I normally use Copper tape as a Sug and Snai deterrant., but I don't know how that would react to the bark of the trees, try 'Fruit Tree grease', you can get it from a garden centre, lay it on thick, it should do the job, however, you would still have to get rid of the ones that are already there.
I don't know where I heard it but I believe they like the taste of beer. Once tempted they fall into the container and drown. Can anyone else confirm this practice?
Its true, Beer does attract them, you can buy Beer Traps, but thats the problem, it attracts them, and you end up with them coming into your garden from miles around.
hi My trick is to go out at night and pick them up by hand and put them into a plastic bag I know its time comsuming but its satifying knowing you have reduced the snail population in your garden thus saving a few more plants You can also do with with slugs but they are sticky be sure to use gloves or wash your hands with a nail brush afterwards