Nematicidal Principles from Neem (Azadirachta indica A. Juss) Part III Isolation and Bioassay of Some Neem Meliacins

Eight pure C-secomeliacirts including azadirachtin, a mixture of isomeric azadirachtins and two fractions of diametrically opposite polarity were derived from neem (Azadirachta indica A. Juss) seed kernels both by isolation and partial synthesis. Different concerttratipns of the C-seco meliacins between 50 and 250 ppm along with carbofuran at 20 ppm as the reference nematicide were evaluated in vitro against root- knot nematode (Meloidogyne incogntta Kofoid and White) Chitwood. The bioassay results expressed as per cent mortality and LC₅₀ values, respectively after 24 and 48 h treatments revealed that all the 8 meliacins were highly nematicidal with LC₅₀ values ranging between 55 and 157 ppm. The non-polar fraction which was tentatively identified as a long chain ketone was inactive and the other two fractions were moderately active. A highly significant positive correlation was obtained between LC₅₀ and Rf of test meliacins, thereby implicating the influence of hydrophilicity of the molecule on nematotoxicity. Another salient observation was the delayed nematicidal action of some meliacins such as nimbin and salannin, which has a striking parallelism to the typical slow mode of action in arthropods.