Caterpillar-induced plant volatiles attract conspecific herbivores and a generalist predator

Plants release volatiles in response to caterpillar feeding that attracts natural enemies of the herbivores, a tritrophic interaction which has been considered to be an indirect plant defence against herbivores. On the other hand, the caterpillar-induced plant volatiles have been reported to either repel or attract conspecific adult herbivores. This work was undertaken to investigate the response of both herbivores and natural enemies to caterpillar-induced plant volatiles in apple orchards. We sampled volatile compounds emitted from uninfested apple trees, and apple trees infested with generalist herbivore the pandemis leafroller moth, Pandemis pyrusana (Lepidoptera, Tortricidae) larvae using headspace collection and analysed by gas chromatography/mass spectrometry. Infested apple trees uniquely release six compounds (benzyl alcohol, phenylacetonitrile, phenylacetaldehyde, 2-phenylethanol, indole and (E)-nerolidol). These compounds were tested on two species of herbivores and one predator in apple orchards. Binary blends of phenylacetonitrile + acetic acid or 2-phenylethanol + acetic acid attracted a large number of conspecific male and female adult herbivores. The response of pandemis leafroller to herbivore-induced plant volatiles (HIPVs) was so pronounced that over one thousand and seven hundred conspecific male and female adult herbivores were caught in traps baited with HIPVs in three-day trapping period. In addition, significantly higher number of male and female obliquebanded leafroller, Choristoneura rosaceana (Lepidoptera, Tortricidae), was caught in traps baited a binary blend of 2-phenylethanol + acetic acid, or a ternary blend contains 2-phenylethanol and phenylacetonitrile + acetic acid. This result challenges the current paradigm hypothesized that HIPVs repel herbivores and question the indirect defensive function proposed for these compounds. On the other hand, a ternary blend of phenylacetonitrile and 2-phenylethanol + acetic acid attracted the largest numbers of the general predator, the common green lacewing, Chrysoperla plorabunda. To our knowledge, this is the first record of the direct attraction of conspecific adult herbivores as well as a predator to the caterpillar-induced plant volatiles in the field.