Red leaf margins indicate increased polygodial content and function as visual signals to reduce herbivory in Pseudowintera colorata

Summary• Red‐pigmented leaf margins are common, but their functional significance is unknown. We hypothesized that red leaf margins reduce leaf herbivory by signalling to herbivorous insects the presence of increased chemical defences. • Leaves were collected from a natural population of Pseudowintera colorata . Margin size, herbivory damage, anthocyanin content and concentrations of polygodial, a sesquiterpene dialdehyde with antifeedant properties, were quantified. Feeding trials involving larvae of Ctenopseustis obliquana , a generalist herbivore, were conducted on red‐ and green‐margined P. colorata leaves in darkness, or under white, green or red light. • Leaves with wider red margins contained higher concentrations of polygodial and anthocyanins, and incurred less natural herbivory. In trials under white light, C. obliquana consumed disproportionately more green‐ than red‐margined leaf laminae. Larvae exhibited no feeding preference when light was manipulated such that leaf colour discrimination was impaired. • Red leaf margins provide a reliable and effective visual signal of chemical defence in P. colorata . Ctenopseustis obliquana larvae perceive and respond to the colour of the leaf margins, rather than to olfactory signals. Our study provides direct experimental evidence for aposematic coloration in red leaves.