Acute toxicity of isomers of the pyrethroid insecticide deltamethrin and its major degradation products to Daphnia magna

The acute toxicities (4‐h, 24‐h and 48‐h) of eight stereoisomers and four major breakdown products of deltamethrin to D. magna were determined. Parent deltamethrin (isomer 1) was the most toxic, with EC50 values in the range of 0.05 to 1.75 μg /L, concentrations that have been observed in the surface microlayer and subsurface water in field studies. The other three isomers (2‐, 3‐ and 4‐deltamethrin) were two‐ to tenfold less toxic than 1‐deltamethrin. Isomers 1′, 2′, 3′ and 4′ plus the four breakdown products studied displayed no significant toxicity. Because 1‐deltamethrin can be converted in natural water to 2′‐, 3‐ and 4′‐ deltamethrin, and because 3‐deltamethrin is toxic to D. magna , this isomerization is only a partial detoxification step as far as some aquatic organisms are concerned.