ASSESSMENT OF RAINWATER-MEDIATED DISPERSION OF FIELD-SPRAYED BACILLUS-THURINGIENSIS IN THE SOIL

The dispersion of field-sprayed Bacillus thuringiensis in the soil was assessed under naturally and artificially irrigated conditions. In tests on two field plots (mulberry plantations), there was no translocation of sprayed B. thuringiensis into the soil down to a depth of 10 cm, under a continuously rainwater-irrigated condition of the summer rainy season for about one month. When B. thuringiensis-sprayed soils were artificially irrigated with water equivalent to a 450-mm rainfall, B. thuringiensis was detected in the soil down to a depth of 3-6 cm. In irrigation tests using soil columns, B. thuringiensis was not capable of passing through the column of volcanic ash soil, but a few bacteria were detected in the flow-through water from a column of alluvium sand. The results suggest that the major factor causing the decrease in the level of B. thuringiensis is not a physical dilution due to the rainwater-mediated dispersion of bacteria into the soil.