Environmental context. Pesticide has greatly contributed to the increased yields and regular production in agriculture; however, its use is nowadays more and more being questioned. Current authorisation procedures are more and more comprehensive (evaluating their environmental fate and impact on ecosystems and on human health) but are not yet fully satisfactory. Here we demonstrate the utility of evaluating the stability of pesticide towards sunlight when applied on crops. Abstract. We developed a methodology to perform laboratory studies that approach field conditions. To demonstrate our methods, we used the herbicide mesotrione. Simulated solar light irradiation experiments were conducted on several cuticular wax films. Adjuvants greatly favoured the rate of mesotrione photolysis. Specifically, the photolytic transformation of formulated mesotrione was 15 times faster than that of pure mesotrione. The morphology and composition of the wax films had a greater effect on the photolysis of formulated mesotrione than of pure mesotrione, which formed aggregates on the waxes. This shows the importance of considering the formulation when studying pesticide photolysis on crops. To corroborate our model, we conducted experiments on detached leaves. The rates of photolysis on leaves and on wax films were of the same order of magnitude. Finally, the mesotrione rate of photolysis on leaves is much higher compared with data from the literature in water and on soils. Photolysis is likely to be a fast dissipation process on crops. The photodegradation of pesticide on crops should be systematically evaluated.