Cooperative microbial metabolism enhances tryptophan-mediated insecticide detoxification in the fall armyworm.

The fall armyworm, Spodoptera frugiperda, is a major global agricultural pest, known for its rapid evolution of insecticide resistance. Although host genetic adaptation contributes to this trait, the role of gut symbiont-mediated metabolic pathways in promoting resistance remains poorly understood. Here, we show that besides direct biodegradation, a generalist symbiont Enterococcus casseliflavus EMBL-3 indirectly promotes chlorantraniliprole resistance by compensating for tryptophan deficiency in a maize-based diet. Metabolomics and isotope tracing identify EMBL-3 as the primary producer of tryptophan, which is subsequently converted by co-resident microbes to indoleacetic acid. Indoleacetic acid activates the aryl hydrocarbon receptor, leading to upregulation of UDP-glucuronosyltransferase, a detoxification enzyme essential for chlorantraniliprole resistance, as confirmed by CRISPR/Cas9 knockout. This tripartite EMBL-3-indoleacetic acid-UDP-glucuronosyltransferase axis defines a hierarchical symbiont-host metabolic network driving chlorantraniliprole resistance. Our findings provide a framework and targets for disrupting pest adaptability by targeting critical symbiont metabolic nodes, positioning microbiome-mediated detoxification as a universal vulnerability in resistant pests.