Interference with the Biological Control of Cottony-Cushion Scale by Insecticides and Attempts to Re-establish a Favorable Natural Balance1

Biological control of the cottony-cushion scale, Icerya purchasi Maks, by the vedalia beetle, Rodolia cardinalis (Muls.), in the desert citrus areas of California was upset so severely by insecticidal drift from surrounding treated areas that chemical control became necessary. Although, in the desert, vedalia activity is restricted from November to March by cold temperatures, Icerya is normally controlled biologically before June unless insecticidal drift interferes with vedalia destruction is most often caused by drift of malathion, parathion, or DDT applied by airplane during the early spring months. Malathion drift is worse than parathion because it produces no balancing destruction of the host. Drift residues from multiple treatments are toxic for long periods. The problem of Icerya on desert citrus was largely eliminated by restricting drift. When vedalia could be obtained, its colonization controlled Icerya outbreaks. Vedalia propagation was found to be impractical, but field-collected pupae or adults may be stored for long periods. Periodic colonization of Cryptochaetum iceryae (Will.) was tested, but considered impractical. Other natural enemies introduced against Icerya were not successfully established.