Host plant colonisation by arbuscular mycorrhizal fungi stimulates immune function whereas high root silicon concentrations diminish growth in a soil-dwelling herbivore

•Silicon (Si) application increased plant growth and decreased insect performance.•Arbuscular mycorrhizal (AM) fungi increase root Si which reduced insect growth.•Effects of the AM symbiosis on insect growth was dependent on plant variety.•The AM symbiosis increased insect immunity, independent of plant quality.•A growth-immunity trade off may explain insect responses to AM colonised plants.

Plant nutritional quality is dependent on soil nutrients and co-evolved soil microbial symbionts. Most plants associate with arbuscular mycorrhizal (AM) fungi, which alter their nutritional quality and silicon (Si) uptake from the soil. High Si concentrations reduce plant nutritional quality and can act as an effective defence both aboveground and belowground. The growth and immune function of insect herbivores is dependent on the quality of their host plants, hence the AM symbiosis and Si concentrations can impact insect growth and immunity via changes in host plant quality. The effects of AM fungi or Si on root herbivores are poorly quantified, while impacts on insect immunity are unknown. We investigated the effects of host plant colonisation by AM fungi and high root Si concentrations on plant quality alongside the growth of a root feeding insect and the immune response to entomopathogenic nematode infection.Two sugarcane varieties (Saccharum species hybrids L.) were grown under fully factorial treatment combinations of ± Si and AM/non-AM. Root feeding insects (Dermolepida albohirtum Waterhouse) fed on the plants and their immune function was assessed in a bioassay, while insect growth and root consumption were assessed in a feeding trial. We found high Si concentrations decreased insect growth and root consumption, the latter by 71%. Insect growth was reduced on plants associated with AM fungi, which was dependent on Si treatment and plant variety. Insect immunity increased by 62% on AM colonised plants, which negatively correlated with insect growth. These results demonstrate that the impacts of the AM symbiosis on root feeding insects can depend on Si availability and plant variety. Our study suggests that AM fungi can prime insect immunity, independent of host plant quality or Si concentrations, and the negative effects of AM fungi on soil dwelling insects involves immune function stimulation which, due to a growth-immunity trade-off, results in growth reduction.