The combined effects of the aquatic herbicide fluridone and the lampricide TFM on non-target fish and invertebrates

Invasive sea lamprey (Petromyzon marinus) and exotic aquatic plants like Eurasian watermilfoil (Myriophyllum spicatum) pose environmental and economic threats to aquatic ecosystems, sometimes within the same water bodies. The lampricide TFM (3-trifluoromethyl-4-nitrophenol) and the aquatic herbicide fluridone [1-methyl-3-phenyl-5-(3-trifluoromethylphenyl)-4(1 H)-pyridinone] are routinely used to control these invasives in the Great Lakes and other water bodies, but the combined toxicity of these two pesticides for non-target aquatic animals has not been reported. Here, we examine the acute toxicity of TFM in combination with fluridone for three non-target species: fathead minnows (Pimephales promelas), brown trout (Salmo trutta), and the freshwater microcrustacean Daphnia magna. The average toxicity of TFM was higher in the presence of fluridone for all three species, although this difference was not statistically significant at all concentrations and time points. Lethal times to 50% were lower for both fish species in the presence of fluridone at the TFM concentrations used in treatments. Brown trout, upon which Maximum Allowable Concentrations of TFM are based, was the least sensitive of the test species to TFM at all time points and concentrations. Although our data indicate that fluridone does increase the toxicity of TFM when close to the maximum concentration recommended by the EPA (12ppm), this increase in toxicity is likely to be within currently accepted limits of non-target mortality in most field application targets (typically <7 ppm). Given the potential presence of other contaminants, however, on-site toxicity tests should still be used whenever possible to evaluate acute mortality of non-target species.