Resistance of the Spotted Alfalfa Aphid to Certain Oreganophosphorus Insecticides in Southern California1

Abstract

During the late summer of 1956 an increasing number of failures were reported in alfalfa fields where parathion and malathion were used to combat the aphid, Therioaphis maculate in southern California. A survey of the troubled area strongly indicated the presence of a resistant strain of aphids. Field tests showed that the aphid has developed a low degree of resistance to parathion and Trithion in localized areas. In one test, malathion, Phosdrin, and demeton (Systox) applied at 9.1, 0.7, and 0.9 ounces per acre, respectively, gave only mediocre control. These same dosages gave excellent control of susceptible aphid populations. Laboratory studies showed that the aphid had developed approximately 4-fold resistance to parathion residues. If time of exposure to mortality is considered, the degree of resistance is probably even greater. Resistant aphids continued reproduction when exposed to the toxicant, and many of their young survived. Reproduction in nonresistant aphids was markedly reduced, and the young died. The developmental pattern of resistance in parthenogenetic spp. does not progress in the same manner as in multivoltine bisexual spp. Resistance probably arose through mutation after the aphid arrived in North American.