CHEMICAL CHARACTERIZATION AND IN VITRO LARVICIDAL ACTIVITY OF AQUEOUS AND ORGANIC EXTRACTS FROM TWO AROMATIC AND MEDICINAL PLANTS FROM NIGER AGAINST MOSQUITO LARVAE OF THE GENUS ANOPHELES

The fight against malaria-carrying mosquitoes is under threat from the emergence of resistance to synthetic insecticides. Plant-based insecticides could provide suitable alternative chemical control methods in the future. Although several plants have been reported to have anti-mosquito activity, only a few have progressed from the laboratory to practical application in the field. In most cases, the active compounds have not been identified, and most studies are limited to preliminary screening. Cymbopogon schoenanthus L. spreng and Endostemon tereticaulis poir are common weeds found in many regions of Niger and possess medicinal properties, but little is known to date about the larvicidal activity of these plants. This study forms part of efforts to promote the use of aromatic and medicinal plants in Niger. Extraction yields for aqueous and organic extracts ranged from 1.76 per cent to 24.74 per cent for Cymbopogon schoenanthus and from 2.14 per cent to 26.34 per cent for Endostemon tereticaulis. Water and methanol yielded the highest extraction yields, at 24.74 per cent and 22.42 per cent respectively for Cymbopogon schoenanthus, and 26.34 per cent and 19.67 per cent for Endostemon tereticaulis, compared with organic solvents (petroleum ether, dichloromethane, ethyl acetate and methanol). Phytochemical screening revealed the presence of sterols and triterpenes, alkaloids, coumarins, reducing compounds, saponosides and tannins in the various organic and aqueous extracts of the two plants. The results of in vitro larvicidal tests on Anopheles larvae showed that, amongst the crude extracts, only the ether and dichloromethane extracts of both plants and the ethyl acetate extract of Cymbopogon schoenanthus exhibited larvicidal activity. The ethereal extract of Cymbopogon schoenanthus, with an LD₅₀ of 0.680 mg/mL, is more active than the ethereal extract of Endostemon tereticaulis (LD₅₀ = 0.970 mg/mL), which is itself more active than the dichloromethane extract of Cymbopogon schoenanthus (LD₅₀ = 1.100 mg/mL). The ether extracts of the two plants and the dichloromethane extract of Cymbopogon schoenanthus are less active than deltamethrin. Although the organic extracts are less active than deltamethrin (the reference chemical insecticide), they could be of great interest in the field of vector control, as, being natural substances, they are biodegradable and less polluting than chemical insecticides.