Mechanisms of resistance in wild riceOryza brachyantha to rice leaffolderCnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae).

A wild rice,Oryza brachyantha, was rated as highly resistant to rice leaffolder,Cnaphalocrocis medinalis (Guenée), while its F1 hybrid from a cross with a high-yielding, leaffolder-susceptible rice, IR31917-45-3-2, was rated resistant in a standard screening test. In comparison with IR31917-45-3-2,O. brachyantha and the F1 hybrid were unsuitable for oviposition, survival, and growth ofC. medinalis. However, growth and survival of larvae in artificial diets containing lyophilized leaf powder of IR31917-45-3-2 andO. brachyantha were comparable. Studies on the feeding and settling preference of the larval stages suggested that the mechanism of resistance ofO. brachyantha is that of antixenosis type. Olfactometer tests with first-instar larvae and electroantennogram responses of adults indicated a lack or low levels of volatile attractants inO. brachyantha and the F1 hybrid in comparison with IR31917-45-3-2. Bioassays of sequential solvent extracts of these plants indicated that larval preference for IR31917-45-3-2 and nonpreference forO. brachyantha and the F1 hybrid were due partly to chemical factors present in hexane and methylene chloride extractables. Closer arrangement of silica cells in the epidermal layer ofO. brachyantha and a higher mandibular wear in larvae reared onO. brachyantha suggested that physical resistance due to silica may be an additional cause of resistance. It was hypothesized that the high levels of resistance observed inO. brachyantha may be due to an additive or synergistic action of the absence of attractants or feeding stimulants, the presence of deterrents, and the physical resistance offered by silica. The significance of these results to a successful wide hybridization program aimed at transferring resistance factors from wild rice to cultivated rice is discussed.