POTENTIAL CONTROL OF HYDRILLA AND EURASIAN WATERMILFOIL UNDER VARIOUSFLURIDONE HALF-LIFE SCENARIOS

Fluridone yl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} efficacy against Eurasian watermilfoil (Myriophyllum spicatum L.) and hydrilla (Hydrilla verticillata (L.f.) Royle) was evaluated by simulating herbi cide degradation/dissipation half-lives (t(1/2)) under controlled-envi ronment conditions. Fluridone treatment rates of 100 mu g/L for 7, 10, and 14 d t(1/2), 50 mu g/L for 14 and 21 d t(1/2), 25 mu g/L for a 28 d t(1/2), and static treatments of 5 and 15 mu g/L for 105 d were tes ted. Chlorophyll content and net photosynthesis were measured at 7, 28 , 56, 77, and 105 d posttreatment, and biomass was collected at 28, 56 , 77 or 84, and 105 or 108 d posttreatment to assess efficacy. The 7 a nd 10 d t(1/2) treatments dissipated to 0 mu g/L at 42 and 62 d respec tively resulting in rapid recovery of both hydrilla and Eurasian water milfoil. The 14 d t(1/2) resulted in exposures of 82 to 84 d, and alth ough hydrilla biomass remained significantly reduced, physiological re covery indicated the potential for biomass recovery. Eurasian watermil foil was completely controlled at 84 d posttreatment following both 14 d t(1/2) treatment. The 21 and 28 d t(1/2) and static treatments main tained low fluridone exposures throughout the 105 d study period. Thes e treatments reduced hydrilla biomass significantly (>90%), and physio logical variables showed no evidence of recovery at completion of the study. These treatments also resulted in near 100% control of milfoil biomass by the 84 d harvest. Results indicate that following initial i njury at higher treatment levels, extremely low levels of fluridone (1 to 3 mu g/ L) prevent recovery. Herbicidal activity of these low conc entrations may explain the long-term control achieved with fluridone i n situations that allow long degradation half-lives (>14 d).