Larvicidal activity and phytochemical composition of crude extracts derived from Vernonia lasiopus, Vernonia auriculifera and Vernonia galamensis against the malaria vector Anopheles gambiae
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Malaria is one of the major parasitic diseases in the tropical and subtropical regions of the world. The World Health Organization (WHO) estimates that each year, malaria causes 300 to 500 million infections and 1.5 to 3 million deaths. The disease accounts for 20% of all global childhood deaths. There are major problems of drug resistance, particularly to chloroquine which has been the mainstay of malaria treatment, especially in Africa because of its low cost and minimal side effects. The artemisinin based antimalarials are more effective but relatively expensive and out of reach for most of the rural poor people. Recently cases of resistance to the artemisinin based drugs have been reported in various places. Vector control is therefore one of the primary public health interventions that can reduce malaria transmission. Botanical larvicides have featured prominently as alternatives to synthetic insecticides. These insecticides are promising in that they are effective, environment friendly, easily biodegradable and also inexpensive. Vernonia lasiopus (muhasha in Swahili), Vernonia auriculifera (muchatha in Kikuyu) and Vernonia galamensis belong to the family Asteraceae. Vernonia lasiopus is locally used to treat malaria and a concoction of the root of V. auriculifera is used to treat joint pains while V. galamensis is known for its use for oil seed. No concrete information on effect of Vernonia spp. on mosquitoes is available despite the documented use against malaria in folkore. In this study larvicidal activity of V. lasiopus, V. auriculifera and V. galamensis against the malaria vector Anopheles gambiae was determined. Dried ground leaves and roots of the three plants were sequentially extracted with hexane, chloroform, ethyl acetate, acetone, methanol and water. After rotor evaporation of the solvents, the extracts were subjected to both phytochemical analysis and larvicidal assays against the third instar larvae of A. gambiae. Phytochemical screening revealed presence of steroids, saponins, flavonoids, terpenoids and cardiac glycosides in all extracts. Tannins were present in methanol and water extracts of all the three plants and in acetone extract of V. lasiopus roots, V. auriculifera root and V. galamensis leaf. Phlobatannins were absent in all extracts. Percent mortality for different concentration 125, 250, 500, 750 and 1000 ppm of the extracts was calculated and subsequently subjected to probit regression analysis to determine LC50 and LC90 values. The most active extract recorded was that of acetone root extract of V. galamensis with an LC50 of 22.85. Vernonia auriculifera and V. lasiopus recorded highest activity in acetone root extract and ethyl acetate root extracts with an LC50 value of 37.7 and 205.9 respectively. One-Way ANOVA test was done to compare the mean mortality of the groups (V. lasiopus, V. auriculifera, V. galamensis). The differences were found to be significantly different at all the hours except at 3 hours. The Tukey post hoc tests indicated that the mortality rate for V. galamensis (mean=7.73) was significantly different from the mortality rates of V. lasiopus (mean=5.29) and V. auriculifera (mean=5.57) after 24 hours. Vernonia lasiopus, V. auriculifera and V. galamensis can be used as alternative sources of new, cheap and readily available larvicides for control of the mosquito vector A. gambiae. Further work is recommended to determine what secondary metabolites in Vernonia extracts are responsible for the larvicidal activity.