Bio-efficacy of Azadirachta indica A. Juss oil extracted from sun- and shade-dried seeds against two stored-product beetles
2015
Tofel, Haman Katamssadan | Nukenine, Elias Nchiwan | Stähler, Matthias | Adler, Cornel
Azadirachta indica is a source of highly effective botanical insecticides, with Azadirachtin A as the major active principle. However, it has been contended that sun-drying reduces its Azadirachtin A content and insecticidal efficacy. This work investigated the Azadirachtin A content, fatty acid contents, adult toxicity, F1 progeny inhibition and damage reduction of A. indica oils from sun-dried kernels, shade-dried kernels, sun-dried seeds and shade-dried seeds, against Callosobruchus maculatus Fab. (Coleoptera: Chrysomelidae) and Sitophilus zeamais Motsch. (Coleoptera: Curculionidae), serious storage pest of cowpea and maize, respectively. Analysis by liquid chromatography-electrospray ionization-tandem mass spectrometry showed that the sun-dried seed, but not the sun-dried kernel oil gave less Azadirachtin A amount 2.89 ± 0.17 g/kg compared to the other drying regimes (3.09 ± 0.09 – 3.69 ± 0.16 g/kg). Gas chromatography equipped with a flame-ionisation detector analysis revealed the presence of palmitic acid, stearic acid, oleic acid and linoleic acid, as the major fatty acids in the oils, with similarity in their concentrations among the four drying regimes. The adult toxicity assay showed that C. maculatus was more susceptible to the A. indica oils than S. zeamais, but without differences linked to the drying regime. All the oils completely suppressed progeny production and grain damage, irrespective of the drying regime and insect species. Our results disagree with the contention that sun-drying diminishes the Azadirachtin A content and insecticidal efficacy of A. indica. Therefore, sun-drying could be adopted by farmers because it may speed up processing of seeds and minimize attacks by aflatoxin producing fungi.
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