Carvacrol acts on the larval midgut of Spodoptera frugiperda by destroying the tissue structure and altering the bacterial community

The fall armyworm, Spodoptera frugiperda, has become a global invasive pest in recent years, threatening agricultural production. Carvacrol (CAR) is a multifunctional plant-derived compound and exhibited significant bioactivities against many pests. Our previous study found that CAR inhibited growth and development in S. frugiperda. However, the effects of CAR on the midgut of this pest remain unknown. In this study, the actions of CAR on the larval midgut of S. frugiperda were investigated. The results found that 1.0 and 2.0 g/kg CAR treatments destroyed the structure of the larval midgut based on hematoxylin-eosin staining and transmission electron microscope observation. In addition, 2.0 g/kg CAR exposure significantly altered the larval midgut bacterial community composition and structure. The results of the linear discriminant analysis effect size (LEfSe) analysis suggest that six bacterial genera contributed to the different structures of the larval bacterial community. Furthermore, PICRUSt2 analysis found that 16 bacterial function categories were altered by CAR treatment, of which 8 were significantly increased. Our results provided new insight into the toxicological mechanisms of CAR against S. frugiperda larvae and laid the foundation for the field application of S. frugiperda control.