Abstract: 【Aim】 To screen Beauveria bassiana strains with high virulence against Holotrichia parallela larvae and to evaluate the simulated field efficacy of the strain with the highest virulence in combination with azadirachtin and phoxim, respectively.【Methods】 The virulence of 10 strains of B. bassiana to the 3rd instar larvae of H. parallela was evaluated based on the corrected mortality and medium lethal time (LT₅₀) value using the insect dipping method. Furthermore, the medium lethal concentration (LC₅₀) values of the screened Bb14 strain against the 1st and 3rd instar larvae of H. parallela were determined. In the pot experiments, the conidial suspension of Bb14 (1×10⁸ conidia/mL) was applied in combination with azadirachtin (3 mg/L) and phoxim (200 mg/L), respectively, and the control efficacy against the 3rd instar larvae of H. parallela on tobacco seedlings was evaluated in greenhouse.【Results】 Among the 10 strains of B. bassiana at the concentration of 1.0×10⁸ conidia/mL, Bb14 had the highest virulence against the 3rd instar larvae of H. parallela, causing the cumulative corrected mortality of 76.67% in 17 d and with the LT₅₀ value of 11.14 d. The LC₅₀ values of Bb14 infecting the 1st and 3rd instar larvae of H. parallela were 0.32×10⁷ and 0.68×10⁷ conidia/mL, respectively. In the pot experiments, the control efficacy of Bb14, azadirachtin and phoxim against the 3rd instar larvae of H. parallela was 42.86%, 64.29% and 82.14%, respectively, at 25 d after application. At 25 d after applying Bb14 in combination with azadirachtin and phoxim, respectively, the control efficacy against the 3rd instar larvae of H. parallela reached 78.57% and 92.86%, respectively. 【Conclusion】 Among the tested 10 strains of B. bassiana, Bb14 has the highest virulence to H. parallela larvae. The application of Bb14 in combination with azadirachtin and phoxim leads to improved control efficacy against H. parallela larvae.

Key words: Holotrichia parallela, Beauveria bassiana, virulence test, field efficacy evaluation, biological control