Bacillus thuringiensis and its pest control potential as endophyte

Background: Bacillus thuringiensis (Bt) is widely employed as bioinsecticide due to its selectivity and efficacy against pest insects. While its entomopathogenic activity has been extensively documented, its ecology and potential role as a plant endophyte have been much less explored. Here we investigate if Bt, isolated from a registered commercial bioinsecticide, can develop as endophyte in tomato plants and the resulting effects on Spodoptera littoralis larvae. Results: Tomato plants inoculated with Bt, obtained from a commercial formulation, were systemically colonized by seedling drenching, with spores ensuring the highest efficiency of colonization. Feeding assays on Bt-colonized plants showed a marked reduction of survival in early instars of S. littoralis larvae, reproducing midgut alterations induced by Bt toxins, such as epithelial disruption, mitochondrial disorganization, and alterations of the peritrophic matrix. Surviving larvae completed the development but displayed reduced immunocompetence, decreased adult longevity, and lower fertility. The reduced immunocompetence increased larval susceptibility to foliar spray applications of Bt-based bioinsecticides, even in later instars poorly sensitive, revealing a synergistic interaction between plant endophytic colonization and spray treatments. Conclusion: This study demonstrates that endophytic colonization of tomato plants by Bt negatively affects S. littoralis fitness, making its larval stages also more sensitive to spray applications of Bt and, likely, to the action of other biocontrol agents. These findings open new perspectives for Bt use in sustainable pest management strategies.