings and shade of colour; while the species show a great general similarity to one another. The fore wings are long and rather narrow, and the hind wings are pale grey. A segetum (see Fig. 134) reaches 1 1/4in. to 1 2/3in. across the fore wings, which are grey or brownish, sometimes with a yellowish tinge; they are usually darker towards the front margin, and bear obscure, darker marks and cross-lines; the hind wings are pearly-grey. The larvae are of the form shown in Fig. 135; they are smoky-brown, tinged with pink or purple, with darker lines and small spots, and a paler line down the middle of the back; the head is small, and much narrower than the body. Owing to their habit of living concealed below the surface of the soil, the larvae are seldom seen. While young, they feed, in the darkness, above ground; and they are especially disposed to gnaw through the base of the stem in young plants, or to remove the bark, if the stems are too large to be gnawed through. They are indiscriminate feeders, devouring almost all kinds of herbaceous plants. In autumn, they often resort to Turnips, boring into the tuberous roots from below, and eating out large cavities, in which they live during the winter, or till food fails. A few become pupae, in cocoons in the soil, in October; but by far the larger number pupate in spring, and the moths emerge in June.
The Heart--and--Dart Moth (Agrotis exclamationis) is very similar to, and almost as destructive as, the Turnip Moth. The larvae of the two species are so much alike that it requires a skilled entomologist to distinguish the one from the other; but this is of little moment to gardeners and farmers, as they are as much alike in habits and in destructive powers as they are in appearance.
The larvae of a good many more species of Noctuae occasionally feed on the roots, leaves, or stems of Turnips, but they are so seldom the cause of perceptible injury to these plants that it is unnecessary to discuss them here.
Remedies. Such larvae as feed on the leaves of Turnips are best combated by the means detailed under Turnip Sawfly. Those larvae that frequent the roots are not easily reached when they have bored into the Turnips. The time when they can be destroyed, or kept in check, with most hope of success, is in autumn, while they are still living concealed during the day, but come out to feed at night. Hand-picking, by the light of a lantern, is too slow, except to protect choice plants. Soot and gas-lime have been found useful, applied round the top of the root of each plant; and the same is true of gas-water poured on the soil. Ploughing or digging the ground, during winter, is useful by turning up the larvae and exposing them to rooks and to other insectivorous birds, which are, in truth, the agriculturist's best allies against foes of this sort.