Michelbacher, A.E., & Hitchcock, S. 1958 Induced increase of soft scales on walnut.. Journal of Economic Entomology 51(4): 427-431.

Notes: One of the most serious problems confronting walnut growers in California is the marked increase in the populatin 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 scales are involved: frosted scale (Lecanium pruinosum Coq.), European fruit lecanium (L. corni Bouché) and the calico scale (L. cerasorum (C'kll.).