Behavior of isoprothiolane and fipronil in paddy water, soil, and rice plants after nursery-box or submerged applications.

We investigated the behavior of isoprothiolane and fipronil after nursery-box application and that of isoprothiolane after submerged application in an experimental paddy field. The concentrations of the pesticides and their metabolites were monitored in paddy water, soil, and rice plants. The distribution profile for isoprothiolane mass in the field differed greatly between the nursery-box and submerged applications. The nursery-box-applied pesticides were mostly distributed in soil near the transplanted rice seedlings (root zone), versus little distribution in paddy water and rice plants (<1.1 and <0.3% of the applied mass, respectively). The residual levels in rice plants were similar to those in the root-zone soil. To estimate the soil pesticide mass, we defined a key parameter: the ratio of the root-zone area to the total area of the paddy field estimated to be 0.1 to 0.15. This parameter is important when evaluating the concentrations of nursery-box-applied pesticides in soil and rice plants.