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Little information is available on the interaction between herbivorous insects and plants storing terpenoids. In this work we describe the response of the essential oil plant Mentha aquatica to the specialist herbivore Chrysolina herbacea. Feeding from C. herbacea induced a significant increase of jasmonic acid (JA) in both local and systemic M. aquatica leaves, whereas the content of the JA precursor, cis-(+)-12-oxophytodienoic acid (OPDA), was increased in local leaves and decreased in systemic leaves. The JA conjugate, (3R,7S)-jasmonoyl-l-isoleucine (JA-Ile), was slightly increased in herbivore wounded (HW) local leaves, whereas its content significantly increased in systemic leaves. Herbivory by C. herbacea did not increase the content of salicylic acid (SA); however, SA showed a two-fold increase in HW systemic leaves. Our results indicate that also in plants producing direct defences, such as the essential oil plant M. aquatica, JA, and SA signalling is triggered by herbivory just like in plants that respond with indirect defence. [ABSTRACT FROM AUTHOR]