A predaceous mite,Typhlodromus occidentalisNesbitt (Acarina: Phytoseiidae), was incapable of maintaining population densities of the twospotted spider mite,Tetranychus urticaeKoch, and McDaniel spider mite,T. mcdanieliMcGregor, beneath levels that produced foliage injury on apples.T. occidentaliswas an effective predator early in the season, but it was unable to maintain itself in sufficient numbers during early summer to prevent strong resurgences of twospotted spider and McDaniel spider mites during –September.By employing selective pesticides in commercial orchards, predatory mites can be maintained and can exert effective late-season control of tetranychid spider mites during August–October. One or more summer applications of selective acaricides which result in a more favorable predator:prey ratio is required to insure effective late-season predation byT. occidentalis.The cyclical nature of the predator:prey relationship may be due to lack of an alternate prey species such as the apple rust mite,Aculus schlechtendali(Nalepa), which is virtually absent in commercial orchards. Attempts to establish the apple rust mite as an alternate prey forT. occidentalisby deleting fungicidal applications for 3 years in 1 orchard were unsuccessful.