Sequential and Simultaneous Interactions of Plant Allelochemical Flavone, Bt Toxin Vip3A, and Insecticide Emamectin Benzoate in Spodoptera frugiperda

Target pests of genetically engineered crops producing both defensive allelochemicals and Bacillus thuringiensis (Bt) toxins often sequentially or simultaneously uptake allelochemicals, Bt toxins, and/or insecticides. How the three types of toxins interact to kill pests remains underexplored. Here we investigated the interactions of Bt toxin Vip3A, plant allelochemical flavone, and insecticide emamectin benzoate in Spodoptera frugiperda. Simultaneous administration of flavone LC₂₅ + Vip3A LC₂₅, emamectin benzoate LC₂₅ + Vip3A LC₂₅, and flavone LC₁₅ + emamectin benzoate LC₁₅ + Vip3A LC₁₅ but not flavone LC₂₅ + emamectin LC₂₅ yielded a mortality significantly higher than their expected additive mortality (EAM). One-day pre-exposure to one toxin at LC₅ followed by six-day exposure to the same toxin at LC₅ plus another toxin at LC₅₀ showed that the mortality of flavone LC₅ + Vip3A LC₅₀, emamectin benzoate LC₅ + Vip3A LC₅₀, and Vip3A LC₅ + emamectin benzoate LC₅₀ were significantly higher than their EAM, while that of flavone LC₅ + emamectin benzoate LC₅₀ was significantly lower than their EAM. No significant difference existed among the mortalities of Vip3A LC₅ + flavone LC₅₀, emamectin benzoate LC₅ + flavone LC₅₀, and their EAMs. The results suggest that the interactions of the three toxins are largely synergistic (inductive) or additive, depending on their combinations and doses.