Insecticidal activity of purified compounds and their derivatives from Ocimum americanum L. against dengue virus vector Aedes aegypti

Abstract

Ocimum americanum is commonly known as hoary basil, wild basil and American basil. It is used in traditional medicine for the treatment of microbial infections, fever, cold, tuberculosis and constipation. The insecticidal activity in Ocimum americanum is very limited. The aim of this study is to isolate the effective mosquitocidal compounds against Aedes aegypti. To isolate the bioassay-guided mosquitocidal compounds and their derivatives against Aedes aegypti larvae. The successive extracts of the aerial parts of Ocimum americanum (Lamiaceae), were tested against the third instar larvae of Aedes aegypti. Bioactivity guided fractionation of the most active extract gave the active constituents. Further, activities of the derivatives, viz., acetate and propionates, were also investigated. Of the successive extracts of the aerial parts of the plant, viz., hexane, chloroform and methanol, the chloroform extract showed the highest activity. The LC₅₀ and LC₉₀ values after 48 h were 337.94 and 918.79 ppm, respectively. Bioassay-guided fractionation of the active chloroform extract resulted in the isolation of the active principles viz., oleanolic acid and ursolic acid. The LC₅₀ and LC₉₀ values of the compounds were 42.41, 78.39 and 26.09, 62.06 ppm, respectively. In the case of the acetate derivative, both compounds showed higher activity; among them, oleanolic acid acetate showed the highest activity; LC₅₀ and LC₉₀ values were 4.97 and 9.31 ppm, respectively. The pentacyclic triterpenoids oleanolic acid and ursolic acid were identified as the active larvicidal constituents against the third instar larvae of Aedes aegypti. Oleanolic acid acetate being the most active derivative.