The western predatory mite (WPM - Galendromus occidentalis) (Acari: Phytoseiidae) is a dominant predator of Pacific spider mite (PAC - Tetranychus pacificus) (Acari: Tetranychidae) in California vineyards. We evaluated the effects of six pesticides that are not used against spider mites on biological control of PAC using grape plants in microcosms with a two-way design: presence and absence of WPM crossed with presence and absence of pesticides. The predator alone significantly reduced PAC populations in all six experiments, whereas the effects of pesticides varied. The insecticide imidacloprid did not affect PAC population growth, but drove WPM close to extinction, such that PAC densities were 70% lower in microcosms treated with WPM alone than with WPM and imidacloprid. In contrast, the fungicide wetable sulfur significantly decreased PAC populations by 50%, and the lack of a significant interaction showed that sulfur acted additively with WPM, such that PAC densities were 40% less in the WPM and sulfur than in the WPM alone treatment. The insecticide buprofezin had no effect on PAC, and although we detected no interaction between buprofezin and WPM, it reduced WPM populations by 70%, indicating its potential for long term disruptive effects. Finally, the fungicides trifloxystrobin and tebuconazole and the insecticide methoxyfenozide had no influence on PAC population growth or WPM effectiveness. Our study demonstrates that simultaneous consideration of pesticide effects on spider mites and their natural enemies is essential for a full assessment of the demographic consequences of pesticide selectivity on pest and natural enemy populations.