Induced Increase of Soft Scales on Walnut

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Abstract
One of the most serious problems confronting walnut growers in California is the marked increase in the population of soft scales induced by insecticides. The trouble developed because of the interference by certain insecticides, such as DDT, with effective parasitism by hymenopterous parasites. Apparently because of their small size the parasites are adversely affected by relatively small amounts of insecticide. As a result, the problem has developed not only from treatments applied directly to walnut orchards, but is also associated with drift of insecticides, through orchards, when adjacent fields or even distant fields are treated. Three species of soft sealed are involved; frosted scale (Lecanium pruinosum Coq.), European fruit lecanium (L. corni Bouché) and the calico scale (L. cerasorum Ckll.). The first two occur as a complex. There appear to be typical forms and atypical forms that more or less blend into one another. The typical form of the frosted scale was more susceptible to parathion than the typical form of the European fruit lecanium. The scales were most easily controlled during July and August at which time 2 pounds of 25% parathion wettable powder plus three-fourths gallon of light summer oil emulsion in 100 gallons of water per acre applied with an air carrier sprayer was effective against all three species. After leaf drop and until about the first of March all three species were effectively controlled with 5 pounds of 25% parathion wettable powder plus 2 to 4 gallons of oil emulsion per acre, applied with an air carrier sprayer. Control with insecticides was not possible following the second molt which usually occurs during March.

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