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

Simple SummaryThe widespread cultivation of genetically engineered crops producing not only toxic proteins from the bacteriumBacillus thringiensis(Bt) but also plant defensive compounds known as allelochemicals, combined with occasional use of insecticides, is the major tactic to manage some economically important pests. Better understanding of the toxicological interactions of the three types of toxins is needed to rationally deploy them to protect crops from pests. The aim of this study is to examine the sequential and simultaneous interactions of the allelochemical flavone, Bt toxin Vip3A, and insecticide emamectin benzoate in the fall armyworm (Spodoptera frugiperda), a worldwide target pest of Bt crops. Bioassays ofS. frugiperdaneonates revealed that all interactions of the three toxins, except for 1-day pre-exposure to a sublethal dose (LC5) of flavone followed by 6-day simultaneous exposure to flavone LC5+ emamectin benzoate LC50, are synergistic or additive. The results suggest that the combined use of the three toxins is basically a great strategy to manageS. frugiperda.AbstractTarget pests of genetically engineered crops producing both defensive allelochemicals andBacillus 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 inSpodoptera frugiperda. Simultaneous administration of flavone LC25+ Vip3A LC25, emamectin benzoate LC25+ Vip3A LC25, and flavone LC15+ emamectin benzoate LC15+ Vip3A LC15but not flavone LC25+ emamectin LC25yielded a mortality significantly higher than their expected additive mortality (EAM). One-day pre-exposure to one toxin at LC5followed by six-day exposure to the same toxin at LC5plus another toxin at LC50showed that the mortality of flavone LC5+ Vip3A LC50, emamectin benzoate LC5+ Vip3A LC50, and Vip3A LC5+ emamectin benzoate LC50were significantly higher than their EAM, while that of flavone LC5+ emamectin benzoate LC50was 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.Keywords:allelochemical;Bt toxin;chemical insecticide;combined toxicity;induced toxicity;synergistic interactions