ACUTE TOXICITY OF THE HERBICIDE BROMOXYNIL TO DAPHNIA-MAGNA

The acute toxicities of technical-grade bromoxynil octanoate (BO) and two commercial formulations, Buctril(R) and Bronate(R), to <24-h-old n eonate Daphnia magna (Straus) were determined in soft, hard, and oligo saline water. In addition, effects of life stage, feeding, aging the h erbicide, and exposure duration on BO toxicity to daphnids were invest igated. Regardless of formulation, life stage, and water quality, BO w as found to be extremely to highly toxic to daphnids in standard tests ; 48-h EC50 values ranged from 41 to 161 mug/L. Bromoxynil octanoate w as the most toxic to neonates in soft water and the least toxic in har d water. The acute toxicities of the three bromoxynil herbicides to a given age group of daphnids were similar within the same water type. O verall, neonates and 7-d-old adults were more sensitive than 14- or 15 -d-old adults to each herbicide. Feeding daphnids during the toxicity test significantly decreased BO toxicity compared to not feeding them. Aging BO (as Buctril) in hard water decreased its toxicity, and the r ate of deactivation was rapid, with an estimated half-life of biologic al activity of 13 h. Daphnids immobilized by exposures to toxic BO con centrations for less-than-or-equal-to 6 h recovered their mobility, wh ereas exposures of 18 and 24 h to BO produced toxic effects in daphnid s similar to those exposed for 48 h. These results indicated that stan dard continuous exposure tests may not adequately predict the acute to xicity of BO to freshwater animals in the field.