Mosquito Repellency, Physiochemical Properties and Chemical Composition of Volatiles of Ocimum tenuiflorum Growing in Different Agro-Ecological Zones in Kenya

Abstract

Concerns over insecticide resistance and environmental impact necessitate effective alternatives to synthetic repellents like DEET. Ocimum tenuiflorum (Holy Basil), traditionally used in Kenya by communities including the Kamba, Kikuyu, and Maasai for repelling insects, presents a promising candidate. However, its efficacy, chemical composition, and yield may vary with growing conditions. This study therefore, assessed how different Kenyan agro-ecological zones (Kajiado, Kiambu, and Kitui Counties) affect the yield, chemical composition, and repellent efficacy of Ocimum tenuiflorum essential oils against Anopheles gambiae. Plants were harvested from these zones, their essential oils extracted via hydro-distillation, analyzed using GC-MS, and tested for repellency via the WHO arm-in-cage bioassay. Essential oil yield varied significantly (F (2,12) =567.3, p<0.001), with Kiambu yielding the highest (0.40% ± 0.0079), followed by Kajiado (0.30% ± 0.0079) and Kitui (0.26% ± 0.0035). Physicochemical properties were consistent across zones. GC-MS revealed linalyl anthranilate and terpinen-4-ol as major constituents, with preliminary trends showing quantitative differences. Repellent efficacy was concentration-dependent. At 10⁻¹ g/ml, all EOs showed >95% PE, with Kitui essential oils achieving 100% protection. At lower concentrations, Kajiado and Kitui essential oils were significantly more potent (p<0.0001). MCPT for all essential oils (2.25–2.50 hours) was significantly shorter than DEET 20% (4.00 hours, p>0.05), but no significant difference in MCPT was found among the essential oils themselves (p>0.05). A combination of essential oils did not enhance efficacy or duration. Soil properties, climate, and topography significantly influence the yield and repellent potency of Ocimum tenuiflorum essential oils, with Kitui and Kajiado essential oils demonstrating superior efficacy at lower concentrations despite lower yields. These findings support the potential of regionally sourced Ocimum tenuiflorum as a natural short-term repellent and highlight the need for formulation strategies to extend protection time.