The toxicity of malathion to unionid mussels: Relationship to expected environmental concentrations

The acute toxicity of malathion to glochidia, juvenile, and adult freshwater mussels was determined at pH 7.5 in soft water and at pH 7.9 in moderately hard reconstituted fresh water at 25 degrees C and 32 degrees C. Nine species were tested in one or more life stages. Glochidia tests were conducted for 4, 24, or 48 h, while juvenile and adult exposures lasted 96 h. Overall, Utterbackia imbecillis was the least sensitive species for all exposure conditions and life stages. The LC50 values for glochidia tested at 25 degrees C ranged from 7 mg/L for Lampsilis siliquoidea (4 h) to 324 mg/L for Ci. imbecillis (48 h). At 32 degrees C, glochidia LC50s were 119 b o mg/L for Villosa lienosa (48 h) and 374 mg/L for U. imbecillis (24 h). Tests with juvenile mussels produced 96-h LC50s ranging from 24 mg/L for Lampsilis straminea claibornensis at 25 degrees C to 219 mg/L for U. imbecillis at 25 degrees C. The 96-h LC50s for three species of adult mussels were greater than the highest malathion exposure concentration of 350 mg/L. These values are considerably higher than the reported 48-h LC50 of 1 mu g/L for Daphnia magna and the 96-h LC50 of 0.76 mu g/L for Gamarus fasciatus but are similar to 96-h LC50s for some fish. Expected environmental concentrations should not be lethal to unionids.