BACKGROUND: The function of cytochrome P450 proteins (P450s) in the metabolism of a variety of compounds by oxidation and reduction is well elucidated, but its interactions with other electron transfer components in the pyrethroid resistance of insect pests have been a mystery for a long time. In previous studies the authors cloned and characterised CYP6B7 and cytochrome b(5) (Cyt-b(5)) in the fenvalerate-resistant HDFR strain of cotton bollworm (Helicoverpa armigera Hübner) and showed that CYP6B7 mRNA was overexpressed and important for resistance to fenvalerate. In the present study, the functional interactions of CYP6B7, NADPH-dependent cytochrome P450 reductase (CPR) and Cyt-b(5) were assessed using RNA interference (RNAi) strategies and monitoring for fenvalerate resistance levels. RESULTS: RT-qPCR analyses indicated that the expression levels of CYP6B7, CPR and Cyt-b(5) mRNA were decreased drastically in the midgut of fourth-instar larvae of the H. armigera HDFR strain after corresponding double-stranded RNA (dsRNA) injection, compared with that of the control. The knockdown of CYP6B7, CPR and Cyt-b(5) transcripts was time course dependent during a 12-48 h period after dsRNA injection. At the earlier time points analysed, significant suppression of CYP6B7 mRNA levels was observed in larvae injected with dsCYP6B7-313 as compared with controls, and further suppression was observed in larvae injected with dsCYP6B7-313, dsCPR-403 and dsCyt-b(5) . The injection of dsCYP6B7-313 together with dsCPR-403 and dsCyt-b(5) increased larval susceptibility of the HDFR strain to fenvalerate. CONCLUSION: The results demonstrated that silencing of CYP6B7 alone or CYP6B7 together with CPR and/or Cyt-b(5) increased the susceptibility of H. armigera to fenvalerate, suggesting that CYP6B7, CPR and Cyt-b(5) collaboratively participated in enhanced metabolism of fenvalerate and played an important role in the resistance of H. armigera to fenvalerate.