Abstract: Silicon (Si) can adversely affect insect herbivores, particularly in plants that evolved the ability to accumulate large quantities of Si. Very rapid herbivore-induced accumulation of Si has recently been demonstrated, but the level of protection against herbivory this affords plants remains unknown. Brachypodium distachyon, a model Si hyperaccumulating grass, was exposed to the chewing herbivore, Helicoverpa armigera, and grown under threeconditions: supplied Si over 34 d (+Si), not supplied Si (-Si), or supplied Si once herbivory began (-Si +Si). We evaluated the effectiveness of each Si treatment at reducing herbivore performance and measured Si-based defenses and phenolics (another form of defense often reduced by Si). Although Si concentrations remained lower, within 72 h of exposure to Si, -Si +Si plants were as resistant to herbivory as +Si plants. Both +Si and -Si ? +Si treatments reduced herbivore damage and growth, and increased mandible wear compared to Si. After 6 h, herbivory increased filled Si cell density in -Si ? +Si plants, and within 24 h, -Si ? +Si plants reached similar filled Si cell densities to +Si plants, although decreased phenolics only occurred in +Si plants. We demonstrate that plants with short-term Si exposure can rapidly accumulate Si-based antiherbivore defenses as effectively as plants with long-term exposure.