Drought events are becoming increasingly frequent and intense, posing major challenges to crop productivity. Beyond direct water stress, drought can indirectly affect plants by enhancing herbivore performance. While arbuscular mycorrhizal fungi (AMF) have been proposed to alleviate drought stress and to enhance plant resistance to herbivory, their role in mediating plant responses to the two combined pressures remains poorly understood. Here, we examined the individual and interactive effects of drought, AMF colonisation, and herbivory by Spodoptera exigua on maize (Zea mays) performance by combining a semi-field experiment with growth chamber assays. Drought reduced maize biomass (by 21.5%) and chlorophyll content (by 8.2%), while AMF improved reproductive traits. In particular, AMF colonisation increased the number of ears (from 1.1 to 1.4) and ear length (from 22.5 to 24.3 cm). Interestingly, drought transiently decreased DIMBOA-Glc levels in maize leaves, an effect that was exacerbated under AMF colonisation. Consistently, drought increased leaf herbivore performance by 32%. However, AMF colonisation mitigated the drought-induced increase in herbivore performance, even though leaf damage levels remained similar, indicating a post-ingestive resistance effect. This study highlights the need to consider multi-stressor interactions to harness AMF benefits in agriculture under increasing drought pressure.
