Effects of alkaloids from yam, Dioscorea hispida Schlussel, on feeding and development of larvae of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae)

Abstract

Rhizome of a tropical yam, Dioscorea hispida SCHLUSSEL, which has been used as a traditional pest control agent in the Philippines, was extracted with methanol and the extract was further separated into alkaloid and non-alkaloid fractions by thin layer chromatography. Each fraction was examined to evaluate feeding deterrent activity, developmental effects and toxicity of the fractions against larvae of the diamondback moth (DBM), Plutella xylostella. The methanol extract showed strong feeding deterrent activity to DBM larvae. Two alkaloids, A and B, were isolated as active components from the alkaloid fraction. Alkaloid A was more active than alkaloid B at 100 to 250 μg/ml, and a binary mixture (1 : 1) of these two alkaloids inhibited larval feeding at lower concentrations (50 to 100 μg/ml) than either alkaloid separately. Both alkaloids significantly retarded larval molting and reduced larval weight gain, and high mortality at the larval stage (70 to 86%) and during emergence (98 to 100%) resulted when DBM larvae were reared on treated radish seedlings. No conspicuous effect was observed against DBM larvae when the alkaloids were incorporated in water medium taken up by the seedlings or when applied topically on larvae and pupae.