Bacillus thuringiensis distribution in soils of the United States.

During a 2-year study, samples of various types of soils were collected from 115 fields that had not previously been tested with Bacillus thuringiensis and which were remote from any large-scale aggregations of lepidopterous insects in rearing or grain-storage areas. An average of about 400 isolates were examined from each soil, and, of 46 373 isolates examined, only 250 (0.5%) were identified as B. thuringiensis. While it was almost impossible to insure that a field had never been treated with B. thuringiensis or that drift from some nearby application had not reached the field, it is noteworthy that of the 250 isolates, 156 (62.4%) were not var. kurstaki, the only variety that has been used commercially in the United States in about 10 years. This is a strong indication that the B. thuringiensis isolates observed were present naturally. To verify the procedures used, samples were taken from two adjacent experimental plots which had been treated about 12 months previously with formulations of var. kurstaki and var. galleriae, respectively. With practically no exception, the variety recovered from each plot was the variety applied, indicating that the varietal status of B. thuringiensis is stable in the soil.