Studies On The Toxic Effects Of Bacillus Thuringiensis On Ovomermis Sinensis

The crystal-formingδ-endotoxins produced by Bacillus thuringiensis(Bt) are the most widely used natural insecticides in agriculture. In this paper, we had studied the selective toxicity of Bt spore-crystal proteins and Bt preparations to Ovomermis sinensis, and the effects of cotton bollworm {Heliothis armigera) feeding on transgenic Bt cotton's leaves to O. sinensis.We had selected 10 Bt strains. The ELISA results indicated that 7 of them contained both Cry1Ac and Cry5A proteins, and others only contained Cry1Ac proteins. We demonstrated that the Bt crystal protein Cry5A from 7 Bt strains with different concentration were lethal in vitro to eggs of the mermithid nematode, O. sinensis which had considerable potential for the biocontrol of H. armigera. The most effective strain's LD₅₀ values were 0.1503,0.1313 and 0.0182μg total protein/ml respectively at 24, 48 and 72h. Pre-parasitic juveniles of O. sinensis were unaffected by Bt toxin in vitro. Exposure of infective juveniles to Bt toxins for 72h had no effect on their infectivity in H. armigera. Parasitic juveniles of O. sinensis could grow normally in H. armigera which were fed on cotton's leaves dealing with low concentration's Bt toxins. Exposure of post-parasitic juveniles to Bt toxins had no effect on their molt and development. The Bt crystal protein Cry1Ac were no toxicity to the whole lifecycle of O. sinensis.The O. sinensis' parasitic rates were influenced by transgenic Bt cotton GK19 and Xinmian338. Compared with Sumianl2 (CK), the parasitic rate with GK19 decreased in 50.6% as well as Xinmian338 decreased in 48.7%. The average infective intensity of them also decreased in 58.1% and 51.9%. And the rates of male with GK19 and Xinmian33⁸ were 92.7% and 94.5% as well as 20.1% with Sumianl2 (CK).The sensitivity of O. sinensis' pre-parasitic juveniles to field concentration of seven Bt preparations was evaluated. Sutai and Luyin fell into deleterious preparations. Bili, Nongtai, Bt/shachongdan and ABG-6444 fell into low toxicity preparations. Bt soluble powder was harmless to O. sinensis. The toxicity of Sutai was the highest, which was 66.67% just with 12h and it could reach 81% after 36h. Studies on sublethal effects of Bt soluble powder on O. sinensis suggested that the parasitic rate of the group P, P-Bt and Bt-P had not significant difference with each other. The highest one was the group of Bt-P. But the death rate in 7 days to cotton bollworm of the three groups had significant difference with each other and the highest one was the group of P-Bt which was 96.8%. The pre-parasitic juveniles of O. sinensis could not distinguish between Bt-treated and untreated larvae of cotton bollworm.