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Genetically engineered crops simultaneously produce defensive allelochemi-cals and Bacillus thuringiensis (Bt) toxin proteins to kill some of the world\'s most devas-tating insect pests.How the two types of toxins,when ingested sequentially or simul-taneously,interact at both lethal and sublethal doses in these pests remains underex-plored.Here,we examined the toxicological interactions between the Bt toxin Cry1Ac and the flavonoid allelochemical flavone in Helicoverpa armigera.Simultaneous exposure of H.armigera neonates to lethal doses (LC25) of Cry1Ac and flavone caused a mortality significantly higher than that of either toxin alone and their expected additive mortality.Preexposure for 24 h to a sublethal dose (LC10) of Cry1Ac followed by 6-d simultaneous exposure to the same dose of Cry1Ac plus a lethal dose (1.6 mg/g diets,LC50) of flavone resulted in a mortality significantly higher than that of the LC50 dose of flavone alone and the expected additive mortality of the LC50 dose of flavone plus the LC10 dose of Cry1Ac.One-day preexposure to the sublethal dose (LC10) of flavone followed by 6-d simultaneous exposure to the LC50 dose (6 ng/cm2) of Cry1Ac plus the LC10 dose of flavone yielded a mortality significantly higher than that of the LC50 dose of Cry1Ac but similar to the expected additive mortality of the LC50 dose of Cry1Ac plus the LC10 dose of flavone.The results suggest that Cry1Ac induces and synergizes the toxicity of flavone against H.armigera larvae.