RESEARCH ON NATURAL STRAINS OF BACILLUS-T HURINGIENSIS IN DIFFERENT BIOTOPES THROUGHOUT THE WORLD

A screening programme to isolate new strains of the entomopathogenic b acteria Bacillus thuringiensis was undertaken on 4887 samples of vario us sources from 101 countries over the world: 1260 strains of the baci llus were isolated. Dust from mills and silos, as well as insects from nature, were more successful sources than soil samples, which emphasi zes the diversity of biotopes where the bacillus is encountered. Elect rophoretic characterization reveals the genetic variability of the spe cies. An analysis of insecticidal properties of the isolated strains w as performed on four insect species: Plutella xylostella (Lepidoptera: Plutellidae), Spodoptera littoralis (Lepidoptera: Noctuidae), Phaedon cochleariae (Coleoptera: Chrysomelidae), and Locusta migratoria or Sc histocerca gregaria (Orthoptera: Acrididae). The most frequent strains (54%) were producing crystals constituted of proteins with molecular masses of 130-140 or 66 and 130-140 kDa and were toxic to Lepidoptera larvae. A significant number of strains (31) were larvicidal to Coleop tera while only one, H14 serotype, was active on Diptera. Numerous str ains synthesize crystals made up of proteins with size differing from the already known toxins. Most of these strains were nonactive against the four insect species tested. One strain showing a protein band at 73 kDa had no insecticidal activity against P. cochleariae while it wa s toxic to P. xylostella.