Photodegradation of bentazon, clopyralid, and triclopyr on model leaves: importance of a systematic evaluation of pesticide photostability on crops.

Photolyses of three herbicides, bentazon, clopyralid, and triclopyr, were studied on plant leaves after crop treatment. The experiments were carried out on cuticular wax films, which are good models for leaf surfaces. The pure compounds and their commercial formulations were investigated under simulated solar light. At the recommended agricultural application rates, the three formulated herbicides photolyzed more rapidly on films than on soil or in water. Their photolysis is likely to be an important dissipation path from crops after treatment. The effects induced by the adjuvants in formulations were varied. Adjuvants slowed the photodegradation of bentazon slightly. In Garlon, in which triclopyr and clopyralid are combined, the adjuvants did not affect the photolysis of clopyralid even though they accelerated the rate of photolysis of triclopyr by a factor of 7. The kinetics were also affected by the application rates. This work also underscores the importance of assessing the photoreactivity of active ingredients in conditions similar to those of their application.