2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridine carboxylate; imazapyr (ISO)

Short-term toxicity to aquatic invertebrates

Description of key information

The toxic effects of the test substance was estimated in both, freshwater (Daphnia magna) and marine species (Penaeus duorarum, Crassostrea virginica). In all acute toxicity assays an EC50 value >100 mg/L was observed.

Additional information

Weight of evidence
Freshwater species 
WoE (American Cyanamid Company, 30098, 1983) 
An acute toxicity study with Daphnia magna under static conditions was performed equivalent to OECD guideline 202. Water fleas were exposed to nominal test item concentrations of 10, 18, 32, 56 and 100 mg/L for a period of 48 hours. No analytical verification of the nominal test concentrations was performed. Under the conditions of this study, the test item was determined to be practically non-toxic to daphnids with an EC50 value >100 mg/L.   

Marine species 
WoE (American Cyanamid Company, 87384-0200-2130, 1988) 
A flow-through saltwater toxicity test was conducted to determine the acute toxicity of the test item to Pink shrimp (Penaeus duorarum). The test organisms were exposed to nominal test item concentrations of 0, 26, 43, 72, 120 and 200 mg/L for a period of 96 hours. The corresponding determined mean measured concentrations were <10.1, 25.0, 38.1, 60.6, 111 and 189 mg/L. The criterion for effect was death. After 96 hours of exposure an LC50 >189 mg/L was determined. One shrimp died at the second highest concentration level (111 mg/L), but mortality did not appear to be related to the treatment. No other signs of toxicity were observed. Therefore, the NOAEC was determined to be 189 mg/L, the highest concentration tested. 

WoE (American Cyanamid Company, 87384-0400-2130, 1988; 954-97-139, 1997)
Two flow-through toxicity tests were conducted to determine the acute effect of the test substance on the Eastern oyster (Crassostrea virginica) after 96-hour of exposure. The criterion of effect was in both tests reduced shell growth of exposed oysters compared to shell growth of the control oysters. In the first study (American Cyanamid, 87384-0400-2130, 1988) Eastern oyster were exposed to nominal concentrations of 0, 25.9, 43, 72, 120 and 200 mg test item/L for 96 hours. These concentrations correspond to the following mean measured concentrations: <10.5, 21.5, 42.4, 65.5, 109 and 173 mg/L. After 96 hours of exposure an EC50 of >173 mg/L was determined. No mortalities were observed in either the treated or control groups. There was a statistically significant decrease in mean shell deposition at 173 mg/L when compared to the control group (p ≤0.05). The NOAEC was determined to be 109 mg/L based on measured concentrations.
In the second acute toxicity test (American Cyanamind, 954-97-139, 1997) Eastern oyster were exposed to nominal test substance concentrations of 0, 16, 26, 43, 72 and 120 mg/L, which corresponds to the mean measured concentrations of <LOQ, 16, 27, 46, 80 and 132 mg/L. Under the conditions of this test, the 96-hour EC50 value for eastern oysters exposed to the test substance was estimated as >132 mg/L, the highest concentration tested. The NOEC was 132 mg/L. No significant adverse effects were observed on shell deposition for any treated group.

Conclusion:
The test item was determined to be practically non-toxic to the freshwater species, Daphnia magna and marine Pink shrimp and Eastern oyster. In all acute toxicity assays an EC50 value >100 mg/L was observed.