The spider mite predator Phytoseiulus persimilis Athias-Henriot and the thrips predator Amblyseius cucumeris (Oudemans) are both commercially mass-reared for use as biological control agents for greenhouse pests. The ability to stockpile these mites in cold storage would greatly facilitate economical mass-production and distribution. In addition, these two species provide an opportunity for comparative studies of cold hardiness because P. persimilis originated in subtropical Mediterranean type climates and is thought to be incapable of entering diapause whereas A. cucumeris is widespread in temperate zones where adult females enter a reproductive diapause for overwintering. Temperature/mortality curves confirmed that both species are freezing intolerant in the traditional sense that supercooling points (SCPs), at which freezing of body fluids occurs, represent absolute lower lethal temperatures. Both species were capable of moderate supercooling, into the range of-20°C to -30°C, and both showed a trend of increasing SCP temperatures during development from egg to adult that suggested an inverse relationship between supercooling capacity and body size within species. The only exception to this trend was adult female A. cucumeris, which may be significant because this is the only life stage that is capable of diapause or survival of temperate winters. On the other hand, diapause induction and low temperature acclimation had little or no effect on supercooling capacity, and survival of nonacclimated mites at subzero temperatures above their SCPs was limited to very short periods of exposure, suggesting that the SCP represents a physical property of the mites rather than an adaptation for survival of exposure to subzero temperatures. Cold-storage survival of both species was optimum at 7.5°C, was greatly enhanced when a source of moisture was provided, and was enhanced even further when food was provided even though the mites were held at temperatures below their theoretical temperature threshold for development. Under these conditions, survival of P. persimilis was 80% after six weeks whereas that of A. cucumeris was only 35% after the same period of time. Furthermore, longevity and fecundity of P. persimilis after eight weeks of cold storage were comparable to mites taken directly from rearing cultures whereas oviposition by A. cucumeris after six weeks of cold storage was low and irregular compared to previously reported values. Temperature data from areas where P. persimilis survives outdoors indicate that this species is capable of surviving prolonged exposure to temperatures below 10°C and occasional exposure to subzero temperatures, and apparently does so without diapausing. On the other hand, A. cucumeris may be capable of surviving prolonged exposures to subzero temperatures, but only as diapausing adult females. The results of the current studies are consistent with these ideas and further suggest that nondiapausing A. cucumeris are less cold hardy than P. persimilis. / Graduate