Effects through Successive Selection with Fenvalerate on Malathion-Resistant Strains of the Rice Brown Planthopper and the Small Brown Planthopper

Abstract

Strains of the rice brown planthopper and small brown planthopper with 360 -and 370-fold resistance to malathion were respectively and successively selected with fenvalerate to trace the changes of LD₅₀ values of malathion and fenvalerate. The LD₅₀ value of malathion markedly decreased during the first five to six generations of selection in both planthoppers, but little changed by further selection (equilibrated state). In both planthoppers, the LD₅₀ value of malathion in the final (19th) generation of selection was about 1/4 that of the parent strains. In the fenvalerate-selected strain of the rice brown planthopper, the LD₅₀ values of fenitrothion, diazinon and phenthoate decreased to 1/2 to 1/5 during the selection of the first seven generations. In the fenvalerate-selected strain of the small brown planthopper, on the other hand, decreases in the LD₅₀ value of these insecticides were minimal. The LD₅₀ value of fenvalerate increased in the fenvalerate-selected strains of the two planthoppers. This increase, however, did not proceed in proportion to the change in the level of resistance to malathion; the LD₅₀ value did not markedly increase before the level of resistance to malathion reached an equilibrium. In the final (19th) generation of selection, the LD₅₀ value of fenvalerate was about 11-times as high as those in the corresponding parent strains in the rice brown planthopper, and about 5-times in the small brown planthopper.