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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 (LC50 = 24.0 ± 0.3 ppm), limonene (LC50 = 34.1 ± 0.7 ppm), α-terpinene (LC50 = 50.0 ± 0.5 ppm), and carvacrol (LC50 = 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. [ABSTRACT FROM AUTHOR]

Simple Summary: Due to the environment and human health concerns of synthetic repellents, essential oils (EOs) as natural alternatives have received increased attention. In this study, the repellent activity of 60 commercial EOs against Aedes albopictus was evaluated. In the initial screening, 8 active EOs including cinnamon, marjoram, lemongrass, bay, chamomile, jasmine, peppermint2, and thyme were selected. Twenty-one major constituents (>5% relative area) in the 8 active EOs were identified via gas chromatography-mass spectrometry (GC-MS) analysis. Cinnamaldehyde, citral and terpinene-4-ol displayed the highest repellent activity with more than 60% RR, which were more active than N,N-diethyl-3-methyl benzoyl amide (DEET). Next, their nanoemulsions were prepared and characterized. In the arm-in-cage assay, cinnamaldehyde- and citral-based nanoemulsions have prolonged mosquito protection time compared with their normal solutions. Mosquitoes are one of the most important disease vectors from a medical viewpoint in that they transmit several diseases such as malaria, filariasis, yellow and Dengue fever. Mosquito vector control and personal protection from mosquito bites are currently the most efficient ways to prevent these diseases. Several synthetic repellents such as DEET, ethyl butylacetylaminopropionate (IR3535) and 1-(1-methylpropoxycarbonyl)-2-(2-hydroxyethyl)piperidine) (Picaridin), have been widely used to prevent humans from receiving mosquito bites. However, the use of synthetic repellents has raised several environment and health concerns. Therefore, essential oils (EOs) as natural alternatives receive our attention. In order to discover highly effective mosquito repellents from natural sources, the repellent activity of 60 commercial EOs against Ae. albopictus was screened in this study. Eight EOs including cinnamon, marjoram, lemongrass, bay, chamomile, jasmine, peppermint2, and thyme, showed a suitable repellent rate (>40%) at the tested dose of 10 μg/cm2. Then, their main constituents were analyzed by GC-MS, and the active constituents were identified. The most active compounds including cinnamaldehyde, citral and terpinen-4-ol, exhibited an 82%, 65% and 60% repellent rate, respectively. Moreover, the nanoemulsions of the three active compounds were prepared and characterized. In the arm-in-cage assay, the protection times of the nanoemulsions of cinnamaldehyde and citral were significantly extended compared with their normal solutions. This study provides several lead compounds to develop new mosquito repellents, and it suggests that nanoemulsification is an effective method for improving the duration of the activity of natural repellents. [ABSTRACT FROM AUTHOR]

• 53 essential oils (EOs) as larvicides and synergists with deltamethrin were screened. • Main components of 9 active EOs were analyzed by GC–MS. • F. vulgare EO has the highest larvicidal activity with LC 50 value of 27.5 ppm. • C. sinensis EO shows more synergy with deltamethrin against adults than PBO. • Relationship between chemical components and anti-mosquito activity was discussed. Although the extensive use of synthetic insecticides to control mosquito vectors have gained significant success, meanwhile resulted in various problems, such as increase in insecticide-resistant mosquitoes, environmental pollution, and health threat to human beings. Hence, there is an urgent need for alternative insecticides and effective synergists to combine with existing insecticides, which are safe to public health and environment. In this work, the larvicidal activity of 53 commercial essential oils (EOs) and their synergistic effect with deltamethrin against Aedes albopictus adults were screened, then the main constituents of active EOs were analyzed by GC–MS. The most potent EOs against the larvae are from Foeniculum vulgare (fennel), Thymus vulgaris (thyme), Citrus medica (lime), and Camellia sinensis (green tea) with LC 50 values of 27.5, 31.6, 51.3, 53.5 ppm, respectively. The most efficient EOs enhancing the efficacy of deltamethrin are C. sinensis (green tea), the co-toxic factor is 316.67, which is more than the positive control PBO (co-toxic factor = 283.35). It suggests that there is significant potential of natural EOs in the application of mosquito control as larvicides or synergists for enhancing efficacy of current adulticides. [ABSTRACT FROM AUTHOR]