Phytochemical evaluation of Ocimum kilimandscharicum GUERKE, Gnidia subcordata MEISN AND Annona mucosa jacq leaves for secondary metabolites and their insecticidal activities against Sitophilus zeamais and Prostephanus truncatus.

Abstract

Global food security of which maize is a major component is threatened by food loss due to storage insect pests. Sitophilus zeamais and Prostephanus truncatus cause stored maize losses of 20% world wide and 45% in Africa. Synthetic insecticides though effective in control of insect pests, insect resistance and the adverse side effects associated with the insecticides calls for search of effective and safe botanical insecticides. Ocimum kilimandscharicum, Gnidia subcordata and Annona mucosa leaves have been used traditionally to control insect pests. Efficacy of their crude extracts and active principles occasioning their uses are however not documented. This work evaluated insecticidal activities of the plants’ leaf extracts against S. zeamais and P. truncatus, isolated and characterised pure isolates and evaluated the pure isolates for their insecticidal activity against the two insects. Powdered plant leaves were separately sequentially soaked in n-hexane, ethyl acetate and methanol then filtered, concentrated and subjected to column chromatography fractionation with eluents of differing polarities and then thin layer chromatography. Structural elucidation of isolated compounds was done using physical and spectroscopic methods including NMR, IR, UV-VIS, mass spectrometry and comparison with literature data. Extracts and pure isolates were assayed for toxicity and antifeedant activities against S. zeamais and P. truncatus. Deltamethrine and azadirachtin were used as positive controls. Gnidia subcordata methanol extracts exhibited the highest contact toxicity and antifeedant activities against S. zeamais: LC50 = 27.03 μg/mL, AFI50 = 20.47 μg/mL and P. truncatus: LC50 =22.55 μg/mL, AFI50 = 20.99 μg/mL, which compared favourably with activities of the positive controls. Gnidia subcordata yielded β-amyrin acetate (199), 3β-hydroxy-11-oxoolean-12-ene (200), dihydronitidine (201), dihydrochelerythrine (202), gedunin (203), obacunone (204), nagilactone (205), quercetin (140), kaempferol-3-O-β-galactoside (206) and 4', 5-dihydroxystilbene-3-O-β-glucoside (207). n-Eicosanol (208), friedelin (209), stigmasterol (6), lupeol (49), 2α-hydroxy-3-oxodammar-20, 24-diene (210), 2α, 3β-dihydroxy dammar-20, 24-diene (211), chrysin (212), apigenin (144), fisetin (213), quercetin (140) and apigenin-7-O-neohesperidoside (214) were isolated from Ocimum kilimandscharicum. Annona mucosa yielded; α-amyrin acetate (215), β-sitosterol (96), 3α, 24-diacetoxy-12-oleanene (216),3-oxo-11β-hydroxyurs-12-ene (217), (3R,20S)-3-acetoxy-20-hydroxydammar-24-ene (218), 3β-acetoxy oleanolic acid (219), 3β-acetoxytirucallic acid (220), quercetin (140), oleanolic acid (4), quercetin 3-O-β-D-arabinoside (221) andquercetin-3-O-β-D-glucoside (222).Compounds 210, 211 and 216 are new. Among the isolated compounds, 203 had the highest contact toxicity activities LC50 15.68 and 16.99μg/mL while 222 had the highest antifeedant activities AFI50 14.93and 16.84 μg/mL., against S. zeamais and P. truncatus respectively, which compared well with activities of the positive controls. This study has validated the traditional use of the plants in stored maize protection against insect pests. It has also identified the individual compounds responsible for the activities. These compounds can be developed into formulations individually or as mixtures to control S. zeamais and P. truncatus in maize.