Comparative laboratory efficacy of three commercial organophosphate insecticides against fall armyworm (Spodoptera frugiperda) larvae
Fall armyworm, Spodoptera frugiperda, is a maize pest whose management remains dependent on synthetic insecticides. This study compared three organophosphate insecticides against third-instar S. frugiperda larvae. Chlorpyrifos, profenofos, and diazinon were evaluated at 5%, 20%, 50%, 75%, and 95% of the maximum label dose using a factorial completely randomized design with three replications of six larvae. Mortality was recorded for 168 h and corrected using Abbott’s formula based on pooled control mortality. Insecticide formulation significantly affected mortality (F₂,₃₀ = 39.06, P < 0.001), whereas concentration (F₄,₃₀ = 0.57, P = 0.687) and the insecticide × concentration interaction (F₈,₃₀ = 1.62, P = 0.162) were not significant. Across concentrations, profenofos and diazinon caused mean mortalities of 90.4% and 93.2%, respectively, and both were more effective than chlorpyrifos, which caused 32.2% mortality. Exploratory probit analysis estimated LC₅₀ values of 2.01% for profenofos and 0.71% for diazinon, but both were below the lowest concentration and had wide confidence intervals. Chlorpyrifos LC values were not estimable because its concentration–mortality slope was negative and non-significant. At 95% of the label dose, profenofos reached 50% corrected mortality within 24 h and diazinon within 48 h, whereas chlorpyrifos did not reach this threshold within 168 h. Profenofos and diazinon showed comparably high efficacy and outperformed chlorpyrifos.