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Plants release volatiles in response to caterpillar feeding that attract natural enemies of the herbivores, a tri-trophic interaction which has been considered an indirect plant defence against herbivores. The caterpillar-induced plant volatiles have been reported to repel or attract conspecific adult herbivores. To date however, no volatile signals that either repel or attract conspecific adults under field conditions have been chemically identified. Apple seedlings uniquely released seven compounds including acetic acid, acetic anhydride, benzyl alcohol, benzyl nitrile, indole, 2-phenylethanol, and (E)-nerolidol only when infested by larvae of the light brown apple moth, Epiphyas postvittana. In field tests in New Zealand, a blend of two of these, benzyl nitrile and acetic acid, attracted a large number of conspecific male and female adult moths. In North America, male and female adults of the tortricid, oblique-banded leafroller, Choristoneura rosaceana, were most attracted to a blend of 2-phenylethanol and acetic acid. Both sexes of the eye-spotted bud moth, Spilonota ocellana, were highly attracted to a blend of benzyl nitrile and acetic acid. This study provides the first identification of caterpillar-induced plant volatiles that attract conspecific adult herbivores under natural conditions, challenging the expectation of herbivore avoidance of these induced volatiles.

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Summary1. Scientists have been fascinated by carnivorous plants for centuries and they have thoroughly investigated how these plants can benefit from insect capture for example through increased growth, earlier flowering, and increased seed production. How prey is actually lured into the traps, however, is less well understood. Trapping prey may be achieved in a random way, for example by camouflaging the traps (hiding them in the surrounding vegetation), so that prey is trapped by accidental landing on the trap leaves or wind drift, or in the other extreme, trapping may involve mimicry of other attractive resources such as fruits or flowers by using specific visual or olfactory signals to attract a specific prey assemblage. 2. We investigated for the first time volatiles of the trapping leaves of carnivorous plant species by dynamic headspace methods. We present data on the venus flytrap Dionaea muscipula, the sundew Drosera binata, and the North American pitcher plants Sarracenia flava, Sarracenia leucophylla, Sarracenia minor, and Sarracenia purpurea. A large number of compounds and relatively high emission rates were found in three of the North American pitcher plants (S. flava, S. leucophylla, and S. minor) with compounds typically found in flowers or fruits. This suggests together with other features (e.g. colour, nectar production) that these traps are possibly flower or fruit mimics. The leaves of S. purpurea, Dionaea muscipula, and Drosera binata emitted much weaker scents with lower numbers of components, consisting mainly of volatiles typically emitted from green leaves. 3. We discuss whether or not the use of volatiles for attracting prey animals is linked with specific trapping mechanisms and whether carnivorous plants can be grouped into specialized 'olfactory syndromes'.