Nicotinamide adenine dinucleotide phosphate (NADPH)-cytochrome P450 reductases (CPRs) function as redox partners of cytochrome P450 monooxygenases (P450s). CPRs and P450s in insects have been found to participate in insecticide resistance. However, the CPR of the moth has not been well characterized yet. Based on previously obtained transcriptome information, a full-length CPR cDNA of () was PCR-cloned. The deduced amino acid sequence contains domains and residues predicted to be essential for CPR function. Phylogenetic analysis with insect CPR amino acid sequences showed that is closely related to CPRs of Lepidoptera. Quantitative reverse transcriptase PCR (RT-qPCR) was used to determine expression levels of in different developmental stages and tissues of . expression was strongest at the sixth-instar larvae stage and fifth-instar larvae showed highest expression in the midgut. Expression of in the midgut and fat body was strongly upregulated when fifth-instar larvae were exposed to phoxim at LC (4 μg/mL) and LC (20 μg/mL) doses. RNA interference (RNAi) mediated silencing of increased larval mortality by 34.6% (LC dose) and 53.5% (LC dose). Our results provide key information on the gene and indicate that expression levels in larvae influence their susceptibility to phoxim and possibly other insecticides.