The environmental effects of safeners, chemicals that protect crops from herbicide toxicity, are largely unknown. Safeners are considered inert ingredients added to many classes of herbicides. We compared the toxicity of the dichloroacetamide safener benoxacor, with its degradation product (mono-chlorinated benoxacor), a typically paired herbicide (S-metolachlor), and a mixture of S-metolachlor and benoxacor, to larvae of Chironomus riparius in benthic microcosms containing natural, iron-rich

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... diment. Larval C. riparius were exposed to these four chemicals in spiked sediments during chronic 28 day experiments. High concentrations (~100 mg/kg, 200 mg/kg mixture) of all four chemicals significantly affected percent adult emergence. These high concentrations of benoxacor and the S-metolachlor + benoxacor mixture reduced adult emergence rates, and high concentrations of S-metolachlor reduced male adult biomass. All four chemicals, at varying concentrations, affected hazard ratios of emerging healthily for both sexes. Benoxacor and its degradation product (mono chlorinated benoxacor) were shown to be more toxic to C. riparius (based on survival, time to-emergence, and emergence rate) at similar concentrations compared to the herbicide S-metolachlor. A transformation rate constant (𝑘 !, ) of 0.06 ± 0.03 d–1 was determined for benoxacor within a spiked-sediment microcosm and the half-life for benoxacor was calculated as 11.6 ± 3.9 d. The toxicity experiments and evaluation of benoxacor persistence under microcosm conditions provides useful insight into the ecotoxicological effects of dichloroacetamide safeners.