Abstract: Two strains of the cotton bollworm, Helicoverpa armigera (Hübner), one is a susceptible laboratory strain and the other is collected from Bt cotton fields, were used as the insect hosts to study the effect of Bt insecticidal protein on the development of the parasitoid, Microplitis mediator (Haliday). The results showed that the larval duration of the parasitoid was delayed, and the pupal weight, body weight of the newly emerged adult and adult longevity decreased significantly when the hosts larvae fed on diet containing Bt protoxin Cry1Ac at the concentrations of 0.5－8.0 μg/g in all time or from 12 h before parasitismtill pupation of the parasitoid . Compared with the control, the larval weight and pupal weight of H. armigera decreased significantly when the larvae fed on diet containing cry1Ac. Pupation rate of H. armigera decreased significantly in the treatment with diet containing Cry1Ac at 4 .0 μg/g. When H. armigera larvae fed on the transgenic cott on leaves expressing both Cry1A and CpTI proteins, the mortality of the laboratory strain was 48.5%－95.8% in 2－5 days after treatment, which was significantly higher than that of the field strain (30.9%－86.1%). It is concluded that feeding on diet containing Bt insecticidal protein, both strains of the cotton bollworm will have significantly negative effects on deve lopment of the parasitoid, M. mediator.

Key words: Helicoverpa armigera, Microplitis mediator, Bt protoxin, transgenic cotton, development