Midgut microbiota and associated metabolites played a synergistic role in the Vip3Aa19 toxicity in <i>Spodoptera frugiperda</i>

Abstract Gut bacteria profoundly influence the physiology and ecology of insects. They may enter the hemocoel after the Bacillus thuringiensis (Bt) toxin impairs the midgut of larvae, potentially leading to sepsis. In our study, we found that antibiotic treatment, which led to a reduction in intestinal bacteria, significantly decreased the susceptibility of fall armyworm (FAW) larvae to the Vip3Aa19 protein. The 16S rRNA sequencing was conducted to analyze the gut microbiota associated with antibiotic treatment and the virulence of Vip3Aa19, and Enterococcus mundtii , Enterococcus faecalis , Enterococcus gallinarum , and Enterococcus casseliflavus were identified. Further studies indicated that E. mundtii and E. gallinarum , particularly the supernatants of these four Enterococcus species, exert a synergistic effect on the insecticidal capacity of Vip3Aa19 proteins. We selected three metabolites from the identified compounds produced by Enterococcus species and investigated their toxicity against FAW larvae. The results revealed that methyl indole‐3‐acetate, 3‐methyloxindole, and 4‐ethyl‐2‐methoxyphenol not only act as gastric poisons but also induce the death of hemocytes, which can accelerate the onset of septicemia. Our findings illuminate the significant role of insect gut microbiota and their metabolites in the context of Bt Vip3Aa19‐killing pressure, providing insights into the importance of septicemia mediated by Bt toxins.