Effects of Bt transgenic cotton lines on the cotton bollworm parasitoid Microplitis mediator in the laboratory

Microplitis mediator (Haliday) is an important endoparasitoid of the cotton bollworm, Helicoverpa armigera (Hübner) in northern China. Interactions among H. armigera, its larval parasitoid M. mediator, and insect-resistant transgenic cotton lines were evaluated under laboratory conditions. Two major transgenic cotton cultivars used in Hebei province of northern China, DP99B (Bollgard), carrying the cry1Ac gene, and SGK321, carrying both cry1A and CpTI (Cowpea trypsin inhibitor) genes, were used in the experiments. The results indicated that there was significant growth inhibition of the H. armigera larvae when they were fed a diet containing Bt transgenic cotton powder. The parasitoid offspring developed more slowly and pupal and adult weight was reduced significantly when the parasitized host larvae fed on the Bt cotton powder leaf diet compared with non-Bt treatment. With an increase of Bt cotton leaf powder concentration in the host larvae diet, the parasitism rate and adult emergence of the parasitoid decreased and the abnormal pupal rate increased. There was no evident difference in the effects on M. mediator between the transgenic single- and two-gene cotton cultivars; however, the parasitized host larval mortality was higher than that of unparasitized larvae in most treatments. The observed effects on M. mediator were probably host-quality mediated rather than direct effects of transgenic cotton because the H. armigera larvae which fed on diet with leaf powder of both transgenic cotton cultivars also experienced a significant decrease in weight, particularly when the host larvae were parasitized.