Onion Fly
(Anthomyia Phorbia ceparum). This insect is at times most hurtful to Onions, the larvae burrowing into, and eating, the bulb scales, near the base, in companies of from two or three to one hundred, or even more. These attacks are very serious in some localities, the greater part of the crop being often severely injured. The bases of the thick leaf-scales rot, and the plants are easily pulled up, leaving the decaying bulb and the maggots in the soil. The flies are not unlike common house-flies in general appearance. They are of a blackish colour, thickly powdered with grey; the sides of the thorax are pale, and there are three dark lines on the back. In certain lights, the abdomen has a whitish lustre on it, with, in the male, a darker median stripe. The eyes are separated, in the male, by a slender, black, white-bordered line; the face has a pale lustre, and the forehead is black, as are the antennae and palpi; the legs are pitch black, the wings being pale grey, and the poisers white. The length of the insect is about 1/4in. In the female, the forehead is broad, with a reddish-brown, vertical median stripe, and the body is more ochreous in colour than in the male.
Life History. The flies emerge, in early summer, from the brown pupae, which have remained all winter underground, sometimes in the Onion, but usually near it in the soil. The female generally lays her eggs, five or six on each plant, on the leaves, just above the soil; and from these emerge larvae, which bore downwards in the soil, and into the leaf bases, causing the whole Onion ultimately to rot under their attacks. They are nearly white, with yellow stigmas, smooth and shining, tapering (but headless) in front, and blunt behind. When fullfed, they burrow into the soil, and there become changed into chestnut-brown wrinkled pupae. The fly, larva, and pupa, are shown in Fig. 742, and a diseased bulb in Fig. 743. The entire development occupies from four to six weeks, so that several broods may appear in a summer. The first sign of attack by the larvae is that the outer leaves become yellow, and fall, withered. The habitat in the bulbs protects the larvae from injury, and from direct means of destroying them, unless the Onions are destroyed. Remedies. Prevention is better than cure; and, for the safety of future crops, it is well to dig up and remove any plants that show signs of turning yellow, especially where the flies have been observed. The diseased bulbs should be dug out carefully; and they, and any larvae that creep out, should be destroyed. If this is done sufficiently early, all the larvae should be got rid of; but if deferred, they will have left the bulbs, and have turned into pupae in the soil, in which situation they are less easily reached. Deep trenching of the soil in autumn is of use to destroy the pupae, and farmyard manure, dug in during the operation, strengthens the plants. Applications of soot or gypsum, and of gas-lime, to the soil have been found useful; as have also soapsuds, and water from pigstyes, and, best of all, the use of paraffin oil, in the proportion of about 1oz. or 2oz. to a gallon of water. Sulphuric acid has been recommended, but is hardly suitable, as it is dangerous to the plants. Scattering the plants in small groups among other crops hinders the passage of larvae from plant to plant.