Adaptation of the brown planthopper (BPH), Nilaparvata lugens (Stal) (Homoptera: Delphacidae), to BPH resistant rice cultivars carrying bph8 or Bph9

Colonies of the brown planthopper (BPH), Nilaparvata lugens (Sta1), virulent against two resistant rice cultivars carrying BPH resistance genes, either bph8 or Bph9, were selected under laboratory conditions. Two BPH resistant indica cultivars, Thai Col. 11 (a bph8 carrier) and Pokkali (a Bph9 carrier), were used as selection hosts on which the wild-type population of RPH was continuously cultured. The virulence or adaptation was monitored on the resistant hosts during the selection process based on the following five parameters; survival rate, wing dimorphism, oviposition, egg hatch and egg hatchability. Selection appeared to be achieved at the 9th-15th generations depending on the parameters and the selection hosts. The colony selected on Thai Col. 11 showed a high level of virulence not only against the selection host but also against another bph8 carrier, cv. Thai Col. 5. The colony selected on Pokkali also showed virulence on the selection host but this virulence was unstable and not effective against another Bph9 carrier, cv. Balamawee. Our results suggested the presence of virulent BPH individuals (forerunners) in the wild population that could effectively be selected on resistant rice hosts with specific major resistance genes. The genetic system of BPH virulence and rice resistance is discussed.