AbstractThe organophosphate veterinary insecticide coumaphos O‐3‐chloro‐4‐methyl‐2‐oxo‐2H‐chromen‐7‐ylO, O‐diethyl phosphorothioate is used by the Animal and Plant Health Inspection Service of the United States Department of Agriculture in its program designed to prevent re‐entry of the southern cattle tick (Boophilus microplusCan.) and the cattle tick (B. annulatusSay) into the United States from Mexico. Large amounts of waste acaricide are generated annually in this program. One event that triggers disposal of coumaphos dips is the accumulation of potasan O, O‐diethylO‐4‐methyl‐2‐oxo‐2H‐chromen‐7‐yl phosphorothioate. This process was shown to occur under anaerobic conditions and to be inhibited by mercury ions, indicating that anaerobic micro‐organisms were responsible for a reductive dechlorination. Maintaining coumaphos suspensions at pH<7 prevented the reductive dechlorination of coumaphos and left a suspension that was amenable to disposal by biodegradation. A field trial on an 11000‐litre capacity coumaphos dipping vat demonstrated the applicability of using acid to extend the life of coumaphos solutions and of biod
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