Triterpenoid saponins synergize insecticidal pea peptides: Effect on feeding and survival of Sitophilus oryzae (Coleoptera: Curculionidae)

The triterpenoid saponins soyasaponin I, dehydrosoyasaponin I, echinocystic acid 3-glucoside, β-escin, glycyrrhizic acid, hederacoside C, and α-hederin were tested alone and in combination with insecticidal PA1b peptide mixtures isolated from peas for their effects on the feeding and survival of a stored-product insect, the rice weevil, Sitophilus oryzae (L.). There were two sources of peptides: a purified extract composed primarily of PA1b peptides and a partially purified extract (C8 extract) that contained mainly peptides and lesser amounts of soyasaponin I, dehydrosoyasaponin I, and other compounds. Dehydrosoyasaponin I, echinocystic acid 3-glucoside, α-hederin, and β-escin were active (causing reduced feeding and increased mortality) when used alone. Soyasaponin I, hederacoside C, and glycyrrhizic acid were inactive when used alone. Purified peptides and C8 extract were active when used alone. The mixtures of the inactive soyasaponin 1 and the active peptides were as active as peptides alone, even when peptides composed only 10% of the mixture. Similar trends were seen with the mixtures of β-escin and PA1b. In general, the mixtures of saponins and peptides were synergistic. Possible modes of synergistic action are discussed. © 2010 Entomological Society of Canada.