The walnut aphid, Chromaphis juglandicola Kaltenbach, is an Old World species which apparently invaded California at about the turn of the present century. Although attacked by native lady beetles, green lacewings, and other natural enemies, the aphid frequently becomes extremely abundant. Consequently, walnut growers in many areas routinely treat their groves with insecticides to control it. For years, nicotine sulfate was the most widely used aphicide, but since the middle 1940's it has been supplanted by a variety of synthetic organophosphates and chlorinated hydrocarbons.