The Cabbage Aphis (Aphis Brassicae), also known as the cabbage louse, has been known in America for over a century, and has become disseminated throughout the country. Its destructiveness varies greatly from year to year, depending upon seasonal conditions and the prevalence of natural enemies. In many states it was especially troublesome and caused heavy losses in 1908 and 1909. Both seasons were drier than usual, and it seemed that neither insect nor fungous foes had much effect in checking the ravages of the aphis. Protracted drouth unquestionably favors the multiplication of this pest, while low temperatures with heavy rainfalls are the most unfavorable conditions.
This insect usually appears in the North the latter part of May or early in June, and feeds upon both upper and under sides of the leaves, which they cause to curl. The pest multiplies with marvelous rapidity, producing from a dozen to a score of broods in a season. (Bulletin 2, Virginia Truck Experiment Station, Norfolk, Va.) The bodies of the mature forms are green, but are covered with a grayish, powdery coating, which at a distance gives the appearance of mildew. The young are ready to reproduce in five or six days.
The eggs, which pass the winter attached to the stems and the refuse leaves, give rise to the first broods next spring. This suggests the most effective means of prevention: all refuse in infested fields should be destroyed in the fall. It is also important to destroy remnants of infested plants from which the summer crop has been harvested.
Spraying with kerosene emulsion, diluted to 15 parts, is a satisfactory means of controlling the insect on growing plants. The first application should be made before the infestation becomes general. A knapsack sprayer, with the proper nozzle and extension attachments may be used to advantage in spraying isolated plants and small plantations. It is important that the material be applied to the underside of the leaves as well as to the upper surfaces. To accomplish this thoroughly seems almost impossible with a power machine.