Abstract • Below-ground organisms influence above-ground interactions in both natural and agricultural ecosystems. Among the most important below-ground organisms are mycorrhizal fungi, comprising ubiquitous and ancient plant mutualists that have significant effects on plant growth and fitness mediated by resource exchange with plants. In the present study, we focus on the effects of arbuscular mycorrhizal fungi (AMF) on crop defence against insect pests. • AMF alter the availability of resources used by crop plants to manufacture defences against pests and to compensate for pest damage. However, AMF also provide plants with nutrients that are known to increase insect performance. Through potentially opposing effects on plant nutritional quality and defence, mycorrhizal fungi can positively or negatively affect pest performance. • Additionally, AMF may directly affect gene expression and plant defence signalling pathways involved in the construction and induction of plant defences, and these effects are apparently independent of those caused by nutrient availability. In this way, AMF may still influence plant defences in the fertilized and highly managed systems typical of agribusiness. • Because AMF can affect plant tolerance to pest damage, they may have a significant impact on the shape of damage–yield relationships in crops. Potential mechanisms for this effect are suggested. • We highlight the need for continuing research on the effects of AMF identity and the abundance on crop defences and tolerance to pest attack. Much work is needed on the potential effects of mycorrhizal colonization on plant signalling and the induction of direct and indirect defences that may protect against pest damage.