Bisdesmosidic Saponins from Securidaca longepedunculata Roots: Evaluation of Deterrency and Toxicity to Coleopteran Storage Pests

Powdered dry root bark of Securidaca longepedunculata was mixed with maize and cowpea and effectively reduced the numbers of Sitophilus zeamais and Callosobruchus maculatus emerging from these commodities, respectively, more than 9 months after treatment. This effect was reciprocated in grain treated with a methanol extract of the root bark, indicating that compounds were present that were oviposition deterrents or directly toxic to the adults or larvae. Two new bisdesmosidic saponins, 3-O-beta-D-glucopyranosyl-28-O(alpha-L-arabinopyranosyl-(1 -> 3)-beta-D-xylopyranosyl-(1 -> 4)[beta-D-apiofuranosyl-(1 -> 3)]-alpha-L-rhamnopyranosyl-(1 -> 2)-[4-O-(4-methoxycinnamoyl-beta-D-fucopyranosyl)])-medicagenic acid (securidacaside A) and 3-O-beta-D-glucopyranosyl-28-O(alpha-L-arabinopyranosyl-(1 -> 3)-beta-D-xylopyranosyl-(1 -> 4)-[beta-D-apiofuranosyl-(1 -> 3)]-alpha-L-rhamnopyranosyl-(1 -> 2)-[4-O-(3,4,5-trimethoxy-(E)-cinnamoyl-beta-D-fucopyranosyl)])-medicagenic acid (securidacaside B), were isolated from the methanol extract of the roots of S. longepedunculata and characterized by spectroscopic methods. Securidacaside A, which occurred as (E)- and (Z)-regioisomers, showed deterrency and toxicity toward C. maculatus and S. zeamais and could contribute to the biological activity of the methanol extract. The potential to optimize the use of this plant for stored product protection using water extracts, which would be appropriate technology for target farmers, is discussed.