abstract. The bioinsecticidal effects of methanolic extracts of the leaves of castor bean, Ricinus communis, and papaya, Carica papaya, on the red flour beetle, Tribolium castaneum, were studied. More specifically, larval weight, larval mortality, percentage pupation, percentage adult emergence and F1 progeny production were recorded. We also studied the effects of the extracts on α-amylase activity and amylase gene expression in T. castaneum. The extracts of R. communis and C. papaya, which were administered in a diet, increased larval mortality and extended the durations of the larval and pupal periods. Feeding them an extract-treated diet also reduced the percentages of larvae that pupated and of adults that emerged and the number of F1 progeny. α-Amylase activity was inhibited and glucose content reduced in the larvae and adults of these insects. This inhibition of α-amylase activity by the R. communis and C. papaya extracts was concentration dependent in vitro. In addition, amylase gene expression was reduced in insects fed a diet containing C. papaya, but not R. communis extract. These results indicate that the bioinsecticidal effects of the leaf extracts are mediated by inhibiting the gene expression and activity of enzymes involved in sugar metabolism.
Abstract. Plant latex contains proteins and other components that defend plants against herbivorous insects. We determined the inhibitory activity of methanolic extracts of latex obtained from three species of plant: mulberry, Morus alba; jackfruit, Artocarpus heterophyllus; and weeping fig, Ficus benjamina, against trehalase in the red flour beetle, Tribolium castaneum. We also determined the changes that occurred throughout the life of the insect in the enzymatic activities of soluble and membrane-bound trehalase and the expression profiles of the genes encoding the two types of trehalase. Soluble trehalase activity was higher than membrane-bound trehalase activity in larvae and adults, whereas there was little difference in eggs, prepupae and pupae. The expression of the trehalases, TcTre-1 and TcTre-2, changed during insect development but did not coincide closely with changes in enzymatic activity, indicating that these changes did not necessarily depend on gene expression. All of three plant latices tested inhibited the activities of both the soluble and membrane-bound trehalase. At the gene expression level, these latices reduced the expression of TcTre-1 but not TcTre-2, indicating that the latices contain component(s) that selectively inhibit gene expression or at least differentially inhibit these two trehalase genes. The inhibition of trehalase activity resulted in a 140% increase in the concentration of trehalose in the beetle and a decrease in glucose concentration to 72% of the control. These findings show that the latices tested contain components that inhibit trehalase activity and TcTre-1 expression and thus, may contribute to the plants' defense against herbivorous insects.
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