Gustatory habituation to essential oil induces reduced feeding deterrence and neuronal desensitization in Spodoptera litura

The tobacco cutworm, Spodoptera litura, is one of the most important agricultural insect pests. Plant essential oils can be considered potential candidates for eco-friendly control agents, as they exhibit insecticidal and feeding deterrent activity. The present study investigated the antifeedant and insecticidal activity of 29 essential oils. Their potential for habituation and association with the gustatory sensilla were also examined. In no-choice tests and contact-fumigation bioassays on third instar larvae, clove bud, fennel sweet, and lemongrass oils exhibited notable activities. Still, no direct correlation between insecticidal activity and feeding deterrence was observed. Second instar larvae were pre-exposed to those active oils to test the habituation effect. Larvae reared with lemongrass and clove bud oils showed gustatory habituation, whereas those with fennel sweet oil did not show any desensitization compared to the control. Comparable outcomes were observed in individuals exposed to the main constituents of the three oils. Additionally, the mixture of fennel sweet and clove bud oils showed a synergistic feeding deterrent effect. However, although statistically insignificant, potential habituation for the mixture was observed, and only robust inhibition of habituation was expected at physiologically high concentrations (FDI₉₀ + FDI₉₀). Electrophysiological studies showed that the response of the maxillary palp to citral decreased in the experienced group, while to trans-anethole, it was maintained at levels similar to the naive group. The reduction in feeding deterrence corresponded to the repeated exposure and desensitization of the maxillary palp, varying with the types of essential oils.