Abstract: [Objective]The indoor virulence and field control test results showed that the mixture of Bt and chlorbenzuron had synergism to the second and third generation of the larvae of Hyphantria cunea. This paper analyzed the influence on the activity of midgut protease and haemolymph proteins on the 4^th instar larvae of H. cunea for exploring its mechanism of synergism.[Method]The biochemical determination method was used to detect the mixture treatment effect on the activity of midgut protease and haemolymph proteins on the 4^th instar larvae of H. cunea. [Result] In 6 h treatments with 5 concentrations of Bt, chlorbenzuron and its mixture to the 4^th instar larvae, the activity of midgut protease was all lower than that of control, but there were significant differences among treatments. Compared with control the larval midgut protease activity decreased significantly after 24 h treatments with different concentrations, and the activity was negatively correlated with treatment times. Test results indicated that the activity of midgut protease was inhibited significantly by the mixture of Bt and chlorbenzuron, which directly affected the feeding of larvae. After being treated for 6 h with 5 concentrations of the mixture of Bt and chlorbenzuron, the content of haemolymph protein was 15.8, 18.0, 19.7, 23.4 and 13.9 mg·mL^-1 of the 4^th instar larvae and was higher than the content of 14 mg·mL^-1of the control. The higher content of haemolymph protein in the treated larvae suggested that the larvae initiated defense responses; After being treated for 12 h the larvae were consumed or damaged the content of haemolymph protein, and the content of haemolymph protein decreased significantly, and the treatment concentration were negatively correlated with the treatment time; The content of haemolymph protein of the treated group was 6.2, 8.9, 2.3, 3.2 and 9.0 mg·mL^-1 respectively after being treated for 72 h, and they were all significantly lower than 13.3 mg·mL^-1of the corresponding control. The content of haemolymph protein significantly decreased compared with the control group (P < 0.01). [Conclusion]The inhibition of the mixture of Bt and chlorbenzur on the activity of midgut protease and haemolymph protein of the 4^th instar larvae of H. cunea was stronger than that of the single agent, the mixture has strong inhibition and synergistic effect so as to accelerate and increase the death of the treated 4^th instar larvae of H. cunea.

Key words: Hyphantria cunea, mixture of Bt and chlorbenzuron, the activity of midgut proteinase, haemolymph proteins, synergistic effect