Influence of Fonofos on the Distribution and Metabolism of¹⁴C-Terbacil in Peppermint

When the insecticide fonofos ( O -ethyl- S -phenylethylphosphonodithioate) was added to a nutrient solution containing 14 C-terbacil (3- tert -butyl-5-chloro-6-methyluracil), movement of total radioactivity into leaves of peppermint ( Mentha piperita L. cv. ‘Todd's Mitchum’) during a 3-day treatment period approximately doubled. In addition, a higher percentage of this total radioactivity was identified as intact terbacil. Total radioactivity in the leaves was not significantly increased when terbacil was added to 14 C-fonofos in nutrient solution. Studies also were conducted to measure the effect of fonofos upon the rate of terbacil metabolism in peppermint. Peppermint plants were treated for 3 days with nutrient solution containing 14 C-terbacil alone or 14 C-terbacil plus fonofos. The percentage of total radioactivity which remained as intact terbacil at the end of the treatment period was five times as high in plants treated with the combination as in those treated with terbacil alone. The relative rate of degradation during the succeeding 11 days was approximately the same in plants subjected to the two treatments. These results could account for damage to peppermint from closely timed applications of fonofos and terbacil in the field.