New arylpicolinate herbicide chemistry under development for rice, aquatic weed management, and other uses was evaluated using five aquatic plants. The herbicide 4-amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxyphenyl)-5-fluoro-pyridine-2-benzyl ester—also identified as XDE-848 BE or SX-1552 (proposed International Organization for Standardization common name in review; active tradename RinskorTM)—and its acid form (XDE-848 acid or SX-1552A) were evaluated on three dicots: (1) Eurasian watermilfoil (EWM), (2) megalodonta, and (3) crested floating heart (CFH), and two monocots: (1) hydrilla and (2) elodea. A small-scale Organization for Economic Cooperation and Development (OECD) protocol developed using EWM for registration studies was utilized. EWM and megalodonta were also evaluated in larger-scale mesocosms for comparison. In-water concentrations between 0.01 and 243 μg ai L−1 as SX-1552 or SX-1552A were applied under static conditions for 14 (growth chamber) or 28 d (mesocosm). EWM was susceptible to both SX-1552 and SX-1552A, with dry-weight 50% effective concentration (EC50) values of 0.11 and 0.23 μg ai L−1 under growth chamber conditions. Megalodonta had EC50 values of 11.3 and 14.5 μg ai L−1 for the SX-1552 and SX-1552A. CFH was more sensitive to SX-1552 (EC50 = 5.6 μg ai L−1 ) than to SX-1552A (EC50 = 23.9 μg ai L−1). Hydrilla had EC50 values of 1.4 and 2.5 μg ai L−1, whereas elodea was more tolerant, with EC50 values of 6.9 and 13.1 μg ai L−1 for SX-1552 and SX-1552A, respectively. For EWM mesocosm trials, EC50 values for SX-1552 and 1552A were 0.12 μg ai L−1 and 0.58 μg ai L−1, whereas the megalodonta EC50 was 6.1 μg ai L−1. Activity of SX-1552 on EWM, hydrilla, and CFH merits continued investigation for selective aquatic weed control properties. Results suggest that the OECD protocol can be used to screen activity of herbicides for multiple aquatic plant species.
Nomenclature: 4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxyphenyl)-5-fluoro-pyridine-2-benzyl ester; crested floating heart, Nymphoides cristata (Roxb.) Kuntze; elodea, Elodea canadensis Michx.; Eurasian watermilfoil, Myriophyllum spicatum L.; hydrilla, Hydrilla verticillata L.f. Royle; megalodonta, Bidens beckii Torr. Ex Spreng.