Insecticidal components from field pea extracts: soyasaponins and lysolecithins

Extracts from field peas (Pisum sativum L.) have previously been shown to have a utility to control insect pests. To identify potentially new bioinsecticides in field crops, we describe the fractionation of impure extracts (C8 extracts) derived from protein-rich fractions of commercial pea flour. The activity of separated fractions was determined by a flour disk antifeedant bioassay with the rice weevil [Sitophilus oryzae (L.)], an insect pest of stored products. Bioassay-guided fractionation showed that the triterpenoid saponin fractions were partly responsible for the antifeedant activity of C8 extracts. Soyasaponin I (soyasaponin Bb), isolated from peas and soybeans, and mixtures of soyasaponins, comprised of soyasaponins I-III and isolated from soybeans, were inactive antifeedants, but dehydrosoyasaponin I (the C-22 ketone derivative of soyasaponin I), a minor component found in C8 extracts, was shown to be an active component. Dehydrosoyasaponin I (soyasaponin Be) and soyasaponin VI (soyasaponin betag) coeluted under conditions of silica gel thin-layer chromatography and C18 high-performance liquid chromatography. However, dehydrosoyasaponin I could be isolated from saponin-enriched fractions with a reversed phase column of styrene/divinylbenzene operated at alkaline pH. Phospholipids of the lysolecithin type were also identified in saponin fractions of C8 extracts from peas. Three of the lysolecithins were inactive alone against rice weevils, but mixtures of these phospholipids enhanced the insecticidal activity of dehydrosoyasaponin I.