Identification of a Triterpenoid Saponin from a Crucifer, Barbarea vulgaris, as a Feeding Deterrent to the Diamondback Moth, Plutella xylostella

Abstract

Larvae of the diamondback moth, Plutella xylostella, a crucifer specialist, refuse to feed on a crucifer, Barbarea vulgaris, because of the presence of a feeding deterrent, which is extractable with chloroform. We isolated a feeding deterrent from B. vulgaris leaves, by successive fractionations with silica-gel, ODS, i.e., C₁₈ reversed phase, and Sephadex LH-20 column chromatographies, and ODS-HPLC, guided by a bioassay for feeding deterrent activity. The structure of the compound was determined to be a monodesmosidic triterpenoid saponin, 3-O-[O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl]-hederagenin, based on FAB-MS, ¹H- and ¹³C-NMR spectra, and hydrolysis experiments. When the compound was applied to cabbage leaf disks at greater than 0.18 μg/mm², consumption of the disks by third instars was less than 11% of control disks treated with the solvent alone. Furthermore, all first instars died on the disks treated with the same concentrations. Because the concentration of the compound in the fresh leaves of B. vulgaris was comparable to the effective dose in the cabbage leaf disk tested, we conclude that the unacceptability of B. vulgaris to P. xylostella larvae is primarily due to this saponin.