The effect of mixing two herbicides mesotrione and nicosulfuron on their photochemical reactivity on cuticular wax film.

There are growing concerns about the environmental impacts of pesticides with a consequent increasing demand for meaningful and accurate information on their consumption and use pattern. Pesticides reactivity towards light is rarely considered at the leaf surface after crop treatment although it is directly impacting the pesticide efficiency. We have established a test on thin wax films which mimic well the leaf surface. Under simulated solar light herbicides mesotrione and nicosulfuron as well as their mixture are photoreactive. For both herbicides considered separately, there is a strong accelerating effect of adjuvants compared to the pure active ingredient. Mesotrione, a triketonic herbicide was very reactive with a half-life of photolysis of 2h. Nicosulfuron, a sulfonylurea, has a half-life of photolysis of 32 h. The mixing of the two formulated herbicides had an accelerating effect on both herbicide decay rates. The products formed when considering the pure molecule were mainly the result of photohydrolysis, oxidation and cyclisation. While pesticide photostability on crops is not evaluated for registration, this study demonstrates that these photochemical tests could be useful to improve the pesticide effectiveness and reduce the applied dose.