Phytochemical and larvicidal evaluation of stem bark of Vernonia amygdalina

The Aedes aegypti mosquito is responsible for the transmission of yellow fever, and several arboviruses, which has led to an increasing disease burden. The development of resistance in mosquitoes has prompted researchers to investigate on biological larvicides that are biodegradable and ecofriendly. This study is aimed at evaluating the larvicidal activity of the stem bark of Vernonia amygdalina using the activity–guided fractionation approach. The powdered stem-bark (300g) was extracted by successive cold maceration first in dichloromethane and then in 70% aqueous ethanol for 72 hours with fresh replacement of solvent after every 24 hours. The extracts were separately concentrated using a rotary evaporator to obtain the dichloromethane extract (DCM) and 70% aqueous ethanol extract (AQE) respectively. The more active AQE was further partitioned with ethyl acetate to obtain the aqueous phase (AQP) and the organic phase (EAP). Larvicidal assay was done following the World Health Organisation protocol with modifications. Dichlorvos (2,3-dichlorovinyldimethylphosphate) was used as a reference larvicidal agent. Phytochemical screening was done using standard phytochemical screening reagents. The result from Larvicidal assay showed that the 70% ethanol extract gave a percentage mortality of 86.5%, at its highest concentration (10mg/ml) and an LC50 of 6.5mg/ml, while the highest percentage mortality recorded from the dichloromethane extract, was found to be 58%, at 10mg/ml. The EAP demonstrated a percentage mortality of 100%, at a concentration of 1mg/ml, while the AQP yielded a percentage mortality of 100% at concentration of 6.5mg/ml. With repect to phytochemical constituents, the crude plant sample contained deoxy sugars, triterpenoids, carbohydyrates, saponin and phenolics. The DCM lacked flavonoids and phenolics, but contained carbohydrates, triterpenoids and deoxy sugars. The AQE lacked free anthraquinones. The AQP contained carbohydrates, saponins, phenolics, whereas the EAP contained deoxy sugars, triterpenoids and flavonoids in addition. In conclusion, the 70% aqueous ethanol extract, as well as its organic and aqueous phases can be developed as bio-larvicides, for vector control of Aedes aegypti mosquitoes.