Enantioselective toxicity, bioaccumulation and degradation of the chiral insecticide fipronil in earthworms (Eisenia feotida)

- R-fipronil was more toxic than S-fipronil to earthworms (Eisenia feotida).
- The bioaccumulation of fipronil in the earthworms was not significantly different from that of the enantiomers.
- The degradation of fipronil in earthworm was enantioselective: S-fipronil was biodegraded preferentially over R-fipronil.
- The degradation of fipronil in two types of soil (each with earthworms and without earthworms) was not enantioselective.

The enantioselective acute toxicity to earthworms of racemic fipronil and its individual enantiomers was studied. R-(−)-fipronil was approximately 1.5 times more toxic than the racemate and approximately 2 times more toxic than S-(+)-fipronil after 72 and 96 h of exposure, respectively. Assays of fipronil enantiomer bioaccumulation and degradation in earthworms were conducted. The bio-concentration factors (BCFs) were slightly different between the two enantiomers. The enantiomeric fraction (EF) values in earthworms in the bioaccumulation period were approximately 0.5, which indicated there was no enantioselective bioaccumulation. In contrast, the degradation of fipronil in earthworms was enantioselective: the t1/2 values for R- and S-fipronil were 3.3 and 2.5 days, respectively, in natural soil, and 2.1 and 1.4 days, respectively, in artificial soil. The results of soil analyses showed that the degradation of fipronil was not enantioselective, which suggested that the enantioselectivity of fipronil in earthworms results from the organism's metabolism. The study also demonstrated that the presence of earthworms could accelerate the degradation of fipronil in soil.