What is the aquatic toxicity of saponin-rich plant extracts used as biopesticides?

Highlights: • Saponins are the main ingredients explaining toxicity of the saponin-rich plant extracts. • Worms, zebrafish and snail embryos are the most sensitive species for all saponins. • The species-specific type of saponins affect their toxicity, making "read-across" between saponins a dubious exercise. • Saponin-based biopesticides might pose a risk to the aquatic environment if not used cautiously. Saponin-rich extracts from Quillaja saponaria and Chenopodium quinoa have been registered by US EPA as active ingredients in biopesticides, and extract from tea seed powder, Camellia oleifera has been proposed for biocidal use. If saponin-rich biopesticides are efficient against pests, they are most likely also bioactive in the aquatic environment against non-target organisms. The aim of this study was to conduct an effect assessment of saponin-rich plant extracts by using species sensitivity distributions based on acute toxicity tests. The maximal concentrations protecting 95% of the aquatic species (HC₅) of saponins extracted from quillaja bark, tea seed coat and quinoa seed coat were 2.91 ± 1.00, 0.22 ± 0.11 and 22.9 ± 5.84 mg/L, respectively. The 100-fold difference in toxicity between the saponin-rich extracts from different plant species, indicate that saponin toxicity depends on the species it origins from, making "read-across" between saponins a dubious exercise. In addition, the predicted environmental concentrations of different saponins are close to or higher than their water quality standard, which means that the extracts might pose a risk to the aquatic environment if not used cautiously.