Earthworm lipid content and size help account for differences in pesticide bioconcentration between species

The uptake and elimination kinetics of pesticides from soil to earthworms are important in characterising the risk of pesticides to soil organisms and the risk from secondary poisoning. However, the understanding of the relative importance of chemical, soil, and species differences in determining pesticide bioconcentration into earthworms is limited. Furthermore, there is insufficient independent data in the literature to fully evaluate existing predictive bioconcentration models. We conducted kinetic uptake and elimination experiments for the earthworms Eisenia fetida, Aporrectodea caliginosa and Lumbricus terrestris which have varying body size (specific surface area (SSA) of 0.70 – 1.45 m2 kg-1) and lipid content (9.21 – 19.4 dry wt%) in five contrasting soils (pH 5.23 – 7.06, organic matter content of 0.97 – 40.0 wt%, clay content 4.0 – 50 wt%) using a mixture of five pesticides (lenacil, flutriafol, dieldrin, hexachlorobenzene and p,p'-DDT) (log Kow 1.69 – 6.63) at bulk soil concentrations in the range 1.2 – 7 mg kg-1, 8 – 100 times below established LC50 values or experimental concentrations reported in previous studies. Bioconcentration of the pesticides based on earthworm and bulk soil concentrations ranged from 0.007 – 18.42 kg kg-1. Bioconcentration increased with the hydrophobicity of the compounds and decreased with soil organic matter. Bioconcentration factors were comparable between earthworm species for hydrophilic pesticides due to the similar water content of earthworm species. Inter-species variations in bioconcentration of hydrophobic pesticides were primarily accounted for by earthworm lipid content and specific surface area. Similar results were found whether soil bulk concentration or soil porewater concentration was used in calculations. Existing bioconcentration models either failed to perform well across earthworm species and for more hydrophilic compounds (log Kow <2) or were not parameterised for a wide range of compounds and earthworm species. Refined models should incorporate earthworm properties (lipid contents and SSA) to account for inter-species differences in pesticide uptake from soil.