Abstract:The prey preference of polyphagous predators to different preys and the interference of the prey density greatly affect their control efficiency. Prey preference of the predator Orius sauteri on the western flower thrips, Frankliniella occidentalis (Pergande) and the two-spotted spider mite, Tetranychus urticae Koch, and effects of density of one prey on the predation on the other prey, were studied in the laboratory maintained at (26±1)℃ and 70% RH. Both 5th instar nymphs and adults of O. sauteri prefer to feed on nymphs than on adults of F. occidentalis, whereas prefer to feed on adults than on nymphs or larvae of T. urticae. The second instar larvae of thrips was the most favorite for O. sauteri among all supplied stages of thrips and spider mites. At a fixed density of spider mites (60/19.63cm2), increasing thrips density from 10 to 60 significantly decreased the predation of spider mites by O. sauteri; however, at a same fixed thrips density, increasing spider mite density didn′t decrease the predation on thrips. These results strongly indicated that F. occidentalis was a more suitable prey than T. urticae for O. sauteri. The western flower thrips, which is a newly invasive species in China, was discussed as to its ecological roles in the intrusive surroundings in context of prey preference of predators.