Compounds from Ageratum conyzoides: isolation, structural elucidation and insecticidal activity

This work aimed at identifying plant compounds with insecticidal activity against Diaphania hyalinata (L.) (Lepidoptera: Pyralidae), Musca domestica (L.) (Diptera: Muscidae), Periplaneta americana (L.) (Blattodea: Blattidae) and Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae). The plant species used were: basil (Ocimum selloi Benth.), rue (Ruta graveolens L.), lion's ear (Leonotis nepetaefolia L.), Jimson weed (Datura stramonium L.), ‘baleeira’ herb (Cordia verbenaceae L.), mint (Mentha piperita L.), wild balsam apple (Mormodica charantia L.) and billy goat weed (Ageratum conyzoides L.). Firstly, the insecticidal activities of hexane and ethanol plant extracts were evaluated against adults of R. dominica. Among them, only the hexane extract of A. conyzoides showed insecticidal activity. The hexane extract of this plant species was therefore fractionated by silica gel column chromatography to isolate and purify its bioactive chemical constituents. Three compounds were identified using IR spectra, 1H NMR, 13C NMR, HMBC and NOE after gel chromatography: 5,6,7,8,3′, 4′, 5′-heptamethoxyflavone, 5,6,7,8,3′-pentamethoxy-4′, 5′-methylenedioxyflavone and coumarin. The complete assignment of 13C NMR to 5,6,7,8,3′-pentamethoxy-4′, 5′-methylenedioxyflavone was successfully made for the first time. 5,6,7,8,3′-Pentamethoxy-4′, 5′-methylenedioxyflavone did not show any insecticidal activity against the four insect species tested. 5,6,7,8,3′, 4′, 5′-Heptamethoxyflavone showed low activity against D. hyalinata and R. dominica and was not toxic to M. domestica or P. americana. In contrast, coumarin showed insecticidal activity against all four insect pest species tested, with the following order of susceptibility: R. dominica < P. americana < D. hyalinata < M. domestica after 24 h exposure