Two spider mite pests, the Pacific spider mite (Tetranychus pacificus) and the Willamette spider mite (Eotetranychus willamettei) frequently cause serious damage to grapes in California. Although these two spider mites are sympatric in their distribution, they usually occur in different vineyards, resulting in a mosaic pattern of occurrence with some vineyards infested with Pacific spider mites and others with Willamette spider mites. The mosaic pattern of occurrence of the two spider mites in the Central Valley of California suggests that grape leaf microclimate may differ markedly even between neighboring vineyards affecting their suitability for mite population growth. Differences in leaf microclimate between vineyards may also influence the effectiveness of the western predatory mite (Galendromus occidentalis), the most important natural enemy of these spider mites. To better understand the factors influencing the distribution and biological control of Pacific and Willamette spider mites in vineyards we monitored water stress, leaf temperatures and mite densities throughout the season in several vineyards in Lodi and Madera. Our preliminary results suggest that Pacific spider mites commonly inhabit water stressed plants with high leaf temperatures, whereas Willamette spider mites prefer less stressed plants with lower leaf temperatures.