CONICET | Buscador de Institutos y Recursos Humanos

Plant volatile organic compounds play a role inselection of host plants by herbivorous insects.Once the insectreaches the plant, contact cues determine host acceptance.Although the willow sawfly Nematus oligospilus(Hymenoptera: Tenthredinidae) can differentiate among willowgenotypes, no knowledge is available on the cues used bythis insect to seek and accept the host plant. In this study, werecorded behavioral orientation in a Y-tube olfactometer ofwillow sawfly females to volatiles of the highly preferredgenotype Salix nigra and the non-preferred genotypeS. viminalis. The volatiles released by undamaged willowsof each genotype were analyzed by coupled gaschromatography-mass spectrometry. Contact cues were evaluatedfirst by oviposition preference bioassays after selectiveleaf wax removal, and then by studying the micromorphologyof abaxial and adaxial leaf surfaces and their chemical composition.Willow sawfly females oriented preferentially toS. nigra volatiles, which contained more than 3 times theamount of volatiles than that collected from S. viminalis.Analysis of volatiles showed significant differences inamounts of (Z) and (E)-β-ocimene, undecane, decanal, andβ-caryophyllene. The adaxial leaf surface of S. nigra was lesspreferred after wax removal, suggesting a role of cuticularwaxes for oviposition acceptance. No differences were foundamong the micromorphology of leaf surfaces between preferredand non-preferred genotypes. The chemical analysisof cuticular waxes showed that the abaxial leaf surface ofS. viminalis, which is completely avoided for oviposition,possessed 97 % of alkanes. The accepted leaf surfacescontained a more diverse wax profile including alcohols,acids, and esters. Thus, non-alkane wax compounds mightbe related to oviposition. In sum, our study suggests that severalcues act in concert to provide oviposition cues for thesawfly N. oligospilus: females are attracted to volatiles froma distance, and once alighting on the plant, they seek specificchemical contact cues in order to lay eggs.