Variations in chemical composition and mosquito repellent efficacy of essential oil of conyza newii from different regions of Kenya

Abstract

In this research work, the chemical composition of essential oil of Conyza newii (Compositae) from seven, regions within Kenya has been determined. The efficacy of three modes of application of the plant as a mosquito repellent is reported. The regions considered in the study are West Pokot, Kilome, Nyakach, Kericho, Naivasha, Nairobi and Webuye. The methods of application assessed are steam distilled oil, thermally expelled plant material and directly burned plant material. Vapors obtained from the plant by headspace trapping were also analyzed. Steam distillation was carried out using Dean and Stark apparatus while thermal expulsion and direct burning of plant material was undertaken, the vapors cooled and collected for bioassay. Headspace trapping was carried out using Porapak S adsorbent. Repellency bioassays were carried out according to established WHO protocols for field evaluation of insect repellents and pesticides. Identification of major compounds in the essential oil and smoke emanations from thermally expelled and directly burned plant material was carried by GC-MS and co-injection with authentic standards. ANOVA of protective efficacy with respect to geographical location of the plant and method of application of the products was carried out using the Student-Newman's Kuels test on SAS system for windows. Significant variation (p = 0.001) in the protective efficacy of essential oil of C:onyza newii from different geographical locations was established. However, protective efficacy of smoke emanation of thermally expelled and directly burned plant material does not vary significantly with geographical location of the plant (p = 0.001 and 0.001, respectively). NOVA revealed significant variations (p = 0.001) between the three modes of application, with steam-distilled oil showing the highest while direct burning had the lowest repellency, Major compounds identified in the essential oil were limonene, peril alcohol, peril aldehyde, geraniol, 1,8-cineole, tians-fl-ocimene, a-caryophylene oxide, apinene and fenchyl alcohol. Most compounds in the essential oil were same as those identified in smoke emanations but in lower concentrations. Major compounds identified in the vapors from headspace trapping were cis-limonene oxide, trans-limonene oxide, cis-dihyrocarvone, 4-methoxyphenol and 3-methoxy-2-methylphenol. Repellency of individual compounds i:. the oil and their blends confirmed that the repellencv of natural oil depends on the concentration of peril alcohol, geraniol, limonene and peril aldehyde.