Intercropping and cover cropping differentially alter maize secondary metabolism and affect herbivore resistance

Abstract Diversified agricultural systems such as intercropping and cover cropping provide a wide range of emergent properties that can positively impact agricultural performance metrics. Both cropping systems by leveraging distinct ecological processes such as associational resistance and plant–soil feedbacks can facilitate soil health while increasing pest resistance and crop productivity. However, the extent to which the cropping system in general and the companion plant species in particular induce changes to plant metabolism as a mechanism mediating plant resistance remains understudied and limits a wider application of companion cropping technologies. To close this gap, we use microcosm experiments that simulate intercropping and cover cropping practices by manipulating plant neighbourhood and soil conditioning to investigate how three different companion legume species change plant secondary metabolite profiles and the resulting effects on herbivore resistance of maize plants. By utilizing a conventional maize variety (genetically modified; bt‐transformed), we further test the potential for such ecosystem services to persist under industrialized agricultural systems. The non‐volatile secondary metabolite profiles of maize leaves and roots differed across intercropping and cover cropping systems, while volatile organic compound emissions remained largely unchanged. Defence‐related compounds such as benzoxazinoids were generally upregulated under intercropping, while the companion plant identity further shaped the maize defence profile with bean intercropping specifically upregulating benzoxazinoid compounds. Resistance bioassays with the generalist herbivore Spodoptera frugiperda revealed that intercropping significantly reduces leaf damage relative to cover cropping. Among the intercrops, beans increase larval mortality while alfalfa decreases larval mortality on maize plants. Synthesis and applications . We demonstrate that intercropping and cover cropping can shape the overall secondary metabolite profiles of maize and mediate herbivore resistance. These findings underscore that crop diversification strategies can specifically modulate the expression of chemical defences linked to herbivore resistance. In practical terms, this means that diversified cropping systems can be optimized to enhance constitutive levels of pest resistance, potentially reducing the reliance on insecticides, increasing sustainability and supporting yield. The fact that positive resistance effects persist even when using pest‐resistant GM crops suggests companion cropping practices as a valid tool to increase the sustainable management of insect pests.