Abstract:
Control of disease vectors by use of synthetic chemical pesticides has been associated with a series of problems, including resistance development, environmental pollution, and safety risks for humans and domestic animals. Plant-based products have been used for generations in traditional practices to control different arthropods. These have been used in space or personal protection either to repel the arthropods from distance or to deter them from blood-feeding on contact. In the present study, the repellence of essential oils of three plants (Ocimum gratissimum L., Vitex keniensis Turill and Hyptis suaveolens (L.) Poit used in Siaya County in western Kenya were screened against the malaria vector Anopheles gambiae sensu stricto. The essential oils were extracted by steam distillation using Clevenger apparatus. The efficacy of protection provided by oil from each plant and some blends of electrophysiologically active components was tested using WHO-approved topical application bioassay. All the oils were repellent, with that of O. gratissimum showing the highest repellence (RD50 = 2.77 × 10-5 mg cm-2, 95% CI), followed by V. keniensis (RD50 = 5.68 × 10-5 mg cm-2 , 95% CI), and then H. suaveolens (RD50 = 6.25× 10-5 mg cm-2, 95% CI). The three essential oils were analyzed by gas-chromatography (GC) and gas-chromatography-linked Mass Spectrometry (GC-MS). The constituents of the essential oils were identified by direct comparison of their mass spectra with Wiley NBS and NIST databases. The major constituents of O. gratissimum were (Z)-ocimene (29.73%), eugenol (21.76%), (R)-(+)-germacrene D (9.65%), β-caryophyllene (5.86%), and β-linalool (4.13%). The constituents that dominated H. suaveolens essential oil were (E)-caryophyllene (21.27), γ-elemene (9.75%), (E)-α-bergamotene (5.07%), (Z)-α-cis bisabolene epoxide (4.54%), and spathulenol (4.35%). That of V. keniensis was dominated by α-cadinol (16.1%), δ-cadinene (12.67%), β-cubebene (10.88%), tau-muurolol (9.79%), and patchulane (4.60%). Gas chromatography-linked elecroantennographic analyses (GC-EAG), followed by co-injections with authentic standards on the GC were deployed to identify the constituents in each oil that were perceived by the antennae of An. gambiae. The active components in O. gratissimum essential oils were α-pinene, β-pinene, hexyl acetate, (Z)-ocimene, (Z)-4,8-dimethyl-1,3,7-nonatriene,(E,E)-2, 4-decadienal, eugenol, and (R)-(+)-germacrene D. α-Pinene, p-cymene, (E)-caryophyllene, spathulenol and α-cadinol in V. keniensis essential oil were found to be electrophysiologically active, and in essential oil of H. suaveolens, β-pinene, p-cumenol, (E)-caryophyllene, α-copaene, α-gurjonene, bicyclogermacrene were active. Subtractive bioassays of blends of selected electrophysiologically active components of O. gratissimum essential oil were then carried out to determine their roles in conferring repellence to An. gambiae. Absence of hexyl acetate, α-pinene and (E,E)-decadienal resulted in the drop in repellency activity against the An. gambiae. The repellency activity reported herein explains and verifies their efficacy in ethno-practices. In addition, the improved repellency of selected blend of constituents warrants further investigation so as to formulate a cheap, affordable and environmental friendly repellent against An. gambiae