Author : Dang Thanh Ho

Summary : The mechanisms of resistance involved in moderately resistant (MR) rice varieties to the brown planthopper (BPH), Nilaparvata lugens were studied. Moderate resistance is found quantitatively expressed and polygenically controlled with minor genes having a small, cumulative effect. Minor gene recombination occurred in the F2 and F3 accumulatively. MR varieties have a low level of resistance at the seedling stage but can tolerate a high BPH population under field conditions at the vegetative stage. The most important component in MR varieties is tolerance. A MR variety continued photosynthetic activity in spite of suffering heavy damage caused by BPH feeding. Yield reduction in a MR variety was significantly less than in a susceptible variety under high insect infestation. Therefore, in epidemics, tolerance operates as a major component of MR varieties to maintain acceptable yields. The second most important component operating in moderate resistance is antibiosis which affects growth and development of the insect population. Although a MR variety allows high feeding activity, it reduces the food utilization for body biomass and egg formation and eventually the insect population growth rate is reduced.