Co-toxicity and field control efficacy of thiamethoxam 21% SC combined with 17 insecticides against Frankliniella intonsa on alfalfa

This study aimed to screen highly effective insecticide combinations and ratios for controlling Frankliniella intonsa on alfalfa.

The toxicity of 18 insecticides to field-collected F. intonsa was tested. The combined toxicity of thiamethoxam 21% suspension concentrate (SC) and 17 other insecticides was evaluated using co-toxicity factor and co-toxicity coefficient. Subsequently, combinations exhibiting relatively strong synergistic effects was selected for field experiments.

The results of toxicity measurement indicated that among the 18 pesticides, the lowest LC₅₀ was spinetoram, while the highest was deltamethrin. The respective values for 48-hour LC₅₀ were determined to be 0.03 mg/L and 189.31 mg/L. With the exception of thiamethoxam combined with deltamethrin, all other combinations displayed co-toxic factors exceeding 20. The synergistic effect of thiamethoxam combined with emamectin benzoate or chlorfenapyr was relatively stronger. Specifically, significant synergistic effects were observed in combinations such as thiamethoxam and emamectin benzoate, thiamethoxam and chlorfenapyr at ratios of 1∶4 and 2∶3 respectively, yielding co-toxicity coefficients of 749.6, 467.8, 572.8 and 390.1; along with corresponding LC₅₀ values of approximately 2.78 mg/L, 3.25 mg/L, 0.59 mg/L, and 1.02 mg/L, respectively. Further field efficacy tests revealed that all combinations reached their maximum efficacy at 3 days post-application. Notably, the combination of thiamethoxam 21% SC and emamectin benzoate 5% ME (in a ratio of 1∶4) exhibited the most pronounced control effect with an efficacy rate as high as 92.18%, representing an increase by 6.31%~10.18% (1~7 days after application). Even after 7 days, the control effect remained above 80%.

This compounding scheme has great potential for promotion and application in the control of thrips.