Arbuscular mycorrhizal (AM) fungi are ubiquitous symbionts of most terrestrial plants that can augment plant defences against insect herbivores. A clearer understanding of the mechanisms underpinning community-specific effects of AM fungi on plant resistance to herbivores is needed. Here, we report how plant (Triticum aestivum) phenolic-based resistance to an insect herbivore is differentially affected by inoculation with different AM fungal communities. Plants inoculated with four AM fungal species or with a field-sourced AM fungal community had significantly greater foliar phenolics than plants inoculated with a single AM fungal species (Rhizophagus irregularis) or with no AM fungi. Correspondingly, herbivore performance (relative growth rate) was lowest when feeding on those plants with greater phenolic concentrations. Furthermore, there was a negative correlation between foliar phenolics and herbivore growth. We propose that AM fungal community assembly can drive insect herbivore performance by affecting phenolic-based resistance mechanisms.