Experimental indications of gardeners' anecdotes that snails interfere with invasive slugs.

The invasive Spanish slug () is an important pest species in agriculture and horticulture in Europe. In the last decades it has spread across the continent where it outcompetes native slug and snail species, thus posing a threat for biodiversity. A popular anecdote suggests to promote Roman snails () in gardens because they are able to control . We examined a potential interrelationship between these two species using a mesocosm experiment with lettuce plants. C-N stable isotope labelling of lettuce allowed us to investigate interactions between and on weight gain/loss and herbivory. Additionally, we wanted to know whether different watering regimes (daily vs. every 3rd day watering of weekly amount) and earthworms alter these interactions. Egg predation of on eggs was further tested in a food-choice experiment. showed a five times higher herbivory per body mass than in a single-species setting. However, in mesocosms containing both species percentage of herbivory per body mass was significantly lower than in -only mesocosms, especially when watered every three days. Overall isotope uptake via eaten lettuce was unaffected by the presence of the other species. Only very little predation (three out of 200 eggs) of on eggs was observed. Our results provide no evidence for a clear dismissal or confirmation of the popular gardener's anecdote that snails have a negative effect on abundance or herbivory.