Arriza Arida, Jedeliza Ferrater, Daisuke Fujita and Finbarr Horgan
International Rice Research Institute, Los Baños, Philippines
Over the last 50 years, researchers have identified and developed rice varieties with resistance against the brown- (Nilaparvata lugens Stål)(BPH) and white-backed plant-hoppers (Sogatella furcifera Horváth)(WBPH). In many cases, the underlying genetics of this resistance has been identified: currently, some 32 genes for BPH resistance and 16 genes for WBPH resistance have been identified. More recently, efficient molecular markers, combined with novel introgression and pyramiding techniques, are aiding in the development of further varieties with increased resistance against planthoppers. But have we learned enough from early experiences in using host-plant resistance against rice planthoppers?
During the early years of resistance technology, poor attention to issues of varietal deployment ultimately led to the rapid breakdown of previously effective resistance genes. Data from the field indicated that resistant varieties had a limited life-time, as hoppers adapted to the genes and became more virulent (Table 1 and picture above). The rate of breakdown for a newly deployed gene will vary according to the specific function of the gene, and the background genetics of the variety (which may include unidentified resistance sources). In laboratory studies, rates of complete breakdown can vary from 8 to 20 generations (Fig. 2). Recent evidence suggests that a large proportion of known resistance genes have already succumbed to increasingly virulent BPH and WBPH populations over much of Asia (Fig. 3). For BPH, resistant genes Bph1, bph2, Bph3, bph4, Bph8, Bhp9, Bph20 and bph21 have become increasingly ineffective.
Under field conditions, breakdown will depend on the extent of varietal deployment (spatial and temporal) and the relatedness of associated rice varieties and resistance genes (i.e., those varieties and genes that are deployed close together in space and time).
It is important to remember that resistance is a two-tier process: plant genetics determines the ability of a variety to defend itself against insect attack, whereas field deployment and the spatial and temporal distribution of varieties slows hopper adaptation to the genes.
Current research aimed at prolonging the life of resistant varieties (that is, increasing resistance durability) is predominantly focused on plant genetics (pyramiding genes, applying QTLs, etc.), whereas issues of field deployment (proper crop management, spatial distribution of varieties, refuge technology, etc.) are generally overlooked. Many new and promising genes and varieties are distributed to national agricultural and rice institutes, and through extension services, to Asian farmers, without regard to deployment issues. At the current rate of breakdown, it appears that most of the recently discovered resistance genes may already be ineffective within a further 40 to 50 years.
Resistance genes are a non-renewable resource that requires effective management and conservation. IRRI in partnership with research institutes in Asia is currently addressing the extent of hopper virulence, the utility of remaining genes and the mechanisms of hopper adaptation in an attempt to better deploy resistant rice varieties. Proper management of the remaining genetic sources of planthopper resistance will effectively increase the durability of resistance. This interdisciplinary research will require cooperation between entomologists, breeders, geographers, social scientists, economists and extension specialists if it is to be truly effective.
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Myint, KKM, Yasui, H, Takagi M, Matsumura M. 2009. Virulence of long-term laboratory populations of the brown planthopper, Nilaparvata lugens (Stal), and whitebacked planthopper, Sogatella furcifera (Horvath)(Homoptera: Delphacidae), on rice differential varieties. Applied Entomology and Zoology 44: 149-153.