Abstract: 【Aim】 Beauveria bassiana is widely used in the biological control of insect pests. This study aims to investigate the effects of spore suspension concentration and host body size on the pathogenicity of B. bassiana against the Japanese pine sawyer, Monochamus alternatus, so as to provide basic information for practical biological control of M. alternatus using fungi B. bassiana. 【Methods】 After the 4th instar larvae of M. alternatus were inoculated with 0.5% Tween-80 (CK) and different concentrations (1×10⁵, 1×10⁶, 1×10⁷, 1×10⁸ and 1×10⁹ conidia/mL) of B. bassiana spore suspension, respectively, the larval mortality and infection rate within 15 d post inoculation were assayed. Furthermore, after M. alternatus larvae in different body size (100-150, 200-220, 300-320, 400-420, 500-520 and 600-650 mg/individual) were inoculated with the optimal concentration (1×10⁹ conidia/mL) of B. bassiana spore suspension, the larval mortality and infection rate within 20 d post inoculation were determined. 【Results】 After inoculation with 1×10⁵-1×10⁹ conidia/mL B. bassiana spore suspension, the 4th instar larvae of M. alternatus moved freely at the beginning, then they showed burned symptom on the head and their body color gradually turned red. Finally, the whole body was covered with hyphae. With the increase of spore concentration of B. bassiana, the corrected mortality and infection rate of the larvae increased significantly. When the 4th instar larvae were exposed to 1×10⁶-1×10⁹ conidia/mL B. bassiana spore suspension for 15 d, their accumulative corrected mortality rates in various treatments reached 100%. After the 4th instar larvae were inoculated with B. bassiana spore suspension at the concentrations of 1×10⁷, 1×10⁸ and 1×10⁹ conidia/mL, the duration before death was the shortest. Similarly, at 15 d post inoculation, the corrected infection rates of the 4th instar larvae exposed to 0 (CK), 1×10⁵, 1×10⁶, 1×10⁷, 1×10⁸ and 1×10⁹ conidia/mL B. bassiana spore suspension were 0, 20.00%, 86.67%, 90.00%, 96.67% and 100.00%, respectively, showing that the higher concentration of B. bassiana spore suspension, the higher infection rate of M. alternatus larvae. Furthermore, the test results of the effects of 1×10⁹ conidia/mL B. bassiana spore suspension on M. alternatus larvae in different body size showed that the larger the larvae, the higher their mortality and infection rate. At 20 d post inoculation with 1×10⁹ conidia/mL B. bassiana spore suspension, the mortality rates of M. alternatus larvae with the body size of 100-150, 200-220, 300-320, 400-420, 500-520 and 600-650 mg/individual were 76.67%, 76.67%, 66.67%, 93.33%, 100.00% and 100.00%, respectively, and the infection rates were 60.00%, 63.33%, 60.00%, 86.67%, 96.67% and 100.00%, respectively. 【Conclusion】 The concentration of B. bassiana spore suspension has significant effects on the mortality and infection rate of M. alternatus larvae, both of which increase with the increasing of spore concentration. Furthermore, the larger the larvae of M. alternatus, the higher their mortality and infection rate by B. bassiana. Our results provide new clues for the management of M. alternatus by using B. bassiana.

Key words: Beauveria bassiana, Monochamus alternatus; spore suspension, body size, pathogenicity, bacterial infection rate, biological control