Walnut Aphid Investigations in Northern California

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Abstract
The walnut aphid ( Chromaphis juglandicola (Kltb.)) is most destructive to walnut from the time the leaf buds begin to unfold until the leaves are fully expanded. Severe infestations during this period greatly stunt the leaves which is reflected in a reduced size of the developing nuts. Schradan is the most effective insecticide that has been tested against the walnut aphid. Except in a region where the aphid has shown resistance to certain phosphate insecticides, 0.5 pound of schradan per acre has effectively controlled the pest for an entire year. Better control has been obtained with dosages of 0.75 and 1.0 pound. For satisfactory results treatments must be made before the foliage hardens. Other factors that influence the effectiveness of treatment are vigor, and thoroughness of application. Although schradan exerts a suppressing action on other sucking pests of walnut, there are some cases, such as the calico scale ( Lecanium cerasorum (Ckll.)) and the European red mite ( Panonychus ulmi (Koch)), where it tends to induce an increase in the population. The vigor of the trees and the exceptional fine quality of foliage on trees treated with schradan may in part result from a stimulus beyond that obtained through the control of the walnut aphid and other pests.