Abstract
The adaptation of the brown planthopper (BPH) to mixture of resistant rice lines was studied. The BPH population which had no ability to infest any resistant rice varieties in Japan was reared on the three plots of single resistant lines and the four mixtures of two to three resistant lines. BPH adapted to the single resistant lines by the fifth generation of selection and to the mixtures by the ninth generation. The BPH colonies selected on the mixtures were more virulent than those on the single resistant lines, which enabled to infest all the component lines. These results revealed that the mixtures of twO to three BPH resistant lines were only effective to delay the development of the varietal resistance-breaking populations for some geneartions but ineffective to prevent it.
