Effects of the aquatic herbicide, Reward®, on the Fathead minnow and Northwestern Salamander

In this study the commercial formulation of the aquatic herbicide Reward® (active ingredient diquat dibromide [DB]) was tested. In larval and adult Fathead minnows (Pimephales promelas; FHM) after two 24 h pulse exposures spaced two weeks apart the LC50’s of 4.19 mg/L and 6.71 mg/L were obtained, respectively. A lowest observed effect concentration of 1.18 mg/L for larval growth in FHMs during a 7 d continuous exposure was also derived. An acute 96 h LC50 of 71.5 mg/L DB was obtained for Northwestern salamander (Ambystoma gracile) larvae, while a 21 d continuous exposure LC50 was dramatically lower (1.56 mg/L DB) for these larvae. Few reports of environmental concentrations of this pesticide could be found for Canadian waters. Whether aquatic and/or terrestrial applications and/or DB’s persistent nature in sediments translate into acute or sub-chronic toxicity exposure scenarios in Canadian aquatic systems is unknown and should be the focus of future studies.