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Bacillus thuringiensis  ( Bt ) and its insecticidal toxins are widely exploited in microbial biopesticides and genetically modified crops. Its population biology is, however, poorly understood. Important issues for the safe, sustainable exploitation of  Bt  include understanding how selection maintains expression of insecticidal toxins in nature, whether entomopathogenic  Bt  is ecologically distinct from related human pathogens in the  Bacillus cereus  group, and how the use of microbial pesticides alters natural bacterial populations. We addressed these questions with a MLST scheme applied to a field experiment in which we excluded/added insect hosts and microbial pesticides in a factorial design. The presence of insects increased the density of  Bt/B. cereus  in the soil and the proportion of strains expressing insecticidal toxins. We found a near-epidemic population structure dominated by a single entomopathogenic genotype (ST8) in sprayed and unsprayed enclosures. Biopesticidal ST8 proliferated in hosts after spraying but was also found naturally associated with leaves more than any other genotype. In an independent experiment several ST8 isolates proved better than a range of non-pathogenic STs at endophytic and epiphytic colonization of seedlings from soil. This is the first experimental demonstration of  Bt  behaving as a specialized insect pathogen in the field. These data provide a basis for understanding both  Bt  ecology and the influence of anthropogenic factors on  Bt  populations. This natural population of  Bt  showed habitat associations and a population structure that differed markedly from previous MLST studies of less ecologically coherent  B. cereus  sample collections. The host-specific adaptations of ST8, its close association with its toxin plasmid and its high prevalence within its clade are analogous to the biology of  Bacillus anthracis . This prevalence also suggests that selection for resistance to the insecticidal toxins of ST8 will have been stronger than for other toxin classes.

Environmental Factors Determining the Epidemiology and Population Genetic Structure of the Bacillus cereus Group in the Field