Snails can be harmful to young trees eating through tender bark or to trees with low-growing leaves. Most often the latter are citrus where they cause the most damage.

We have had snail infestations on citrus trees which required some serious hand-picking on an almost daily basis. However, the tree still produced quite well.

If these are young fruit trees and not yet well established, you are well advised to continue picking the snails off. If the tree is older and larger, it's not as big an issue. We currently have a very mature large apple tree which often has slugs and snails on the lower trunk, but these do not seem to cause a problem to the tree.