Silicon reduces herbivore performance via different mechanisms, depending on host-plant species

There is mounting evidence silicon (Si) can alter plant nutrient dynamics and is an important functional trait in plant defence and plant-insect ecology. Despite this, there remains a paucity in our understanding of how Si-driven changes in nutritional quality can impact herbivore performance across different plant species. We investigated how Si alters plant nutritional quality and the concomitant effects on the performance of the Australian native generalist herbivore Helicoverpa punctigera feeding on three economically significant plant species of varying Si-uptake ability: Brassica napus (non-Si accumulator), Cucumis sativus (intermediate Si accumulator) and Sorghum bicolor (high Si accumulator). Si supplementation reduced the nutritional quality of B. napus but increased phosphorus concentrations in S. bicolor. Si reduced herbivore performance in all host-plant species, which correlated directly with Si concentrations in Si-accumulating host plants C. sativus and S. bicolor. However, on B. napus, Si affected herbivore performance indirectly by reducing nutritional quality (foliar carbon:nitrogen ratio and phosphorus concentration). This suggests Si availability can affect herbivore performance directly via Si concentration on Si-accumulating hosts, and indirectly via nutritional quality in a non-Si accumulator. The resistance-enhancing effects of Si on multiple species offer opportunity for agriculture to utilise this abundant element in sustainable management practices.