I know its hard to believe, but, ducks do eat snails.
If the snails die, either the ducks will have to find something else to eat or they will also die. But ducks are omnivores, so they eat a combination of plants and animals. Some of the plants that ducks eat are grass, weeds, algae, seeds, grain, berries, fruit and aquatic plants. Some of the animals that ducks eat are snails, mollusks, worms, frogs, salamanders, small fish and fish eggs. Ducks also eat insects.
The ducks may be eating the snails on your hosta.
ducks eat wild grain, grasses, vegetation. such as water snails and insects
Beavers, muskrats, ducks, porcupines, and snails eat water lilies
Ducks eat a variety of things that include aquatic vegetation, grain, and insects. They are foragers and eat caterpillars, snails, and slugs.
They're bird eaters
ducks, harons, robins, rodents
Ducks eat grasses, wild grains, and other other vegetation. Also water snails and some insects. Ducks eat what any aquatic bird eats. Seeds, grasses and bugs on the ground and aquatic plants and little snails and other organisms in the water.
Many species of ducks will eat snails. In fact, some species are adapted to seek out and eat predominantly such aquatic creatures as snails.
Birds such as chickens, geese and ducks eat snails. Some wading birds such as ibises, brolgas, cranes and even some spoonbills eat snails. Some songbirds, such as the thrush, eat snails, as long as their size allows for it, and some of these birds have even learnt how to crush a snail's shell.
Beavers, muskrats, ducks, porcupines, and snails eat water lilies. Other animals, such as deers eat the seeds inside.
Neverminding the poor grammar in the question, ducks certainly eat flies, and snails, and slugs. They are not strict herbivores as the previous answer stated.