Isolation, Structure Elucidation and Larvicidal Activity of Laggera alata Extracts

The plant; Laggera alata is in the Asteraceae family, and has been shown to have bioactivity against several diseases. Malaria is by far the most important insect transmitted disease and so far no vaccine claims to prevent infection transmitted by A. gambiae mosquito. The malaria parasite; Plasmodium falciparum is continually developing resistance to the available drugs. The only viable preventive measure is vector control. Dried, ground and weighed 600 g of aerial parts of this plant (L. alata) were sequentially extracted with hexane, ethyl acetate, chloroform, acetone and methanol. The solvents were removed by rotor evaporation under vacuum to give five extracts of non-volatile components. Fresh whole plant of L. alata was subjected to hydro-distillation in a modified Clevenger-type apparatus to extract the volatile components. The essential oils obtained were 5g (2.78% w/w) after drying over anhydrous sodium sulphate. The oils were subjected to GC, GC-MS to determine the phytochemical composition and the major compounds were: 2, 5-dimethoxy-para -Cymene 24.4%, and cis- Chrysanthenol 11.8%. The bioassays were performed with third instar larvae of A. gambiae s.s. The LC₅₀ and LC₉₉ of the L. alata oils were found to be 273.38 and 507.75 mg/l respectively. The hexane fraction showed significant larvidal activity and gave an LC₅₀ and LC₉₉ of 1161 and 2734.91 mg/l respectively. The hexane fraction was further characterized by standard chromatographic and spectroscopic techniques and two new eudesmane compounds were elucidated: 3β-angeloyloxy-4β-hydroxy-eudesm-7, 11-en-8-one (1) and 3β- angeloyloxy-4β-acetoxy-11-hydroxy eudesm-6-en-8-one (2). The isolated compounds can be used as lead compounds in the search for environmentally friendly and biodegradable larvicides. Application of these extracts to larval habitats may be useful in malaria and mosquito management programmes.