Presence of tungro vectors and their natural enemies in different rice habitats in Malaysia

Abstract

Rice Oryza sativa L. habitats were evaluated for their propensity to sustain rice tungro virus during the fallow period. Leafhopper vectors and associated natural enemies were sampled by a D-Vac suction machine from rice crops, nurseries, and fallow fields in Malaysia between December 1983 and May 1984. Leersia hexandra Sw. vegetation in wet, fallow rice fields harbored significantly higher densities of Recilia dorsalis (Motschulsky). Nephotettix virescens (Distant) was relatively abundant in second-stage rice nurseries. Nephotettix nigropictus (Stål) and N. malayanus Ishihara et Kawase were only moderately abundant or absent in the habitats sampled. Nephotettix parvus Ishihara et Kawase, a reported vector, was not caught. Spiders, in particular hunting spiders and orbweb weavers, numerically comprised the major natural enemy guilds in all habitats except in direct-seeded rice and single-stage nurseries, where predatory insects were more abundant. Egg parasitoids were the only parasitoid guild in all habitats except in L. hexandra vegetation where low numbers of nymphal-adult parasitoids were recovered as well. Oryza sativa, Fimbristylis miliacea (L.) Vahl and L. hexandra were the most commonly encountered tungro hosts. All habitats with rice potentially perpetuate tungro, particularly ratoon and volunteer rice which carry tungro and vectors to the next season crop and nurseries which are more susceptible to vectors and tungro infection. Fimbristylis miliacea may act as a tungro reservoir when rice is absent but cannot support vectors. Leersia hexandra harbors high densities of vectors but its role as a tungro host is more limited. Elimination of fallows with rice or tungro hosts is recommended as a preventive practice. However, weedy habitats without tungro hosts are beneficial as they harbor a variety of natural enemies of tungro vectors. Such vegetation should be tolerated where possible. Resistant varieties or selective insecticides should be used to prevent tungro infection in the nursery stage without destroying the diverse natural enemy complex.