A methanol extract of the pod surfaces of Cajanus cajan, a feeding
stimulant for fifth-instar Helicoverpa armigera, was shown to contain four
main phenolic compounds. Three of these were identified as isoquercitrin,
quercetin, and quercetin-3-methyl ether, by comparing UV spectra and HPLC
retention times with authentic standards. The fourth compound was isolated by
semipreparative HPLC and determined to be 3-hydroxy-4-prenyl-5-
methoxystilbene-2-carboxylic acid (stilbene) by NMR spectroscopy and
mass spectrometry. Quercetin, isoquercitrin, and quercetin-3-methyl did not
affect the selection-behavior of fifth-instar H. armigera. However, larvae were
deterred from feeding on glass-fiber disks impregnated with the stilbene. Furthermore,
larvae exposed to quercetin-3-methyl ether consumed significant amounts
of both disks. In a binary-choice bioassay, a combination of quercetin-3-methyl
ether and the stilbene on one disk and pure quercetin-3-methyl ether on the other
disk resulted in increased consumption of both glass-fiber disks by larvae. In
contrast, consumption was reduced if the combination was presented to larvae
on one disk with purified stilbene on the other disk. Cajanus cajan cultivars that
varied in their susceptibility to H. armigera were surveyed for the presence of the
four phenolic compounds. An absence of quercetin and higher concentrations
of isoquercitrin than the cultivated variety characterized pod surface extracts of
pod-borer-resistant cultivars. In addition, the ratio of the stilbene to quercetin-
3-methyl ether was greater in the pod-borer-resistant cultivars. These findings