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Long-term toxicity to aquatic invertebrates

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Description of key information

The long-term toxicity of cloquintocet-mexyl to Daphnia magna was investigated in two guideline studies. The most reliable endpoint is the NOEC of 0.437 mg/L reported by Bätscher (2003). 
The EC50 of Daphnia magna to CGA153433, a metabolite of cloquintocet-mexyl, was of 32 mg/L.

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater invertebrates:
0.437 mg/L

Additional information

Two GLP compliant studies investigated the long-term toxicity of cloquintocet-mexyl to aquatic invertebrates.

Vial (1990) investigated the toxicity of cloquintocet-mexyl to Daphnia magna according to OECD 2020 guideline Part 2 (1984), which is a 21 day reproduction test. The 21-day reproduction NOEC was reported as 0.002 mg /L.

In addition, a 21-day semi-static reproduction test with D. magna was also performed by Bätscher (2003). The study was conducted to OECD guideline 211 and showed that cloquintocet-mexyl had no toxic effects on survival, reproduction, and on the growth of Daphnia magna at concentrations up to 0.437 mg/L (the rate tested). Therefore the 21-day NOEC was at least 0.437 mg/L.

The selection of the endpoint for use in the chemical safety report is dependent upon the reliability of the studies concerned. Formally, both tests fulfil the validity criteria of the respective guidelines used. However, the test media were produced using completely different methods. In the Bätscher (2003) test Dimethylformamide (DMF) was used as solvent (max. 0.1 mL/L medium), which is acceptable according to OECD 23 guideline for testing of difficult substances. In the Vial (1990) test, a stock solution was prepared 3x/week making up 10 mg test substance to 1L with test water, but the study report does not indicate how this was achieved (e.g. intensive stirring, ultrasonication).

Since the nominal test concentration were at or below the water solubility limit of cloquintocet (0.6 mg/L) in both tests, it can be assumed that the substance was more homogenously distributed in the Bätscher (2003) where solvent was used to prepare the test media. Indeed, the statement in the Vial (1990) report that “(3) the Day 14 (in freshly renewed medium) recoveries were too high, probably due to heterogenicity of the distribution of the test substance in the test vessels”, appears to support this assumption. Further evidence is provided by the study results. In the Bätscher (2003) test no/little mortalities of adult daphnids were found across the whole concentration range. By contrast, Vial (1990) reported that mortality increased (only from day 8 onwards) on a variable basis between the concentrations and large changes from one day to the next were observed. It is suggested that this was a result of the inhogeneous distribution of the test substance, perhaps even the availability of small particles which were taken up by the daphnids (daphnids are filter-feeders) resulting in these “over-night”/acute events of toxicity. Due to these uncertainties, the study was repeated using a different method of test medium preparation.

As a result of the argument presented above, it is proposed that the Vial (1990) test should be considered to be unreliable (3) and that the repeat performed by Bätscher (2003) should be considered to reliable without restriction. Accordingly, the 21 day NOEC of 0.437 mg/L reported in the Bätscher (2003) should be used in the Chemical Safety Assessment.

 

Additionally, the effect of CGA153433, a metabolite of cloquintocet-mexyl, on the survival and reproduction of Daphnia magna was determined in an OECD 211 study (Hertl, 1997). This study was GLP compliant and is considered to be reliable without restrictions (1). Based on nominal test item concentrations, the 21-day NOEC of CGA153433 was 32 mg/L.