The existence of strains of mites resistant to parathion has been well established (Garman, 1950; Lienk, Dean, & Chapman, 1952; Newcomer & Dean, 1952; Smith & Fulton, 1951). Resistant strains of the European red mite, Metatetranychus ulmi (Koch), first occurred in orchards of the Pacific northwest in 1950 (Newcomer, 1951; O'Neill & Hantsbarger, 1951), approximately three years after parathion was first used as an acaricide. Two other species of orchard mites, the Pacific mite, Tetranychus pacificus McG., and T. mcdanieli McG., were subsequently reported to have developed parathion-resistant strains in the same area (Newcomer & Dean, 1953). Though parathion is lethal to most predacious mites and insects, Huffaker and Kennett (1953) found a difference in tolerance between species of Typhlodromus in the field and in the laboratory: T. reticulatus Oudms. was very susceptible to parathion whereas T. occidentalis Nesbitt was not appreciably affected by it.