Cross-virulence of N ephotettix virescens (Homoptera: Cicadellidae) Biotypes Among Some Rice Cultivars with the Same Major-resistance Gene E. A. HEINRICHS AND H. R. RAPUSAS Department of Entomology, International Rice Research Institute, Box 933, Manila, Philippines Environ. Entomol. 14: 696-700 (1985) ABSTRACT By rearing Nephotettix virescens (Distant) populations on resistant 'Pankhari 203' and 'IRS' for 10 generations, colonies virulent to the resistant cultivars, as indicated by plant damage ratings, N. virescens survival, and population growth, were selected. Testing the colonies on cultivars that have the same major gene for resistance as the cultivar upon which they were selected indicated increased virulence on some cultivars but not on others. Minor genes play an important role in determining the degree of cross virulence of N. virescens on different cultivars. THE GREEN LEAFHOPPER, Nephotettix virescens (Distant), is the most important Nephotettix species attacking rice in South and Southeast Asia. Al- though N. virescens populations seldom reach levels that cause severe direct damage through the re- moval of plant sap, N. virescens is an extremely serious vector of the rice tungro virus, which caus- es severe yield reductions (Palomar and Ling 1966, Heinrichs 1979). A major strategy in the management of N. vi- rescens populations and tungro virus is the devel- opment of N. virescens-resistant rice cultivars. In the genetic evaluation of 50,000 accessions in the International Rice Germplasm Collection, using the standard seedbox method for screening 7-day-old seedlings, 1,200 have been selected for N. vires- cens resistance (Heinrichs et al. 1985) and several have been used as resistance sources in breeding programs at the International Rice Research Insti- tute (IRRI) and in national programs. All but 1 of 27 commerciallRRI cultivars, '1R5' to '1R62', have moderate-to-high levels of N. virescens resistance; these resistant cultivars have been commercially grown in the Philippines since 'IR8' was released in 1966. Insect biotypes have been of great concern to rice scientists responsible for the breeding of resis- tant cultivars since the selection of a brown plant- hopper Nilaparvata lugens (Stal) biotype on 'IR26' resulted in severe yield losses (Pathak and Hein- richs 1982). A major goal of IRRI's breeding pro- gram is to develop cultivars with stable resistance to insects. To support that program, studies have been conducted to understand the processes lead- ing to selection of biotypes of N. virescens and the relationship of biotype selection to tungro virus infection. Heinrichs and Rapusas (1984) reported that, after selection on resistant cultivars for 19 generations, N. virescens survival increased and there was a shift in feeding from the xylem to the phloem. After selection on N. virescens-resistant cultivars, efficiency of tungro virus transmission increased on tungro virus-susceptible but not on tungro virus-resistant cultivars. In the study, only one cultivar for each of the major N. virescens resistance genes was included. As a complemen- tary study, the cross virulence of biotypes selected on one cultivar to that of other cultivars with the same major resistance genes was determined. Materials and Methods N. virescens biotypes were selected on 'Pank- hari 203', which has the gene Glh-l for N. vires- cens resistance, and 'IR8', which has the Glh-3 gene for moderate resistance (Athwal et al. 1971). Adults 3-5 days after the final molt, were placed on potted plants (30-40 days old) of 'Taichung Native' ('TN1'), a susceptible cultivar. Newly emerged nymphs were transferred to 30-day-old potted plants of the resistant cultivars, where they became adults. When adults were 3-5 days old, they were again transferred to 'TN1' plants for oviposition and the newly emerged nymphs were again transferred to the resistant cultivars 'Pank- hari 203' and 'IR8'. The process was continued for 10 generations. The original source of the insects selected on 'Pankhari 203' and 'IR8' was the same as that described in Heinrichs and Rapusas (1984). Insects originally field-collected and continuously reared on 'TNI' for at least 10 years (100 gener- ations) were considered the 'TNI' colony and those reared on 'Pankhari 203' and '1R8' for 10 gener- ations were the 'Pankhari 203' and 'IR8' colonies. Three tests were conducted to evaluate the vir- ulence of each colony. In all tests, 'TN1' was used as the susceptible check and 'IR29' (resistance gene undetermined), the resistant check (Heinrichs et al. 1985). Cultivars with the same major gene for N. virescens resistance were compared for their 696