Contrasting impacts of herbivore induction and elevated atmospheric CO₂ on silicon defences and consequences for subsequent herbivores

Plants are often sequentially attacked by multiple herbivores; feeding by one herbivore can alter host plant quality that affects the performance of subsequent herbivores. Previous studies suggest that silicon (Si) is a highly inducible defence in grasses (Poaceae) following herbivory, so could mediate such temporally separated insect herbivore interactions. Elevated atmospheric CO₂ concentrations (eCO₂), in contrast, often reduce Si accumulation, which potentially weakens this interaction. We examined the effects of prior feeding by Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on the performance of crickets, Acheta domesticus (L.) (Orthoptera: Gryllidae), subsequently feeding on the same plant. We used Brachypodium distachyon (L.) P. Beauv. grass supplemented or non-supplemented with Si and grown in the glasshouse maintained at ambient (aCO₂) or eCO₂ concentrations. As hypothesized, prior feeding by caterpillars induced Si accumulation. Despite eCO₂ reducing Si accumulation, initial herbivore induction of Si negated the effects of eCO₂. Both, previous caterpillar herbivory and Si supplementation reduced cricket performance. Si induction by successive herbivory was additive. Plant biomass was similar in plants attacked by caterpillars alone or both herbivores, suggesting that initial Si induction by caterpillars deterred feeding by crickets. Our results demonstrate that Si induction by one herbivore negatively impacts successive herbivores, as has been demonstrated for secondary metabolite-mediated interactions. Uniquely, however, Si induction is an irreversible defence and potentially a stronger, or perhaps longer-lasting mediator of such herbivore interactions in some plant taxa.