Two Isoquinuclidine Alkaloids of a Tropical Yam, Dioscorea hispida (Dioscoreaceae) as Antifeedant and Toxin Against Lepidopteran Insects.

ALEXIE B. BANAAG1, KEIICHI NAGATA AND HIROSHI HONDA

Institute of Agriculture and Forestry, University of Tsukuba 1-1-1, tennodai, Tsukuba, Ibaraki 305-8572, Japan

1Department of Biology, Mindanao State University, Iligan Institute of Technology.9200 Iligan City, Philippines

Biopestic. Int. 1 (1,2): 46-53 (2005)

ABSTRACT

Toxic compounds in the rhizome of a tropical toxic yam, Dioscorea hispida Schlussel were extracted and evaluated for biological activity against larvae of the diamondback moth (DBM), Plutella xylostella (Linnaeus) and rice armyworm, Psudoletia saparata (Walker) (RAW). Two alkaloids, dioscorine and dioscorine N-oxide showed antifeedant activity, retarding of development and insecticidal activity against the larvae of DBM. Individually, the alkaloids significantly inhibited larval feeding at 50 µg/ml and in combination based on a natural ratio (dioscorine and dioscorine N-oxide, 75:25) also deterred feeding of insects at 25 µg/ml. They also significantly retarded larval molting and reduced larval weight when DBM larvae were reared on alkaloid-laden radish seedlings. LC50 values of dioscorine and dioscorine N-oxide to DBM larvae were 25.6 and 31.9 µg/ml, respectively. Typical symptom in RAW larvae injected with the alkaloids suggested that these chemicals could act as depressants in the nervous system.

KEY WORDS : Dioscorea hispida, alkaloids, diamondback moth, rice armyworm, antifeedant, toxicity