Exogenous double-stranded RNA (dsRNA) is a potent initiator ofgene silencing in diverse organisms.In the present study, we used genetically engineered bacterial strain HTll5 to express dsRNA corresponding to CYP6B6 of Helicoverpa armigera, which is an economical way to produce large quantities of dsRNA.After we investigated the effect of pH, saline solution, hemolymph plasma and time on the stability of the dsRNA from the bacteria in vitro, we tested the effect of the exogenous dsRNA on the transcription and translation of larval CYP6B6, larval growth, development and the insecticide tolerance of the cotton bollworm after ingestion of the engineered bacteria.The stability analysis showed that the dsRNA from the engineered bacteria remained unchanged for 24 h in pH 7.0 KH2PO4/K2HPO4 buffer at room temperature.Both the qPCR and immunohistochemistry results showed that obvious decrease in CYP6B6 decreased compared with the corresponding controls, and the larval growth and development were significantly retarded, the rate of pupation declined, and insecticide tolerance decreased.Thus, the data show that CYP6B6 silencing can disturb the growth and development of cotton bollworm and also reduce its insecticide tolerance.These results provide a good foundation for applying RNAi strategies targeting insect P450 genes by transforming cotton or other plants for protection against the cotton bollworm.