Abstract:  【Aim】 The application of natural enemies and biocontrol bacteria are effective means of pest biological control, and the combined application of the two kinds of agents have strong application prospects and potential of synergistic effect on biological control in the production. However, it is essential to evaluate the safety of biocontrol bacteria on natural enemies. This study aims to assess the biocontrol potential of combined release

of Beauveria bassiana and Orius sauteri for biological control of pests. 【Methods】 We measured the emergence rate of O. sauteri adults after the 5th instar nymphs were treated with two different concentrations of B. bassiana spore suspensions (routine concentration: 1×10⁷ conidia/mL; low concentration: 5×10³ conidia/mL) and spore powder (1×10⁷ conidia/individual) and the predation ability of the 5th instar nymphs of O. sauteri

subjected to the above treatments to female adults of Tetranychus urticae under different prey densities. 【Results】 The emergence rate of O. sauteri adults was significantly reduced after the 5th instar nymphs were treated with B. bassiana spore powder (1×10⁷ conidia/individual), and there was no significant difference in adult emergence rate between the other treatments and control. The predation of the 5th instar nymphs of O.

sauteri in the three treatment groups on female adults of T. urticae fitted with Holling Ⅱ functional response models, and the searching efficiency reduced with the increasing of prey densities. Specifically, the 5th instar nymphs of O. sauteri treated with B. bassiana spore suspension at a low concentration (5×10³ conidia/mL) showed the highest daily maximal predation amount (N_a-max) and the shortest handling time (T_h), and the  gression of searching efficiency showed the lowest decreasing trend with the increasing of prey density. However, the 5th instar nymphs of O. sauteri treated with B. bassiana spore powder had the lowest N_a-max and the longest T_h. 【Conclusion】 Low concentration of B. bassiana spore suspension does not affect the predation ability of the 5th instar nymphs of O. sauteri on female adults of T. urticae, but spore powder treatment causes adverse effect to the predation ability of the 5th instar nymphs of this natural enemy on female adults of T. urticae. This study preliminarily demonstrated the feasibility of the combined application of low concentration of B. bassiana spore suspension and O. sauteri on the control of T. urticae.

Key words: Orius sauteri, Tetranychus urticae, Beauveria bassiana, biocontrol bacterium, natural enemy, Holling model, searching efficiency, biological control