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Specifically, leaf damage severity positively affected the abundance of defensive metabolites in monoculture plots, whereas leaves with greater damage severity exhibited higher diversity of defensive metabolites in mixture plots. With increasing leaf damage severity, bacterial diversity and the number of keystone taxa in co-occurrence networks increased, while network complexity decreased. Notably, compared to fungal communities, bacterial diversity, bacterial keystone taxa, and bacterial interactions potentially mediated changes in defensive metabolites.Conclusions Our findings highlight plant diversity-dependent synergies between herbivory pressure and phyllosphere bacterial communities in shaping plant chemical defences, providing insights into plant-microbe-herbivorous insect relationships.