*3.4. Half-Lives*

According to the best fitting models, difenoconazole and spinosad dissipated following a pseudo 1st-order kinetics that only was a function of the concentration of the compounds. That means that the concentration declined exponentially and their half-lives, (the time needed to reach half of the initial concentration) remained constant and the clearance times, if needed, could be easily estimated. The half-life of difenoconazole and spinosad were 19.2 and 5.8 day, respectively. Imidacloprid and pyracostrobin half-lives determined with the best fitting models were 4.1 and 39.8 day, respectively. In these cases, the mathematical description of the pesticide dissipation were RF 1st-order and 2nd-order, respectively. These types of decay are concentration-dependent, meaning that the time needed to reach half of the concentration in the fruit (the pesticide half-life) will be a function of the pesticide level on the fruit at the starting time considered.

This is the first report of difenoconazole, spinosad and pyraclostrobin dissipation on Clementine mandarins. Imidacloprid showed almost the same half-life as that previously determined in the rind of sweet orange, which was 3.87 day [35].

Previous reports determined 3.5–5.3 day for the half-life of pyraclostrobin in fruit such as banana [45], strawberry [46], tomato [47], values that are 7 to 10 times lower than those reported in the present work (39.2 day). These results may partially be due to differences between the species, application doses, formulation, local environment, and crop growth characteristics [17–19]. It should be taken into account that citrus fruit have an important wax layer where due to the high octanol/water coefficient (Kow) of this pesticide, it could be retained. These facts could explain the deviation from a normal 1st-order kinetics behavior. It also must be taken into account that pyraclostrobin (pKow = 3.99) could dissolve in the essential oil vesicles of the citrus fruit. The dissolution of the pesticide in the oil sacs would hamper its direct contact with the environment, to which it dissipates. The pesticide that is not solubilized in the essential oil would dissipate quickly from the fruit surface and, afterwards, a slow release from the oil would change the speed of decay of pyraclostrobin.

The determined half-life for spinosad was 5.8 day. The short t1/2 of this insecticide is influenced by light, especially the ultraviolet (UV) component [48]. Also, residue decline may be attributed to hydrolysis, biodegradation, or growth dilution [49]. Although there are no specific studies of dissipation of spinosad on mandarins reported in literature similar values of half-lives are reported for this pesticide in other vegetables and fruit. Values in the range of 3.5–3.9 day for zucchini [50], 1.7 day for tomato [51] and 1.4 day for cabbage [52] have been reported.

#### *3.5. Estimating Storage Effect on Pesticide Level*

The equations of the dissipation models allowed the estimation of the influence of the postharvest process through the comparison between the pesticide levels obtained after washing, imazalil and wax application and the final cold storage and the calculated ones after 32 d of postharvest storage. The expected concentration values, applying the best fitting models versus actual ones after cold storage at 74 d after spraying were: 0.005 versus 0.012 mg kg−<sup>1</sup> for difenoconazole, 0.035 versus 0.080 mg kg−<sup>1</sup> for imidacloprid and 0.091 versus 0.142 mg kg−<sup>1</sup> for pyraclostrobin, respectively. Therefore, cold storage slowed down the dissipation of these pesticides.
