Objective The study aims to understand the susceptibility variation of different instar larvae of Spodoptera frugiperda to commonly used insecticides, with a view to providing scientific basis for chemical control of this pest.

MethodS The control efficacy of 10 insecticides to the larvae of the S. frugiperda was determined in the laboratory. The susceptibility of the first, third and fifth instar larvae to four insecticides with the control efficacy of over 85% was compared by the bioassay method.

ResultS The control efficacy of chlorantraniliprole, emamectin benzoate and chlorfenapyr to the third instar larvae of S. frugiperda was 100% seven days after treatment, while that of lufenuron was 91.67%, and that of cyantraniliprole was 88.89%. The remaining five insecticides had a low control efficacy, ranging from 44.44% to 69.44%. Bioassay results showed that the LC₅₀ values of emamectin benzoate to the first, third and fifth instar larvae were 0.019, 0.0234, and 0.3918 mg/L respectively 48 hours after treatment. The LC₅₀ of chlorantraniliprole to the first, third and fifth instar larvae were 0.2167, 0.2763, and 3.3718 mg/L 72 hours respectively after treatment, , while that of chlorfenapyr was 1.1286, 3.0194 and 19.2726 mg/L, and that of lufenuron was 1.2213, 2.1445 and 7.8419 mg/L. It could be seen that the susceptibility of S. frugiperda to insecticides was decreased with the increase of instar. The fifth instar larvae of S. frugiperda had significantly lower susceptibility to the four tested insecticides than the first and third instar larvae.

Conclusion The chlorantraniliprole, emamectin benzoate, chlorfenapyr and lufenuron were recommended for the control of first to third instar larvae of S. frugiperda. The emamectin benzoate and chlorantraniliprole were recommended to control fifth instar larvae.