Potential of using flavonoids, latex and extracts from Calotropis procera (Ait.) as grain protectants against two coleopteran pests of stored rice

Abstract

The leaf methanolic extract, latex protein fraction (Lp) and flavonoids of Calotropis procera (Ait.) (Gentianales: Asclepiadaceae) were extracted. Their effects on the survival and feeding behavior of Sitophilus oryzae (L) (Coleoptera: Curculionidae) and Rhyzopertha dominica (F) (Coleoptera: Bostrichidae) were evaluated. A bioassay-guided fractionation of the crude flavonoid fraction (Cf) of MeOH extract led to the isolation of four flavonoid glycosides as the bioactive constituents. Compounds were found to be the 3-O-rutinosides of quercetin, kaempferol and isorhamnetin, and the flavonoid 5-hydroxy-3,7-dimethoxyflavone-4′-O-β-glucopyranoside. Their structures have been elucidated using physical and spectroscopic methods including (UV, IR, 1H, 13C NMR, DEPT, 2D 1H–1H COSY, HSQC, HMBC and NOESY). The tested natural products showed considerable toxicity against both insects using the treated-filter paper bioassay. At a concentration of (5mLcm−2), mortality percentages of 86.0, 77.6 and 61.0 were achieved after seven days of exposure of adults S. oryzae to (Cf), (Lp) and methanol extract, respectively. R. dominica was less susceptible, where % mortality were ranged between 53.8–64.2%. The F1 progeny of both insects were greatly affected, where a reduction in adult progeny of 83.8, 74.3 and 67.9% was recorded for S. oryzae after parental exposure to a concentration of 5mgcm−2 of (Cf), (Lp) and methanol extract, respectively, meanwhile R. dominica was less susceptible. Results of nutritional study indicate that at sublethal concentrations, the test products significantly reduced the relative growth rate (RGR), food consumption rate (RCR) and food utilization (ECIF) by adults of both insects. The overall feeding process of S. oryzae and R. dominica was deterred by 77.6 and 86.6% with (LP), respectively, while perecentages of fedding deterrnce of 61.2 and 54.2 was recorded in case of (Cf) against S. oryzae and R. dominica, respectively. When tested individually, C. procera flavonoids showed weak to moderate activities.