Larvicidal effects of crude methanolic and n-hexane extracts of Cymbopogon citratus and Ocimum gratissimum on Anopheles gambiae mosquito

Abstract

Malaria is a major cause of economic loss and high morbidity especially in sub-Saharan Africa. Elimination of anopheline vectors using plant extracts is one of the best methods of controlling malaria. The methanolic and n-hexane extracts of two plants, Cymbopogon citratus (lemon grass) and Ocimum gratissimum (scent leaf), were investigated for their effectiveness in control of Anopheles mosquito larvae from July 2010 to March 2011. The tests were carried out following the standard WHO larval susceptibility test. Different concentrations of the extracts were prepared fresh in distilled water for each test. Three replicates comprising 10 larvae each were exposed to 25-300 parts per million (ppm) of the extracts at room temperature. Results were taken at 24 h intervals for 72 h. Acute toxicity tests of the extracts were carried out using modified Lorke method. The results showed that mortality increased with increase in time of exposure and concentration for all the extracts. Mortality was recorded all through the exposure period for all concentrations of the 2 plants, except for the 25 ppm of n-hexane extract that had no mortality at 24 hours. The extracts of C. citratus were found most effective with LC₅₀ value of 55 ppm. The acute toxicity study revealed no sign of toxicity and an LD₅₀ greater than 1000 mg extract/ kg body weight in rats. The study shows that C. citratus and O. gratissimum have lavicidal activities against Anopheles gambiae mosquito and can be used as part of the malaria control programme.

Keywords: Methanolic extract; n-hexane extract; Cymbopogon citrates; Ocimum gratissimum; larvae; Anopheles mosquito