Short-term peripheral sensitization by brief exposure to pheromone components in Spodoptera littoralis

In insects, the olfactory system displays a high degree of plasticity. In Spodoptera littoralis, pre-exposure of males to the sex pheromone has been shown to increase the sensitivity of the olfactory sensory neurons at peripheral level. In this study, we have investigated this sensitization effect by recording the electroantennographic responses of male antennae to the major sex pheromone component (Z,E)-9,11-tetradecadienyl acetate and to the minor components (Z,E)-9,12-tetradecadienyl acetate and (Z)-9-tetradecenyl acetate. Responses to the conjugated diene acetate at 1 and 10 µg and to the unconjugated ester at 10 µg at three different times (11, 22 and 33 min) after pre-exposure (T = 0 min) were significantly higher than those at T = 0, whereas no increase of sensitivity to the pheromone was elicited by any dose of the minor monoene acetate. In addition, pre-exposed antennae to sub-threshold amounts (0.1, 1 and 10 ng) of the major pheromone component also induced an increased response to the chemical at different times (5 and 15 min) after exposure. Our results revealed that pre-exposed isolated antennae display a short-term higher sensitivity at the peripheral level when compared to naive antennae. In addition, we provide evidence of a peripheral sensitization mediated not only by the major pheromone component, but also by the minor unconjugated diene acetate, and the induction of this sensitivity appears to be dependent on the pre-exposure dose and the time span between pre-exposure and subsequent recordings. Possible implications of the sensitization effect displayed by the minor component for a more effective discrimination of the pheromone bouquets of other closely related species are highlighted.