The Synergism Of Bacillus Species On Larvicidal Activity Of Bacillus Thuringiensis

Chapter 1: This is the first investigation devoted exclusively to analyze the distribution of chitinase in Bacillus thuringiensis strains, and the enhancing effects of the chitinase-producing Bt strains on insecticidal toxicity of active Bt strain against Spodoptera exigua larvae It infers that the chitinase produced by Bt might play a role in the activity of the biopesticide. The chitinolytic activities of Bt strains representing the 70 serotypes were investigated by the whitish opaque halo and the colorimetric method. Thirty-eight strains produced different levels of chitinase at pH 7.0, and so did 17 strains at pH 100 The strain T04A001 exhibited the highest production, reaching a specific activity of 355 U/ml in liquid medium. The chitinase gene in some chitinolytic strains was tested by PCR. SDS-PAGE and Western blotting showed that the chitinase produced by some Bt strains had a molecular weight of about 611cDa, and they had high homology to that of Ha SNPV. The bioassay results indicated that the chitinase-producing Bt strains could enhance the insecticidal activity of Bt strain DL5789 against S. exigua larvae, with an enhancing ratio of 2 35-fold. Chapter 2: This chapter is devoted to create a mixed culture technique between the active B. thuringiensis and B. cereits, or active B. thuringiensis and chitinolytic B. thuringiensis based on the screening of synergistic strains enhancing the toxicity of DL5789, an active B. thuringiensis strains against S. exigua. We found that the B. cereiis strain Ym2102 could grow with DL5789. The LCo of final whole mixed-culture reached 0.218nL/g against larvae of .V. exigua, in comparison with 1.047uL/g of the DL5789 preparation alone, with enhancing ratio of 4.81-folds. It has been proved that the toxicity of the preparation of DL5789 against 2nd and 3nd instar larvae of S. exigua was very low, with LC.io values of only 2.151uL/g and 4.115uL/g, respectively; whereas the toxicity of mixed culture was relatively high, having LCso values of 0.387uL/g and 0.743uL/g to S. exigua, respectively. In addition, the mixedculture can also enhance the toxicity of the strain against larvae of the Heliolhis armigera, inferring that the preparation of mixed culture might have synergist against different pest The fermentation property studies indicate that the number of bacteria, pH and toxicity development in mixed culture are different from those in DL5789 culture alone The preliminary research on the mechanism states that the synergism may contribute to the augmentation of the active bacteria in final mixed culture. The supernatant of culture of Ym2102 has some enhancement on the toxicity of DL5789, but limitative. The further research of the mechanism of mixed culture is in progress...