Abstract: A middleresistance strain (15.06 folds) of Helicoverpa armigera was used to study the influence of fenvalerate on resistance development of different instar larvae. Four selection lines SM1, SM2, SM3 and SM4 were generated by treating the strain with fenvalerate in successive generations on neonate, 2nd, 3rd and 4th instar larvae respectively. After ten generations, the resistance of SM3 was found to develop most rapidly, followed by those of SM4 and SM2, while that of SM1 developed most slowly. The SM1 and SM3 were selected in succession for another 3 generations and the resistance level of the former line increased to about 9 folds, and that of the latter was about 70 folds. The realized population heritability of resistance to fenvalerate based on the selection data obtained in SM3 (0.4419) was higher than that in SM1 (0.2346). Further experiment showed that PBO, an inhibitor of MFO, gave greater synergic effect on fenvalerate in the high instar selection lines and higher instar larvae of the same selection line. But TPP, an inhibitor of esterase, made no significant effect. Knockdown experiment showed that knockdown resistance was the main resistance selected in the neonate stage. But in 3rd instar larvae,both knockdown and MFO metabolic resistances were selected.The result infers that applying the pesticide in neonate stage would benefit the prevention and postponement of resistance development.

Key words: Helicoverpa armigera, fenvalerate, resistance development