Acute toxicity and repellent activity of the Origanum scabrum Boiss. & Heldr. (Lamiaceae) essential oil against four mosquito vectors of public health importance and its biosafety on non-target aquatic organisms

The recent outbreaks of dengue, chikungunya, and Zika virus highlighted the pivotal importance of mosquito vector control in tropical and subtropical areas worldwide. However, mosquito control is facing hot challenges, mainly due to the rapid development of pesticide resistance in Culicidae and the limited success of biocontrol programs on Aedes mosquitoes. In this framework, screening botanicals for their mosquitocidal potential may offer effective and eco-friendly tools in the fight against mosquitoes. In the present study, the essential oil (EO) obtained from the medicinal plant Origanum scabrum was analyzed by GC-MS and evaluated for its mosquitocidal and repellent activities towards Anopheles stephensi, Aedes aegypti, Culex quinquefasciatus, and Culex tritaeniorhynchus. GC-MS analysis showed a total of 28 compounds, representing 97.1 % of the EO. The major constituents were carvacrol (48.2 %) and thymol (16.6 %). The EO was toxic effect to the A. stephensi, A. aegypti, C. quinquefasciatus, and C. tritaeniorhynchus larvae, with LC50 of 61.65, 67.13, 72.45, and 78.87 μg/ml, respectively. Complete ovicidal activity was observed at 160, 200, 240, and 280 μg/ml, respectively. Against adult mosquitoes, LD50 were 122.38, 134.39, 144.53, and 158.87 μg/ml, respectively. In repellency assays, the EOs tested at 1.0, 2.5, and 5.0 mg/cm2 concentration of O. scabrum gave 100 % protection from mosquito bites up to 210, 180, 150, and 120 min, respectively. From an eco-toxicological point of view, the EO was tested on three non-target mosquito predators, Gambusia affinis, Diplonychus indicus, and Anisops bouvieri, with LC50 ranging from 4162 to 12,425 μg/ml. Overall, the EO from O. scabrum may be considered as a low-cost and eco-friendly source of phytochemicals to develop novel repellents against Culicidae