Firefly larvae prey on slugs and snails with estimates that one larvae can eat 40 to 60 snails during its development The decollate snail is the only predatory snail commercially available and its availability is restricted to Southern California for fear that' it will become a pest. It is found across the southern United States and along the eastern seaboard as far north as Pennsylvania. The decollate snail provides good control of the brown garden snail, but has much less interest in devouring slugs. Decollate snails move at a snail's pace, avoiding sunny areas. They dislike crossing concrete so they do not easily spread from block to block.
Humans are another predator.
Those brown garden snails were originally brought over from France to be cultivated and prepared as escargot. Rototilling thoroughly can reduce slug populations. The tilling apparently kills the slugs as well as reducing the crevices in the soil where slugs hide. Handpicking can reduce snail populations, but slug populations may not be as effectively controlled. Water lightly in the late afternoon to bring slugs and snails out of hiding.