Silicon and arbuscular mycorrhizal fungi differentially regulate structural defence and nutritional quality in maize–herbivore interactions

Background and aims. Plant susceptibility to insect herbivores is shaped by interacting soil-based biotic and abiotic factors, particularly arbuscular mycorrhizal (AM) fungi and silicon (Si) availability. AM fungi influence plant nutrition and defence signalling, while Si enhances structural and inducible defences. However, how these factors interact to influence herbivore performance, especially in maize systems, remains poorly resolved. Methods. Maize ( Zea mays ) plants were grown in a factorial glasshouse experiment manipulating Si availability (high and low) and AM fungal inoculation (+/-AMF). Fall armyworm ( Spodoptera frugiperda ) larvae were introduced to assess herbivore performance. Plant biomass, foliar chemistry (Si, C, N, P), total phenolics, and AM colonisation were measured alongside insect relative growth rate (RGR) and mandible wear. Results. Si supplementation increased foliar Si concentrations by approximately ~ 128% and increased herbivore mandible wear but had minimal impacts on RGR. AM fungi increased foliar N concentrations by ~ 31.46% and enhanced AM colonisation under high Si, while having little effect on foliar Si. AM fungi increased herbivore RGR by ~ 73% relative to non-mycorrhizal plants. RGR was positively associated with foliar N and negatively associated with total phenolics, but unrelated to foliar Si. Conclusion. Si and AM fungi exert contrasting but largely independent effects on maize–herbivore interactions. Si strengthened structural defences as evidenced by increased mandible wear but did not reduce RGR. In contrast, AM fungi enhanced herbivore performance, primarily via improved host nutritional quality (foliar N). Overall, mycorrhizal-driven nutritional gains can offset Si-based structural defences, with implications for managing herbivory in agroecosystems.