Larvicidal, acetylcholinesterase inhibitory activities of four essential oils and their constituents against Aedes albopictus , and nanoemulsion preparation

The potential for essential oils (EOs) to act as a natural alternative for controlling mosquito populations has aroused extensive interest in recent years. Four EOs ( Thymus vulgaris (thyme), Foeniculum vulgare (fennel), Citrus medica (lime), and Camellia sinensis (green tea)) were selected in our previous larvicidal screening assays against Aedes albopictus . In this study, the active constituents of each EO were identified. Next, their acetylcholinesterase (AChE) inhibitory activity was evaluated, and the binding mode was studied by molecular docking. To improve EO stability and duration of activity, nanoemulsions were prepared and characterized. Thirteen constituents of the four EOs at the 100 ppm level led to 100% mortality of the larvae, the four most active constituents being α -hexyl cinnamaldehyde (LC 50 = 24.0 ± 0.3 ppm), limonene (LC 50 = 34.1 ± 0.7 ppm), α -terpinene (LC 50 = 50.0 ± 0.5 ppm), and carvacrol (LC 50 = 54.7 ± 0.6 ppm). These compounds, with the exception of α -hexyl cinnamaldehyde, also showed potent AChE inhibitory activity. Nanoemulsions with droplet size of 56.8–224.9 nm were prepared by emulsification at a 1:1 (v/v) ratio of EO and Tween 80. These EO nanoemulsions showed equally potent larvicidal activity as EOs themselves; moreover, they showed better stability and were active for longer time. These results suggest that natural EOs and their constituents have potential as larvicides for mosquito vector control and that nanoemulsification is an effective method for improving their physicochemical properties and efficacy.