I'v never observed Leopard Slugs eating my plants, but I have observed the small gray slugs eating plants. If you don't want slugs in your garden you don't have to kill them- just collect them and put them out in the woods. Leopard Slugs do eat the small gray slugs.

Are leopard slugs beneficial?

Here again, the leopard slug plays the field: Though it is fond of eating decaying vegetable matter, it also preys on other slugs; for this reason, it may ultimately be more beneficial than detrimental for gardeners.

Are leopard slugs a pest?

Threat. Leopard slugs are considered a major horticultural pest since they are willing to eat just about any plant. They will eagerly eat the leaves and roots of most plants, which often kills the plant.