The Spanish slug, Arion vulgaris Moquin-Tandon, 1855, is a serious pest in agriculture and private gardens around the world [1]. Saponins, which are secondary metabolites of plants, have previously been shown to have strong molluscicidal effect [2]. The use of saponins-rich plant extracts is gaining importance as are safe (as saponins residues) in agriculture products [3]. However, despite of the proven safety in food, they still do not have a widespread application due to their liquid form and absence of more accurate knowledge of effects on other organisms. White worm, Enchytraeus albidus Henle, 1837, is an important model of decomposer organisms in the terrestrial ecosystem [4]. 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 saponins extract from the bark of the soap tree, Quillaja saponaria Mol., was used in repeated 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 slugs' age group and 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°C) 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°C and lower). The LC50 for worms at -1°C was also significantly lower than at 6°C (Table 1) [5].
| Species | Life stage | Temperature (°C) | LC50 | x2 (df, P) |
|---|---|---|---|---|
| A. vulgaris | Adult | +15 | 68.52±8.38 | 15.68 (27, 0.9588) |
| A. vulgaris | Juvenile | +15 | 96.94±13.84 | 16.731 (17, 0.4727) |
| A. vulgaris | Adult | +2 | 33.52 ±7.49 | 29.077 (30, 0.5135) |
| A. vulgaris | Adult | -1 | 18.38 ± 3.64 | 20.986 (31, 0.9123) |
| E. albidus | Adult | +6 | 2.72 ± 0.09 | 22.792 (29, 0.7857) |
| E. albidus | Adult | -1 | 2.07 ± 0.11 | 51.252 (46, 0.2753) |
Therefore, we can conclude: 1) that Q. saponaria saponins may be a successful slug control tool used during the colder time of the year, but its concentration should be selected according to the age group of A. vulgaris; 2) this measure was more toxic than expected to white worms, therefore, more research should be done with other groups as well.