CHEMICAL CHARACTERIZATION AND THE EFFICACY OF THE ESSENTIAL OILS OF TAGETES MINUTA L. AND CYMBOPOGON CITRATUS STAPF. AGAINST PHLEBOTOMUS DUBOSCQI NEVEU- LEMAIRE

Abstract

Phlebotomine sandflies transmit leishmaniases, a group of diseases which, the World Health Organization (WHO) estimates that over 2.3 million new cases occur each year and that, at least 12 million people are presently infected worldwide. In Kenya, Turkana, Baringo, Kitui, Machakos, Meru, West Pokot and Elgeyo Marakwet Counties are endemic for the disease with serious debilitating effects and which is spreading fast to new areas. The sandfly Phlebotomus duboscqi is a major vector for cutaneous leishmaniasis. The current management strategy for leishmaniasis is mainly chemotherapy of cases and use of insecticides in vector control. However, usage of highly persistent and toxic synthetic insecticides has led to development of resistance in vector populations and environmental pollution. Thus, the harmful side effects of these chemicals on both animals and humans have progressively limited their usage and have led to increased interest in new natural products that are environmentally safe, affordable and effective in leishmaniases control. This study sought to evaluate the chemical composition and the insecticidal activity of the essential oils from Cymbopogon citratus Stapf and Tagetes minuta L. on the eggs, larval and adult stages of Phlebotomus duboscqi. Leaves of C. citratus were collected from the equatorial rainforest in Kakamega while the aerial and foliar parts of T. minuta were collected from the leishmaniasis endemic focus of Marigat, Baringo County. The essential oils from the two botanicals were extracted by hydro-distillation and then analysed by Gas Chromatography–Mass Spectrometry (GC-MS) to identify the constituent compounds. Each essential oil (EO) extract was tested at graded concentrations of 0.125; 0.250; 0.500; 0.750 and 1mg/ml along with a negative control Tween 80 and a positive control of dimethyl-3-methylbenzamide (DEET) for their ovicidal, larvicidal, adulticidal and repellent activity against P. duboscqi. Chemical analysis revealed the presence of a wide range of compounds, including terpenes. The extract from C. citratus at 1mg/ml demonstrated significant inhibitory activity of 100% on the hatching of sandfly eggs while. T. minuta oil achieved 84.44% inhibition at the same concentration. Both the essential oils of T. minuta and C. citratus were highly potent against adult sand flies with C. citratus and T. minuta causing mortality levels of 100.00 and 82.22 % on female sandflies and 100.00 and 88.89 % on male sandflies, respectively at 72 hours post treatment. C. citratus was significantly more potent (P < 0.05) than T. minuta on killing of male and female sandflies. Repellency increased with increasing doses of the essential oils, demonstrated by biting rates which decreased with increasing concentrations of the oils. Further, the oil of C. citratus was more potent than that of T. minuta with regard to protection time and biting deterrence. The effective doses at 50% (ED50) and at 90% (ED90) for the oil of C. citratus were 0.04 and 0.79 mg/ml, respectively while those of T. minuta were 0.10 and 12.58mg/ml. In addition, the percentage repellency of C. citratus and T. minuta against the sandflies was 100% and 88.89%, respectively after 180 minutes exposure. The oils showed concentration dependent larval mortality. However, C. citratus EO achieved higher repellency, ovicidal, larvicidal and adulticidal rates than T. minuta at all tested concentrations. In conclusion, these findings, together with previous studies by other researchers, lend strong credence to the consideration of C. citratus Eos as potentially valuable agents for the control of phlebotomine sandflies at the egg, larval and adult stages. The study further recommends for more investigations to identify the specific active chemical constituents of the extracts of T. minuta and C. citratus that are responsible for the larvicidal, adulticidal, ovicidal and repellent activities as well as their specific mechanisms of action.