The Stability of Resistance to Three Pyrethroids in Spodoptera exigua Hübner

Regression of resistance to pyrethroids (cyhalothrin,fenvalerate,alpha-cypermethrin)in Spodoptera exigua Hübner following several generations of insecticide-free laboratory rearing was evaluated.Although laboratory selected Cyh-R、Fen-R and Cyp-R strain had high level resistance to cyhalothrin (522.9 fold),fenvalerate (669.7 fold)and alpha-cypermethrin (2 737.2 fold),the resistance was unstable and decreased to 60.8,90.8 and 180.5 fold respectively after 10,8 and 9 generations when selection was suspended.Studied on field-collected resistant strain showed that the resistance was also unstable,it declined rapidly without contact with the pyrethroids at first and then stablized at a 6~10 fold level.It seems that it is very difficult to recover susceptibility completely to the three pyrethroids.