Comparative Evaluation of the Larvicidal Properties of Methanol Extracts and Fractions of Ocimum gratissimum L. and Ocimum basilicum L. Leaves (Lamiaceae) on the Fourth Instar Larvae of Culex quinquefasciatus L. and Control of Filariasis

Abstract

Background: Botanical insecticides may serve as suitable alternatives to synthetic insecticides because they contain bioactive chemicals. They are relatively safe, biodegradable and readily available in many areas of the world. Objective: The aim of this study is to comparatively evaluate the larvicidal properties of methanol extracts and fractions of Ocimum gratissimum and Ocimum basilicum leaves on fourth instar larvae of Culex quinquefasciatus, the vector of filariasis. Methods: The plants were identified, collected, air-dried and pulverized. The powdered leaves were macerated in 100% redistilled methanol for 72 hours, filtered and concentrated to thick extracts. After the preliminary work was done as activity-guided process, the crude extracts were partitioned into different solvents and again concentrated to thick fractions. The percentage yields were calculated and recorded. The larvae were collected from a location at Ewet Housing Estate, Uyo, Akwa Ibom State. Toxicity was evaluated by exposing fourth instar larvae of Culex quinquefasciatus to a concentration range of 0.0625 to 1.000mg/mL of the methanol crude extracts and fractions. The larval mortalities were recorded after 24 hours of exposure and LC50 values were determined using the non-linear regression analysis of a statistical package graph pad prism®. Results: The results of percentage mortalities of methanol crude extracts ranged from 2.5±0.50 to 67.50±1.05% for O. gratissimum and 2.5±0.50 to 100±0.00% for O. basilicum with their LC50 of 0.79mg/mL and 0.18mg/mL respectively.  The solvent partitioned fractions also showed that the percentage mortalities of n-hexane and chloroform of O. gratissimum ranged from 5±1.00% to 100±0.00% and 17.5±1.5% to 97.5±0.5% with their LC50 of 0.29mg/mL and 0.32 mg/mL respectively, while those of O. basilicum ranged from 0.25±0.5% to 100±0.00% for n-hexane and 7.5±0.5% to 92.5±1.5% for chloroform fractions with their LC50 of 0.42mg/mL and 0.39 mg/mL compared to that of Nicotine (positive control) with LC50 of 0.01mg/mL with a percentage mortality range of 10±1.00 to 100±0.00%.  Conclusion: O. basilicum leaf methanol crude extract was more than four times active than that of O. gratissimum, but n-hexane and chloroform solvent partitioned fractions of O. gratissimum were more active than those of O. basilicum considering their percentage mortalities and LC50.                   Hence, both methanol crude extracts of O. basilicum and n-hexane fraction of O. gratissimum     have greater potentials as larvicides which can also be used in the control of C. quinquefasciatus larvae.