Efficacy of AVAST! Fluridone Formulation Against Eurasian Watermilfoil and Nontarget Submersed Plants

Experiments were conducted in a laboratory and an outdoor mesocosm system to evaluate the liquid AVAST!® fluridone formulation for control of Eurasian watermilfoil (Myriophyllum spicatum). In addition, physiological assays were used to monitor plant injury during herbicide exposure. Eurasian watermilfoil was planted in 52-L aquaria, grown to precanopy condition (21 days), then dosed with 0, 10, 20, 40, and 80 μg L active ingredient (ai) fluridone for exposure times of 45 and 90 days. Elevated phytoene concentrations indicated herbicide exposure in all treated plants at 7 days after treatment (DAT). Visually, plants did not manifest symptoms of injury from fluridone until 14 DAT. β-carotene concentrations suggested that fluridone disrupted photosynthesis as soon as 7 DAT. Both formulations were effective in controlling Eurasian watermilfoil. Biomass decreased by 90 percent at all application rates following the 45-day exposure and decreased by 99 percent following the 90-day exposure time. No significant differences occurred between application rates at either exposure time. Based on these results, rates of 6, 12, and 24 μg ai L fluridone were applied to Eurasian watermilfoil and four nontarget submersed aquatic species in an outdoor mesocosm system for a 56-day exposure time. Gradual herbicide dissipation in the mesocosms resulted in half-lives ranging from 23 to 24 days. Again, the AVAST!® fluridone formulation was effective in controlling Eurasian watermilfoil. Biomass was reduced by >85 percent at all doses compared to the untreated reference. The AVAST! formulation did not significantly decrease biomass from wild celery (Vallisneria americana) and Illinois pondweed (Potamogeton illinoensis) at any application rate. Biomass levels from elodea (Elodea canadensis) and sago pondweed (Stuckenia pectinata) were reduced at 12 and 24 μg ai L. Symptoms of injury were evident in these plants by depressed β-carotene pigment concentrations at 28 DAT; however, β-carotene levels increased in sago pondweed by 56 DAT, suggesting recovery. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.