CONICET | Buscador de Institutos y Recursos Humanos

Nematus oligospilus is a highly specialized sawfly feeding on selected willows (Salix spp). In the southern hemisphere, females are parthenogenetic. Adults do not seek for food, conspecifics or mating partners. As soon as they hatch, females look for oviposition sites. This specialization for egg laying suggests a sensory system highly tuned for recognition of suitable oviposition sites. Here we studied the oviposition behavior of N. oligospilus on two willow genotypes, a preferred (S. nigra) and a non-preferred one (S viminalis). First, we performed an ethogram during the recognition of the leaf surface before oviposition on both S nigra and S viminalis genotypes. Second, in order to estimate the importance of leaf cuticular waxes for species recognition, we offered willow leaves as oviposition sites with their cuticular waxes intact or removed by means of gum Arabic treatment. In addition, we analyzed the polar and non-polar components of the cuticular waxes by chromatography following dichloromethane and methanol extractions of the leaves. Finally, we performed a morphological study by means of SEM of the antenna and ovipositor of sawflies, in order to look for chemosensory structures. Sawflies spent ca. 40 % of their time on the leaf surface of the preferred genotype while antennating, fanning, tapping with the abdomen end or ovipositing. Conversely, sawflies spent only 20% of their time on the less preferred S. viminalis leaves, where they remained mainly quiet or antennating. According to chemical cues, both polar and non- polar extracts showed qualitative differences among willow genotypes, suggesting that oviposition cues can be polar, non polar or either kind. Putative chemoreceptor sensilla were found in both, the cerci of the ovipositor and the antenna. Our results suggest the presence of contact cues on the leaf surface of preferred genotypes, which seem to increase locomotor activity and trigger egg laying in sawflies.