<p>Silicon (Si) has important role in mitigating diverse biotic and abiotic stresses, mainly via silicification of plant tissues. However, environmental changes such as reduced atmospheric CO<sub>2</sub> concentrations may affect grass Si concentration which, in turn, can alter herbivore performance. Recently, we demonstrated that pre-industrial atmospheric CO<sub>2</sub> increased Si accumulation in a grass, however, how Si is deposited and whether this affects insect herbivores performance is unknown. We, therefore, investigated how pre-industrial (reduced) (rCO<sub>2</sub>, 200 ppm), ambient (aCO<sub>2</sub>, 410 ppm) and elevated (eCO<sub>2</sub>, 640 ppm) CO<sub>2</sub> concentrations and Si-treatments (Si+ or Si-) affect Si accumulation in the model grass, <em>Brachypodium distachyon</em> and its subsequent effects on the performance of the global insect, <em>Helicoverpa armigera</em>. rCO<sub>2</sub> caused Si concentrations to increase by 29% and 36% compared to aCO<sub>2</sub> and eCO<sub>2</sub>, respectively. Furthermore, increased Si accumulation under rCO<sub>2</sub> decreased herbivore relative growth rate (RGR) by 120% relative to eCO<sub>2, </sub>whereas<sub></sub>&#160;rCO<sub>2</sub> caused herbivore RGR to decrease by 26% compared to eCO<sub>2</sub>. Moreover, Si supplementation increased the density of trichomes, silica and prickle cells, and these changes in leaf surface morphology reduced larval feeding performance. The observed negative correlation between macrohair density, silica cell density, prickle cell density and herbivore RGR supports this. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO<sub>2</sub> environment reduced the performance of this generalist insect herbivore performance.<strong> </strong>Contrastingly, we found &#160;reduced Si accumulation under higher CO<sub>2</sub>, which suggests &#160;that some grasses might become more susceptible to insect herbivore under the projected climate change scenarios.</p>
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