α-Amylase Activity of Stored Products Insects and Its Inhibition by Medicinal Plant Extracts

The experiment was conducted to determine α-amylase activity and the effect of seven plant species extracts including Punica granatum L. (Punicaceae), Rheum officinale B. (Polygonaceae), Rhus coriaria L. (Anacardiaceae), Artemisia sieberi B. ( Compositae), Peganum harmala L. (Nitrariaceae) , Datura stramonium L. (Solanaceae) and Thymus vulgaris L. (Lamiaceae) on α-amylase activity of four stored insect pests including Callosobruchus maculatus F. (Coleoptera: Bruchidae), Rhyzopertha dominica F. (Coleoptera: Bostrichidae), Sitophilus granarius L. (Coleoptera: Curculionidae), and Trogoderma granarium E. (Coleoptera : Dermestidae). Also, gut pH and optimum temperature for α-amylase activity of these insects were determined. It was found that α -amylases midgut pH of all four insect species was acidic and optimum temperature was between 30 and 40 °C. Beyond these temperatures, the α-amylases activities sharply decreased. Plant extracts caused inhibitory activity on insect α-amylases varying from nearly 4% to 95% inhibition. D. stramonium and R. officinali extracts had the highest amylase inhibitory activity among the tested extracts, while methanolic extracts of P. harmala, and T. vulgaris (except for S. granaries α-amylase) showed the lowest inhibitory activity. Gel assays revealed that more than one isoform of α -amylase detected in midgut crude extracts of the four insect pests examined could be inhibited by the plant extracts.