Screening Bacillus Thuringiensis Strains Active Against Beet Armyworm And Studies On Strain DL-5789

To screen for highly active Bacillus thuringiensis strains against Beet Armyworm (Spodoptera exigua), the toxicity of 100 wild-type B. thuringiensis isolates to S. exigua was determined by bioassays, among which 23 were highly-toxic strains. By the regression analysis, 10 isolates exhibited activity to the neonate S. cxi gua larvae with high correlation to the concentrations exposed to the larvae. The minimum 50% lethal concentration (LC5Q ) value of the isolates, which was that of CN72, was 2.393 ii L culture per gram diet (u L/g). The mortality of other 13 isolates was also determined and showed that 13 isolates were highly active, among which CN33 produced 100% mortality when added at 5 Is L/g. The data laid the foundations for further study.On the other hand, the studies were conducted on Bacillus thuringiensis strain DL-5789 isolated by this lab , which was outstandingly toxic to Tobacco Cutworm (Prodenia litura) and had an unusual excreted insecticidal protein. The LC50s of the DL-5 789 fermented broth were 1.04-, 1.14- and 0.47 IsL /g, respectively, to neonate and one-day-old S. exigua larvae, and neonate Cotton Bollworm (Helicoverpa armigera) larvae by bioassays at 26(2. The bioassays also showed that DL-5789 had the potency of 2245 IU/ is L to neonate H armigera larvae. The bipyramidal and cuboidal crystals were observed in transmission electron microscope obtained from strain DL-5 789 alongside the sporangium during sporulation. The SDS-PAGE indicated that the toxins produced by this strain were maily composed of 135-, 62-, 35- and 23.7 kDa proteins.In comparison of all the components in the DL-5789 fermented broth, the supernate singly showed high toxicity to S. exigua larvae. However, this component was heat-sensitive deduced by the fact that nearly 90% mortality was lost when treated at 80C for 20 minutes. Only one 64 kDa protein band appeared in SDS-PAGE gel when extracted and purified from the supernate.Moreover, strain DL-5789 contained crylC gene, neither crylE nor cryl F, identified by the electrophoretic pattern of the PCR products obtained by using specially designed primers to probe crylC, cry iF and crylF genes.