ACUTE TOXICITY AND HAZARD ASSESSMENT OF RODEO(R), X-77 SPREADER(R), AND CHEM-TROL(R) TO AQUATIC INVERTEBRATES

The herbicide Rodeo(R) provides waterfowl managers with an effective c hemical tool for creating open water habitats in wetlands if its use d oes not adversely affect native invertebrate communities. The survival of caged Chironomus spp. (midge), Hyalella azteca (amphipod), Stagnic ola elodes (pond snail), and Nephelopsis obscura (leech) was assessed in prairie pothole wetlands treated by air with a tank mixture of Rode o(R), the surfactant X-77 Spreader(R), and the drift retardant Chem-Tr ol(R) at a rate recommended for controlling cattails. Laboratory studi es were then conducted to determine the acute toxicities of Rodeo(R), X-77 Spreader(R), and Chem-trol(R), individually and in simulated tank mixtures, to the same invertebrates and to Daphnia magna in reconstit uted water representative of these wetlands. There was no difference i n the survival of caged invertebrates between treated and reference we tlands after 21 days. Based on nominal concentrations of the formulati ons, X-77 Spreader(R) (LC50s = 2.0-14.1 mg/L) was about 83-136 times m ore toxic than Rodeo(R) (Lc50s = 218-1216 mg/L) to aquatic invertebrat es. Chem-Trol(R) killed less than or equal to 10% of the animals at 10 ,000 mg/L and less than or equal to 50% of the animals at 28,000 mg/L. Daphnia magna were more sensitive than the other species to X-77 Spre ader(R), Rodeo(R), and the simulated Rodeo(R) tank mixture (RTM). The joint toxic action of the RTM was additive for amphipods and midges, g reater than additive for leeches, and was less than additive for daphn ids. X-77 Spreader(R) was the major toxic component in the RTM. Binary mixtures of X-77 Spreader(R), Rodeo(R), and Chem-Trol(R) at tank mixt ure and equitoxic ratios also showed additive toxicity to amphipods. T he use of Rodeo(R) (applied as tank mixture with X-77 Spreader(R) and Chem-Trol(R)) as a management tool in wetlands does not pose an acute hazard to native aquatic invertebrates because the concentrations of R odeo(R), X-77 Spreader(R), and Chem-Trol(R) found to be acutely toxic to these invertebrates were much higher than their expected or measure d concentrations in water from wetlands treated with the RTM.