Metaseiulus occidentalis (Nesbitt) was found to be capable of controlling Tetranychus pacificus McGregor populations in vineyards not subject to annual pesticidal treatments. The presence of Eotetranychus willamettei (McGregor) as an alternative prey is believed to be a critical factor in the ability of M. occidentalis to control T. pacificus . The presence of E. willamettei allows early season increase of M. occidentalis populations and season-long control of T. pacificus . Natural control of T. pacificus appears to have a seasonal carryover, since vines on which T. pacificus were controlled one year supported fewer spider mites and displayed improved vigor the following year even in the absence of predators.