Acute toxicity of selected herbicides and surfactants to larvae of the midge Chironomus riparius

The acute toxicities of eight commercial herbicides and two surfactants to early fourth instar larvae of the midgeChironomus riparius were determined under static conditions. The formulated herbicides tested were Eradicane^® (EPTC), Fargo^® (triallate), Lasso^® (alachlor), ME4 Brominal^® (bromoxynil), Ramrod^® (propachlor), Rodeo^® (glyphosate), Sencor^®(metribuzin), and Sutan (+)^® (butylate); the two surfactants were Activator N.F.^® and Ortho X-77^®. In addition, technical grade alachlor, metribuzin, propachlor, and triallate were tested for comparison with the formulated herbicides. The relative toxicity of the commercial formulations, based on percent active ingredient, varied considerably. The EC50 values ranged from 1.23 mg/L for Fargo^® to 5,600 mg/L for Rodeo^®. Fargo^®, ME4 Brominal^®, and Ramrod^®were moderately toxic to midge larvae; Lasso^®, Sutan (+)^®, and Eradicane^® were slightly toxic; and Sencor^® and Rodeo^® were practically non-toxic. The 48-hr EC50 values of the two surfactants were nearly identical and were considered moderately toxic to midges. For two of the herbicides in which the technical grade material was tested, the inert ingredients in the formulations had a significant effect on the toxicity of the active ingredients. Fargo^® was twice as toxic as technical grade triallate, whereas Sencor^® was considerably less toxic than technical grade metribuzin. A comparison of the slope function values indicated that the toxic action of all the compounds occurred within a relatively narrow range. Published acute toxicity data on these compounds for other freshwater biota were tabulated and compared with our results. In general, the relative order of toxicity toC. riparius was similar to those for other freshwater invertebrates and fish. Maximum concentrations of each herbicide in bulk runoff during a projected “critical” runoff event were calculated as a percentage of the application rate lost in a given volume of runoff. A comparison between estimated maximum herbicide concentrations in runoff and results of acute tests indicated that Ramrod^®, ME4 Brominal^®, and Lasso^® pose the greatest direct risk to midge larvae during a storm event.