Lethal Doses of Saponins from Quillaja saponaria for Invasive Slug Arion vulgaris and Non-Target Organism Enchytraeus albidus (Olygochaeta: Enchytraeidae)

The slug, Arion vulgaris Moquin-Tandon, 1855, is a serious pest in agriculture and private gardens. White worm, Enchytraeus albidus Henle, 1837, is an important model of decomposer organism in the terrestrial ecosystem. Saponins, which are secondary metabolites of plants, have previously been shown to have some molluscicidal effect. We investigated which doses of saponins are lethal to the slug, A. vulgaris, and to the non-target organism, E. albidus. An aqueous solution with different concentrations of saponin extract from the bark of the soap tree, Quillaja saponaria Mol., was used in repeat treatments. Slugs were tested in filter paper contact tests as they are naturally exposed to soil contact while crawling. Worms were tested in soil contact tests as they live below ground. It was found that lethality of saponins depends on the slug age group and the environmental temperature. The median lethal concentration (LC50, at 15 °C) on adults was 68.5 g/L, and on juveniles, 96.9 g/L. The slugs were significantly more sensitive at 2 and −1 °C compared to 15 °C. The LC50 (at 6 ℃) on E. albidus was 2.7 g/L (or 0.5 g/kg dry weight of soil), far below those in A. vulgaris (at 15 ℃ and lower). The LC50 for worms at -1℃ was also significantly lower than at 6 ℃. Therefore, we can conclude: (1) that Q. saponaria saponins may be a successful slug control tool used during colder times of the year, but its concentration should be selected according to the age group of A. vulgaris; (2) this measure is more toxic than expected to white worms, which limits its use.