Photodegradation of Bentazon, Clopyralid, and Triclopyr on Model Leaves: Importance of a Systematic Evaluation of Pesticide Photostability on Crops BORIS EYHERAGUIBEL,ALEXANDRA TER HALLE, AND CLAIRE RICHARD* Laboratoire de Photochimie Mole ´culaire et Macromole ´culaire, UMR CNRS-Universite ´ Blaise Pascal no. 6505, 63177 Aubie `re Cedex, France 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. KEYWORDS: Bentazon; clopyralid; triclopyr; photolysis; cuticular wax; adjuvants; pesticides INTRODUCTION Photolysis is an important degradation process influencing the fate of pesticides in the environment. The abundant literature on pesticide photodegradation reports various reactions pathways (1). The mechanisms can differ according to the nature of the compound, the environmental segment studied (soil, water, plant, vegetation), and the experimental conditions (pH, incident photon flux) (2). However, there is relatively little information on the photochemical decomposition of the active ingredient (AI) once it is deposited on the leaf surface. Most pesticides are sprayed onto cultures. Once the droplets are dried, the AI remains sorbed on the leaf surface, where it is subjected to different environmental variables. Information about the pho- todegradation on leaves under various environmental conditions is essential to estimating the importance of this dissipation path in the field. The photolability of pesticides on crops has some important consequences. First, photolability is sometimes counterbalanced with an increase in application frequencies or with higher application rates (3). Second, the photoproducts can accumulate on the crops, making the evaluation of their potential side effects imperative. In the present climate of reduction in pesticide use and Good Plant Protection Practice, a systematic evaluation of the photostability of ingredient on crops is warranted. Pesticide photolysis on vegetation occurs mainly on the leaf surface (4-7). Once the compound is inside the leaf, photo- chemical reactions are slowed due to light attenuation and plant metabolism. Systems such as isolated cuticles or thin cuticular wax films are often used to model the leaf surfaces (5, 7-11). In the field, agrochemicals are used as formulated. Adjuvants are employed to improve the performance of the active ingredients (12). They enhance herbicidal activity and charac- teristics of the application (better distribution, drift reduction) but must be used with caution to avoid volatility or shelf-life alteration (13). Very few studies have considered the effect of additives on the transformation rate of the AI on leaves. Nevertheless, it has been shown that the herbicide formulation could significantly promote photoreactions (10, 14, 15). In the present study, three herbicides were selected due to their widespread application in agriculture for broadleaf weed control (figure 1). Bentazon is a member of the thiadiazine group, acting as a photosynthetic electron transfer inhibitor (16). Triclopyr and clopyralid are from the chemical class of pyridine; they affect both plant cell respiration and growth (17-19). Triclopyr and clopyralid are sometimes used in combination, as in the case of the commercial formulation Garlon. The three molecules show distinct spectral absorption proper- ties in water (Figure 1). They all absorb in the sunlight spectral region and are all likely to be phototransformed under outdoor conditions. However, their spectra overlap with sunlight very differently. Bentazon has the broadest absorption spectrum, up to 375 nm, whereas clopyralid absorbs only slightly above 290 * Corresponding author (telephone 00 33 04 73 40 71 42; fax 00 33 04 73 40 77 00; e-mail Claire.Richard@univ-bpclermont.fr). 1960 J. Agric. Food Chem. 2009, 57, 1960–1966 10.1021/jf803282f CCC: $40.75  2009 American Chemical Society Published on Web 02/17/2009