Evidence for a defensive ceiling in the effectiveness of plant silicon defences against an insect herbivore

Many plants take up silicon (Si) from the soil and deposit it in their tissues to resist insect herbivory via physical and chemical mechanisms. Si-supplemented wheat, for example, negatively impacts insects such as the cotton bollworm (Helicoverpa armigera). Si-based defences are rapidly deployed, but it is unclear whether levels of Si defence affect herbivore performance linearly (i.e., increasing levels of Si continue to decrease performance), or whether resistance saturates at a certain level of Si accumulation. We examined how a gradient of Si supplementation in wheat affected relative growth rates (RGRs) and mandible wear of H. armigera. Plants were either not provided with Si or supplemented with one of four levels of Si (0.25 mM, 0.5 mM, 2 mM and 4 mM) in hydroponic solutions and fed to second-instar H. armigera larvae. RGR declined most with higher levels of Si compared to the lowest levels of Si supplementation, and there was a significant negative correlation between plant Si and RGR. However, significant reductions in RGR were generally achieved midway through the Si gradient (2 mM). Si supplementation increased plant biomass overall and ameliorated biomass losses due to herbivory. The wearing of insect mandibles was lower on larvae that had fed on silicified plants. Our study also revealed that wheat can take up Si at concentrations beyond its polymerization threshold of 2 mM. We demonstrate that although wheat can acquire Si at relatively high concentrations, accumulation beyond this level does not confer further benefits in terms of herbivore defence.