Abstract
Insecticide susceptibility of the rice leafroller larvae, Cnaphalocrocis medinalis GUENEE, has been surveyed by the leaf-dipping method since 1979. The ratio of LC50 of 5th-instar larvae to that of 1st instar has ranged from 4.8 of fimethylvimphos to 14.5 of chlorpyrifosmethyl. The LC50 values of larvae for several insecticides did not change with advacement of the generations except for chlorpyrifos-methyl in 1981. Insecticide susceptibility of different migration waves in Nagasaki populations in 1983 and 1985 did not differ.LC50 values of Ishigaki and East China Sea populations for several insecticides were smaller than Kyushu main island populations. Ratios of average LC50 values for diazinon, acephate, isoxathion, monocrotophos and cartap obtained in 1985 to those obtained in 1981 were 9.4, 2.7, 1.6, 1.6 and 1.2, respectively, but the ratios for chlorpyrifos-methyl, dimethylvinphos and tetrachlorvinphos were less than 1.
