Soil chemistry determines whether defensive plant secondary metabolites promote or suppress herbivore growth

Significance This study demonstrates that the protective effects of multifunctional maize secondary metabolites against a major pest are dependent on soil chemical composition. By functioning as both digestibility reducers and siderophores, benzoxazinoids link soil chemistry to plant–environment interactions. Given that many plant secondary metabolites have multiple functions in roots and leaves, such links are likely widespread and may govern community composition and pest dynamics across different (agro)ecosystems. The presented findings also illustrate the limits and context dependency of using multifunctional plant secondary metabolites to combat major herbivore pests. The latter is particularly important in the context of the threat that the fall armyworm poses for global maize production.