Abstract
Upon damage by herbivores, plants release herbivory-induced plant volatiles (HIPVs). To find their prey, the pest's natural enemies need to be fine-tuned to the composition of these volatiles. Whereas standard methods can be used in the identification and quantitation of HIPVs, more recently introduced techniques such as PTR-ToF-MS provide temporal patterns of the volatile release and detect additional compounds. In this study, we compared the volatile profile of apple trees infested with two aphid species, the green apple aphid Aphis pomi, and the rosy apple aphid Dysaphis plantaginea, by CLSA-GC-MS complemented by PTR-ToF-MS. Compounds commonly released in conjunction with both species include nonanal, decanal, methyl salicylate, geranyl acetone, (Z)-3-hexenyl acetate, (Z)-3-hexenyl butanoate, (Z)-3-hexenyl 2-methyl-butanoate, (E)-β-caryophyllene, β-bourbonene and (Z)-3-hexenyl benzoate. In addition, benzaldehyde and (E)-β-farnesene were exclusively associated with A. pomi, whereas linalool, (E)-4,8-dimethyl-1,3,7-nonatriene were exclusively associated with D. plantaginea. PTR-ToF-MS additionally detected acetic acid (AA) and 2-phenylethanol (PET) in the blends of both trees attacked by aphid species. In the wind tunnel, the aphid predator, Chrysoperla carnea (Stephens), responded strongly to a blend of AA and PET, much stronger than to AA or PET alone. The addition of common and species-specific HIPVs did not increase the response to the binary blend of AA and PET. In our setup, two host-associated volatiles AA + PET appeared sufficient in the attraction of C. carnea. Our results also show the importance of combining complementary methods to decipher the odor profile associated with plants under pest attack and identify behaviourally active components for predators.