Abstract
In rice field of Saga prefecture, Kyushu, where BHC formulations had been applied frequently until 1969, the brown planthopper showed remarkable seasonal fluctuation of the susceptibility to lindane in 1969-1970. The autumnal generations were 14- to 19-fold resistant to lindane as compared with topical LD50's of those of the immigrant summer generation. The resistant laboratory strains originating from the autumnal generations were also resistant to dieldrin and fenitrothion, their resistance level being>19 times, and 6-7 times, respectively, when compared with the immigrant summer generation. While, there was no apparent difference in the susceptibility to carbaryl and DDT. No correlation was observed between the rate of absorption of the insecticides by the hoppers, as determined by gaschromatographic analysis, and the resistance to lindane and dieldrin. Environmental temperature, photoperiod (shorter days) and selection of brachypterous females had little effect in producing lindane resistant colony from the susceptible immigrant hopper in laboratory tests. These seasonal fluctuations were considered to be due to the alternate effects of the selective pressure of insecticides and replacement of the resistant population, which could not overwinter, with susceptible one which had migrated from distant areas not treated with insecticides.