LARVICIDAL EFFICACY OF FRACTIONATED ESSENTIAL OILS OF TAGETES MINUTA, OCIMUM KILIMANDSCHARICUM, AND ARTEMISIA AFRA AGAINST ANOPHELES GAMBIAE S.S

The growing concern over environmental ills caused by synthetic insecticides employed to control malaria vectors has renewed interest in the search for ecofriendly alternatives. This study evaluated the larvicidal efficacy of fractions obtained from essential oils (EOs) of Tagetes minuta, Ocimum kilimandscharicum and Artemisia afra against Anopheles gambiae s.s. A total of 23, 14 and 7 fractions were isolated from EOs of T. minuta (coded 1 to 23), O. kilimandscharicum(coded A to N) and A. afra(coded X-1 to X-7), respectively. Preliminary tests were done to eliminate fractions with less potent compounds from which one fraction (2), two fractions (A and F), and two fractions (X-1 and X-2) were selected for subsequent bioassays. Chemical analysis of the five fractions with potent compounds was done using Gas Chromatography-Mass Spectrometry (GC-MS) with the major constituents being β-ocimene (56.29%) in fraction 2, α-cubebene (29.39%) in fraction A, β-caryophyllene (31.48%) in fraction F, and β-thujone (37.41%) in fraction X-1, and ethyl-2-octynoate (40.80%) in fraction X-2, respectively. Data obtained were subjected to probit analysis using STATISCA 6.0 statistical software to model the relationship between the response variables and factors. Fraction F was the most efficacious under laboratory conditions, followed by X-1, A, X-2, and 2, respectively. A similar trend of larval mortalities was observed under simulated field conditions (SFC) with significantly higher (P ≤ 0.05) mortalities recorded under SFC than under laboratory conditions. The outcome in all the tests performed were both dose and time depended. Two fractions, Fraction F- a slightly polar eluate from O. kilimandscharicum, and Fraction X-1- a nonpolar eluate from A. afra, exhibited high potency and should be explored further with the aim of formulating commercially viable larvicides.