Mosquito larvicidal compounds from the plant Fagaropsis angolensis (engl. dale) against Anopheles gambiae

Abstract

Anopheles gambiae mosquitoes transmit malaria resulting into about 500 million infections globally every year. Recent studies estimate that more than 50% of the world‟s population is at risk of malaria infections. Apart from the development of insecticide resistance, the use of synthetic insecticides to control mosquito vectors continues to cause adverse effects to the environment, human health and non-target organisms. Plant derived larvicidal compounds are increasingly being explored as possible alternatives in vector control methods. The plant Fagaropsis angolensis has been used in the past as traditional medicine in treating various ailments. This study investigated the potential larvicidal activity of both volatile and non-volatile compounds from F. angolensis leaves. The chloroform extract of the non-volatile compounds was prepared and subjected to a series of bioassay guided fractionation (100% and 7:3 Chloroform/ethyl acetate solvent mixture) and purification steps using column chromatography technique. The essential oil from the leaves of F. angolensis was obtained by hydro-distillation method and analyzed by GC/GC-MS technique. Three larvicidal compounds namely a phenanthrene carboxylic acid derivative (32), hexyl-9,10-dihydroxydec-5-enoate (33) and methyl -10-(3-phenylpropanoyloxy)-7-hydroxy-19-methylhenicosa-4, 13, 16-trienoate (34) were identified using mass spectrometry, 1D and 2D NMR spectroscopy. The compounds 32, 33 and 34 exhibited LC50 values of 245.5 ppm, 147.6 ppm and 144.4 ppm respectively when tested against the third instar of An. gambiae larvae. Their LC90 values were 471.6 ppm, 292.1 ppm and 259.4 ppm respectively. The oil showed an LC50 of 83.7 ppm and LC90 of 324.0 ppm at 95% confidence interval. Only 2.64% of the essential oil components were fully identified, 67.83% partially identified and unknown (29.0%). The oil contained mainly new compounds whose mass spectra could not be found in the GC – MS databases used. The isolated compounds and the oils can be used in the development of natural mosquito larvicides. Results of this study indicate that the three naturally occurring larvicidal compounds and the essential oil of F. angolensis leaves could have the potential applicability in the control of the larval stages of the malaria vector - An. gambiae mosquitoes.