Andean Flora as a Source of New Repellents against Insect Pests: Behavioral, Morphological and Electrophysiological Studies on Sitophilus zeamais (Coleoptera: Curculionidae).

Sitophilus zeamais (Motschulsky) is considered as one of the most destructive foodstuff pests. Due to their efficiency, low toxicity for mammalians and low environmental impact, plant-derived essential oils (EOs) are promising tools for pest control. In particular, the OEs extracted from Lamiaceae are considered among the most bioactive in terms of repellent and/or insecticidal effect. Here, we investigated the repellence of the EOs extracted from two aromatic plant species typical of the flora of the Ecuadorian Andes, Clinopodium tomentosum and C. nubigeum, against adults of S. zeamais. The behavioral assays carried out at concentrations ranging from 0.7 to 23.9 μL L-1 air in a two-way static olfactometer showed a significant repellent effect starting from the concentration of 8.4 mL L-1 air both for the EO of C. nubigenum and C. tomentosum. We also carried out a complete structural analysis of the antenna of S. zeamais using scanning (SEM) and transmission electron microscopy (TEM), in order to characterize the olfactory sensilla equipment. In this species, there is no sexual dimorphism also as regards to the antennal morphology and the sensilla type and distribution. We identified six type of sensilla, among which at least three types (Sensillum Trichoideum 1, Sensillum Trichoideum 2 and Grooved Peg Sensillum) can be considered as olfactory. Electroantennography (EAG) recordings carried out on S. zeamais revealed a positive dose-response to both EOs, without differences between the two sexes.