Shallow water is apparently not an effective barrier.
Water barriers in the test
To test this idea, I set up a simple experiment.
As proposed, I turned a slug collar upside down and filled it with tap water. This created a trench about 1 in wide and deep (2 to 3 cm).
Then I placed freshly collected slugs and snails in the middle and recorded what happened in a time-lapse video.
Video: Snail barriers as a ditch | Time lapse
It was astonishing that a leopard slug (Limax maximus) passed the trench without any problems right at the beginning.
The large slug even crawled unimpressed a bit under the water surface before reaching the other side.
But as it progressed, it became apparent that slugs do shy away from water.
Saucers for pot plants can also serve as water obstacles.
From minute 1:05, you can observe how a slug partially slides into the water. It loses its grip with its back end and can only pull itself out of the water with difficulty.
Finally, it falls completely into the water—from minute 1:30. It then begins to squirm in the water until it regains its footing and, with a bit of luck, finally reaches the other side.
Other slugs crawl along the water’s edge without sliding into the water, but also without any chance of reaching the other side.
At minute 1:42, one slug uses another one as a bridgehead to reach the other bank.
Finally, tree slugs have managed to cross the water. One slug has also managed to reach the other bank with great difficulty. Eight others, however, failed.
Conclusion
It turns out that water is really an obstacle for slugs.
Leopards slugs, on the other hand, manage to find a foothold even underwater and can overcome small water ditches.
However, since leopard slugs are usually not a danger to plants, this method is absolutely recommendable.
Slug collars can easily be converted into a water barrier.
It is also possible to create wider water obstacles for larger areas using rain gutters or similar.
