Volatile organic compounds (VOCs) mediate plant–plant signalling and may contribute to phenotypic differentiation among populations. However, the extent to which VOC-mediated signalling varies among locally adapted populations, and how VOC traits relate to major fitness-related traits, remain poorly understood. We conducted a greenhouse experiment using two genetically and phenologically divergent Iberian populations of Arabidopsis thaliana . Plants were exposed to herbivory by Spodoptera exigua