Abstract: M. occidentalis was released repeatedly every year in an area of 0.6 ha during 1983-1987 in several apple orchards of Lanzhou and Yuzhong to control the leafmite, E. pruni.. Studies in these orchards in 1988-1989 confirmed that the predator has established its natural populations in this region, and played an important role in suppressing the leaf mite populations.The predator was found to have dispersed to 1.4 ha by 1988, and to 2.7 ha by 1989. Comparing to the non-released orchards, chemical spray against E. pruni. in the released orchards was reduced by 2-3 times annually. The predator overwintered in ground crevices, under dry leaves and plant debris, and became the population source for the following spring. The nutrient availability pre- and post-overwintering, as well as pesticide applications in growing seasons were found to be critical in influencing the population fluctuations of the predator in the orchards.