Current status of insecticide resistance in the small brown planthopper, Laodelphax striatellus, in Japan, Taiwan, and Vietnam

The small brown planthopper, Laodelphax striatellus, is one of the most serious pest insects of rice plants. A large migration of the insects from overseas was reported in western parts of Japan in June 2008. Insecticide resistance to imidacloprid, fipronil and BPMC was compared among local populations in these western regions after migration. The insecticides were applied to the insects using a topical application method. In some populations, the resistance status coincided with that of the immigrant insects just after migration, i.e., resistance to imidacloprid but susceptibility to fipronil. In other populations, resistance was observed not only against imidacloprid but also fipronil. It is likely that the status of the latter populations resulted from intercrossing between domestic populations of the insects and migrants. Insecticide resistance was also assessed in other areas of northern and eastern parts of Japan. In general, these populations showed relatively low resistance, although resistance to fipronil was high in the eastern part of Japan where the density of domestic populations has recently increased. Insecticide susceptibilities were also assessed in several sites in Taiwan and the northern parts of Vietnam. Although susceptibilities differed among these sites or countries, they have recently seen a decline for all three insecticides.