Our previous studies showed that enhanced cytochrome P450-mediated detoxification is a major mechanism responsible for pyrethroid resistance in a laboratory-selected strain (YGF) of Helicoverpa armigera (Hübner). Two new cytochrome P450 genes, CYP9A12 and CYP9A14 (encoding 531 and 530 amino acid residues, respectively) were isolated from the fat body of the YGF strain of H. armigera. The mRNA expression levels of these two CYP9 P450 genes, together with CYP6B7 and CYP4G8 (previously reported to be overexpressed in pyrethroid-resistant strains), were assessed by real-time quantitative reverse transcription-polymerase chain reaction in the final instars of a field-derived YG strain (with seven-fold resistance to the pyrethroid fenvalerate) and the YGF strain (selected from the YG strain with fenvalerate in the laboratory, with 1,690-fold resistance to fenvalerate). The mRNA expression levels of CYP9A12, CYP9A14, and CYP6B7 in the fat body of the YGF strain increased to 433-, 59-, and 9.3-fold, respectively, compared with the YG strain, whereas no overexpression was revealed for CYP4G8. CYP9A12 and CYP9A14 had 19- and 4.3-fold overexpression in the midgut of the YGF strain compared with the YG strain, respectively, but CYP6B7 and CYP4G8 were not overexpressed. The current study provided evidence that constitutive overexpression of multiple cytochrome P450 genes (CYP9A12, CYP9A14, and CYP6B7) is associated with pyrethroid resistance in H. armigera.

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The relative contribution of oxidases and esterases to pyrethroid resistance was studied in a YS-FP strain of Helicoverpa armigera from China. The YS-FP strain was derived from a field-collected strain (YS) by 16 generations of selection with a mixture of fenvalerate and phoxim. Compared with the YS strain, the YS-FP strain showed 1850-to >7140-fold resistance to four ester-bonded phenoxybenzyl alcohol pyrethroids (fenvalerate, deltamethrin, cypermethrin and cyhalothrin), >205-fold resistance to a non-ester phenoxybenzyl alcohol pyrethroid (etofenprox) and only 19-fold resistance to an ester-bonded methylated biphenyl alcohol pyrethroid (bifenthrin). The oxidase inhibitor piperonyl butoxide eliminated most the of resistance to fenvalerate, deltamethrin, cypermethrin, cyhalothrin and etofenprox, whereas the esterase inhibitor S,S,S-tributylphosphorothioate had a small synergistic effect for fenvalerate and cyhalothrin only. This suggests that the resistance to these pyrethroids in the YS-FP strain was mainly because of enhanced oxidative detoxification. The monooxygenase activities of the midguts of sixth-instar larvae of the YS-FP strain to substrates p-nitroanisole, ethoxycoumarin and methoxycoumarin were 3.7-, 4.7-and 10-fold, respectively, compared with that of the YS strain. Glutathione S-transferase activity and esterase activity were not significantly altered in the YS-FP strain. This confirms that enhanced oxidative detoxification was a major mechanism contributing to pyrethroid resistance in the YS-FP strain.

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