High nitrogen in maize enriches gut microbiota conferring insecticide tolerance in lepidopteran pest Spodoptera litura

Abuse of chemical fertilizers and insecticides has created many environmental and human health hazards. We hypothesized that high nitrogen (N) in crops changes insect gut microbiota leading to enhanced insecticide tolerance. We investigated the effect of high N in maize on gut microbiota and insecticide tolerance of the polyphagous pest Spodoptera litura. Bioassays showed that high N applied in both maize plants and artificial diets significantly enhanced larval growth but reduced larval sensitivity to the insecticide methomyl. High N promoted the gut bacterial abundance in the genus Enterococcus. Inoculation with two strains (E. mundtii and E. casseliflavus) isolated from the larval guts increased larval tolerance to methomyl. Incorporation of antibiotics in a high-N diet increased the larval sensitivity to methomyl. These findings suggest that excessive application of N fertilizer to crops can increase insecticide tolerance of insect pests via changing gut microbiota, leading to increased use of insecticides worldwide.