BIOACTIVITY OF CRUDE ESSENTIAL OILS AND BLENDS OF Artemisia afra, Ocimum kilimandscharicum AND Tagetes minuta AGAINST Anopheles gambiae S.S.

Abstract

Malaria causes heavy morbidity and high mortality in most parts of the world. Synthetic insecticides mostly employed to control the mosquito vector have detrimental effects to the environment, necessitating the search for ecofriendly alternatives. This study determined the larvicidal activity of crude essential oils (EOs) and blends of Artemisia afra, Ocimum kilimandscharicum, and Tagetes minuta against Anopheles gambiae s.s. using treatment solutions whose concentrations ranged from 25 to 200 parts per million (ppm). Chemical analysis of the crude EOs was done using gas chromatography- mass spectrometry (GC-MS) to determine their chemical composition. Synthetic blends of their major constituents were evaluated for their larvicidal, ovicidal, pupicidal, repellent and oviposition deterrent activities against An. gambiae s.s. in selective bioassays. Data obtained were subjected to probit analysis using STATSTICA 6.0 and means were compared using one-way analysis of variance (ANOVA). Among the crude EOs, O. kilimandscharicum was the most efficacious with LC50 and LC90 of 48.50 and 91.40 ppm after 24 hours, respectively. The crude EO of T. minuta had the least efficacy with an LC50 of 58.80 ppm. The larvicidal activity of selected blends of the plant extracts against An. gambiae s.s. was significantly lower (P <0.001) than that of crude EOs with LC50 and LC90 values of blend OK-4 (derived from O. kilimandscharicum) being 59.50 and 93.80 ppm, after 24 hours under laboratory conditions. The LC50 (40 ppm) and LC90 (84.9 ppm) values obtained when larvicidal activity of the blends were tested under simulated field conditions (SFCs) were comparatively lower than those under laboratory conditions. Among the juvenile stages, ova were found to be the most susceptible while pupae were the most tolerant to the treatments. All the blends negatively influenced oviposition site selection by gravid female mosquitoes, and effectively repelled blood starved female mosquitoes, with blend OK-4 being the most effective. From this study, it is evident that the three plants are potential sources of insecticidal compounds that can be used to control the malaria vector. Formulation and commercialization of products from these plants should, therefore, be explored.