1. Enhanced cytochrome P450 monooxygenase activity associated with fenvalerate resistance in Helicoverpa armigeraA field strain collected from Yanggu County, Shandong Province in 2001 was divided into three sub-populations. The YG strain was originated from one sub-population which was maintained in the laboratory without exposure to insecticides. The YGF and YGFP strains were derived from the other two sub-populations by laboratory selection with fenvalerate and fenvalerate+PBO respectively for 13 generat...