Resistance evolution and change of detoxification enzyme activity in a strain of Helicoverpa armigera selected with a mixture of fenvalerate and phoxim

The resistant YS-FP strain of Helicoverpa armigera was selected from the susceptible YS strain with a mixture of fenvalerate and phoxim (a.i., 1∶10) for 16 generations. Compared with the YS strain, the YS-FP strain developed 14.7-fold resistance to the mixture itself, 2 170 -fold resistance to fenvalerate and 3.1-fold resistance to phoxim. During 16 generations of continuous selection with the mixture, the co-toxicity coefficient decreased gradually after a brief increase at F_2, and the interaction between fenvalerate and phoxim against the YS-FP strain changed from synergism to antagonism. The YS-FP strain had significant cross resistance to three pyrethroids (cypermethrin, deltamethrin and cyhalothrin), triazophos and methomyl, but no cross resistance to endosulfan, spinosad and emamectin. The cytochrome P450 oxidase O-demethylation activity in midguts of 6th instar larvae from the YS-FP strain was 10-fold of that from the YS strain; and the glutathione S-transferase activity (CDNB conjugation) and esterase activity (to the substrate α-naphthyl acetate) of 3rd instar larvae from the YS-FP strain were 1.7-and 2.4-fold respectively compared with the YS strain. The selection of the fenvalerate and phoxim mixture in the YS strain of H. armigera resulted in broad-spectrum cross resistance, multiple metabolic resistance mechanisms (especially enhanced cytochrome P450 oxidase) and antagonistic effect between fenvalerate and phoxim. The results suggested that the role of such fenvalerate and phoxim mixture in resistance management of H. armigera could be limited and temporary.