Increased Resistance to Organophosphorus Insecticides in the Parthenogenetic Spotted Alfalfa Aphid, Therioaphis maculata, in California1

Abstract

The spotted alfalfa aphid, Therioaphis maculate (Buckton), was discovered in California in 1954, and by 1956 thousands of acres of alfalfa were treated with organophosphorus insecticides. During the fall of 1956, failures to control with insecticides were reported in a number of areas in southern California. Field and laboratory studies showed that the aphid had developed a low degree of resistance to certain organophosphorus insecticides. After 1957, insecticide treatments for aphid control were markedly reduced because of: (1) increased activity by native predators, (2) establishment of aphid parasites from the Old World, (3) use of the selective insecticide, demeton (4) and the planting of resistant alfalfa varieties. However, in the Antelope Valley, California, the nonselective insecticide, parathion, was still widely used and many fields were treated more often than elsewhere in California. In 1960 and 1961, a large number of failures to control with chemicals were reported in the Antelope Valley. In 1961, very poor results were obtained when the following materials and dosages were applied per acre: demeton at 2 ounces, phosphamidon at 4 ounces, parathion at 4 and 6 ounces and Meta-Systox-R®-(O,O-dimethyl S-2-(ethylsulfinyl)ethyl phosphorothioate) at 4 ounces. Poor results were observed also, where Phosdrin® (a mixture of the alpha isomer of 2-carbomethoxy-1-methylvinyl dimethyl phosphate (not less than 60%) and related compounds (not more than 40%)) at 4 ounces, malathion at 12 ounces and parathion at 8 ounces were applied per acre. Laboratory studies in 1961 using parathion as the selective toxic agent. showed that. the spotted alfalfa aphid had developed an approximate 183-fold resistance at the LD-50 and an approximate 183-fold resistance at. the LD-90. This increased resistance from an approximate 4-fold level in 1956 to the high degree in 1961 is somewhat. unusual since in California this insect is apparently totally parthenogenetic and resistance developed by mutation. The most logical solution (or control of the resistant aphid would appear to be greater utilization of the resistant. alfalfa varieties.