ARBUSCULAR MYCORRHIZAL FUNGI REDUCE CANEGRUB PERFORMANCE VIA MULTIPLE MECHANISMS INCLUDING INCREASED SILICON CONCENTRATIONS

AS BELOWGROUND HERBIVORES and arbuscular mycorrhizal (AM) fungi share the soil environment, there is potentially strong selection pressure for AM fungi to support plant defences against root herbivores. AM fungi negatively impact root feeding insects, yet the mechanisms remain unknown. Plant silicon (Si) is an effective defence against root feeding insects, and AM fungi have been observed to increase Si in plants. This highlights the potential role of Si within defences against root herbivores mediated by AM fungi. We grew sugarcane (Saccharum spp. hybrids) in high and low Si soils, associated with native AM fungal communities, a commercial AM fungal community or with no AM fungi. Canegrub (Dermolepida albohirtum) performance was measured in a feeding assay. Within low Si soil, both commercial and native AM communities reduced canegrub growth rates by 107 and 81%, respectively, while increasing root Si concentrations by 70% and 41%, respectively. Within high Si soil, AM fungi had no impact on plant Si concentrations or canegrub growth. Our evidence suggests the negative impacts of AM fungi on root herbivores are associated with an increase in plant Si, when soil Si is limited. The results also highlight that AM fungi can impact root herbivores through other mechanisms independent of Si. Therefore, further research is required to better understand this complex interaction.