Do natural or synthetic excito-repellents work better? A study on coastal malaria vector Anopheles epiroticus in Ko...

Malaria remains a significant public health concern in Ko Chang, an island in Trat Province, eastern Thailand. The island has historically experienced low to moderate malaria transmission, particularly in forested and coastal zones. Transmission is driven by local Anopheles vectors such as Anopheles epiroticus Linton & Harbach. In this study, we employed the excito-repellency (ER) assay system to evaluate behavioral responses of laboratory and field strains of An. epiroticus to four synthetic and two natural repellent agents. Mosquito escape responses (% escape) were recorded under contact and non-contact conditions to determine irritant and repellent properties. For the laboratory strain, deltamethrin (42%) and permethrin (35%) at LC₅₀ levels induced the highest escape responses in contact trials. In contrast, alpha-cypermethrin (4-14%) induced limited irritancy and repellency. Citronella oil (>50%) at 5.0% (v/v) elicited the strongest non-contact repellency, outperforming DEET (18-26%) and vetiver oil (26-40%). However, the field strain showed a different pattern: DEET (64%) and deltamethrin (60%) were most effective in contact trials, while citronella and vetiver oils (both ∼14%) had minimal non-contact effects. Our findings indicate differences in excito-repellency between field and laboratory strains of An. epiroticus in Ko Chang. Deltamethrin, permethrin, and DEET were more effective as contact irritants against the field population, whereas citronella oil demonstrated greater efficacy as non-contact repellents against the laboratory strain. These results indicate the need to account for behavioral variation when designing vector control strategies and inform both malaria prevention for travelers and local control programs in coastal Thailand.