Abstract: 【Aim】 The study aims to evaluate the toxicity and mechanism of action of spinetoram against the fall armyworm, Spodoptera frugiperda. 【Methods】 The LC₅₀ and LC₉₀ values of spinetoram against the 2nd, 3rd and 4th instar larvae of S. frugiperda were determined by diet-overlay bioassays with chlorantraniliprole as the control. The activities of mixed-functional oxidase (MFO), glutathione-S-transferase (GST), carboxylesterase (CarE) and acetyl cholinesterase (AchE) in the 3rd instar larvae of S. frugiperda treated by different concentrations of spinetoram (0.127, 0.183, 0.250, 0.400 and 0.572 mg/L) at 48 h post treatment were determined by enzyme-linked immunosorbent assay (ELISA). 【Results】 Compared with chlorantraniliprole, spinetoram had higher toxicity to S. frugiperda larvae. The LC₅₀ values of spinetoram against the 2nd, 3rd and 4th instar larvae at 48 h post treatment were 0.21, 0.34 and 0.59 mg/L, and the LC₉₀ values were 0.59, 0.75 and 2.01 mg/L, respectively. When the 3rd instar larvae were treated with spinetoram, the MFO and AchE activities increased significantly as the treatment concentration increased, and reached the highest (52.23 and 23.98 U/mg pro, respectively) at the concentration of 0.572 mg/L. When the 3rd instar larvae were exposed to the low concentrations of spinetoram (0.127 and 0.183 mg/L), the CarE activity showed no significant change as compared with the vehicle control (0.1% Tween-80), but significantly increased as the concentration increased to 0.400 and 0.572 mg/L. The GST activity increased significantly as the concentration of spinetoram increased, but showed no significant difference between the treatment concentrations of 0.400 and 0.572 mg/L. 【Conclusion】 Spinetoram is highly effective against S. frugiperda, especially to its 4th instar larvae, as compared with chlorantraniliprole. The activities of CarE, MFO and AchE in S. frugiperda larvae increase after exposure to different concentrations of spinetoram.

Key words: Spodoptera frugiperda, spinetoram, toxicity test, detoxification enzyme, target enzyme