Abstract:  【Aim】In order to clarify the control efficacy of biogas slurry on Spodoptera frugiperda, and its mode of action. 【Methods】Under three treatments including original biogas slurry (BGS) based on cow dung (biogas slurry containing ammonium), chemical agent NH₄Cl at the concentration of 708 mg/L and ammonium-removed biogas slurry (BGS-N), the selection behavior, and growth and development of S. frugiperda were determined, and the structure of peritrophic membrane (PM) of larvae was observed under scanning electron microscopy (SEM). 【Results】Selection behavior result indicated that both BGS and NH₄Cl showed extremely significant repellent effect on S. frugiperda larvae, and the selection response rates of S. frugiperda larvae to BGS and NH₄Cl were 17.77% and 22.16%, respectively. However, S. frugiperda larvae showed no significant behavior response to BGS-N, with the selection response rate of 41.09%. BGS significantly decreased the survival rate of S. frugiperda larvae compared with the control treated with clear water, and the larval survival rate was 38.67% on the 10th day after treatment. Moreover, BGS significantly decreased the larval weight of S. frugiperda by 32.84%, and decreased the pupation rate by 35.12%, compared to the control. The insecticidal activity was directly related to ammonium nitrogen (NH₄⁺-N). NH₄Cl treatment also showed inhibitory effects on the growth and development of S. frugiperda. NH₄Cl significantly decreased the larval weight of S. frugiperda by 24.08%, and significantly decreased the pupation rate by 41.53%, compared to the control. However, BGS-N had no significant effect on the growth and development of S. frugiperda, and the increment of larval weight and the pupation rate of S. frugiperda did not differ significantly between BGS-N treatment and the control. Besides, pot assay also verified the insecticidal effect of BGS, and the control efficacy of BGS against the 2nd instar larvae of S. frugiperda was over 60% at 5 d after application. In addition, BGS caused the PM of S. frugiperda larvae to shrink and thicken unevenly, while NH₄Cl also led to the abnormality of PM.【Conclusion】NH₄⁺-N should be the key active ingredient of BGS that shows repellent and insecticidal effects, and the damage of NH₄⁺-N to the physical structure of PM of S. frugiperda larvae is the main physiological factor that promotes the repellent and insecticidal effects. Therefore, NH₄⁺-N-containing biogas slurry can be used for the green control of S. frugiperda.
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Key words: Spodoptera frugiperda, biogas slurry, ammonium nitrogen, repellent and insecticidal effect, peritrophic membrane