A comparative study of the toxicity of some plant extracts and some xenobiotic pesticides towards selected insect species: laboratory bioassays and field validation.

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The toxicity of Turraeanthus africana stem bark and the leaf of Mitragyna stipulosa were investigated for their insecticidal properties and use as alternative pesticides. Phytochemical screening indicated that the petroleum ether (pet ether) extract of the Turraeanthus africana contained mainly terpenoids/ steroids. The methanol extract of Turraeanthus africana contained saponins, steroids/terpenoids, tannins, and alkaloids. The pet ether extract of Mitragyna stipulosa contained steroids/terpenoids and cardeolides (cardiac glycosides) whiles the ethanol extract of M. stipulosa (MSE) contained saponins, steroids/terpenoids, tannins, alkaloids cardeolides and flavonoids. The crude ether extract of T. africana (AVP) was analysed on a glass column constituting 3% OV- 17. Forty-one components were resolved with the major component constituting 21.4% of the crude and having a retention time of 80.58 relative to hexane. Isothermal runs of the column-eluted fractions of (AVP) at an oven temperature of 255 °C indicated a relation between the percentage composition of compound with kovats index of 3600 and their brine shrimps lethality. The second fraction was having the highest lethality of LC50 of 1.21 μg/ml and a percentage composition of 55.23%. AVP crude extract (yield 5.3%) was found to be active against brine shrimps, Oreochromis niloticus and storage pests i.e. Sitophilus zeamais and calusobruchus maculatus with LC50 values of 39.56 μg/ml, 2.75 μg/ml, 292.28 μg/ml, and 229.67 μg/ml respectively. LC50 of pirimiphos methyl on S. zeamais and C. maculatus was 0.002 μg/ml and 7.11 μg/ml respectively. The pet ether extract of M. slipulosa (MSP) was observed to be active to Balanogastris kolae with LC50 of 1225.56 μg/m whiles the lethality to B. kolae by Gammalin 20 had LC50 90.85 μg/ml. The 24hr brine shrimp lethality of MSP was 15.31 μg/ml. Simulation of the laboratory bioassays on field trials revealed that, AVP applied at a concentration of 0.60%, was effective in the control of S. zeamais and C. maculatus over a 2 month and 3 month period respectively. 0.60% concentration of the methanol extract of T. africana deterred oviposition in C. maculatus sufficiently and controlled C. maculatus over a 3 month period but was not effective against S. zeamais.
A thesis submitted to the Department of Chemistry of Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the award of Master of Science in Chemistry, 2003