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

SIMPLE SUMMARY: The widespread cultivation of genetically engineered crops producing not only toxic proteins from the bacterium Bacillus 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 of S. frugiperda neonates revealed that all interactions of the three toxins, except for 1-day pre-exposure to a sublethal dose (LC(5)) of flavone followed by 6-day simultaneous exposure to flavone LC(5) + emamectin benzoate LC(50), are synergistic or additive. The results suggest that the combined use of the three toxins is basically a great strategy to manage S. frugiperda. ABSTRACT: 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 LC(25) + Vip3A LC(25), emamectin benzoate LC(25) + Vip3A LC(25), and flavone LC(15) + emamectin benzoate LC(15) + Vip3A LC(15) but not flavone LC(25) + emamectin LC(25) yielded a mortality significantly higher than their expected additive mortality (EAM). One-day pre-exposure to one toxin at LC(5) followed by six-day exposure to the same toxin at LC(5) plus another toxin at LC(50) showed that the mortality of flavone LC(5) + Vip3A LC(50), emamectin benzoate LC(5) + Vip3A LC(50), and Vip3A LC(5) + emamectin benzoate LC(50) were significantly higher than their EAM, while that of flavone LC(5) + emamectin benzoate LC(50) was significantly lower than their EAM. No significant difference existed among the mortalities of Vip3A LC(5) + flavone LC(50), emamectin benzoate LC(5) + flavone LC(50), 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.