What works for me is to let them into the garden for about an hour in the morning. They (Indian Runners) run around and eat slugs, potato bugs, mushrooms and some leaves they can reach. If left any longer they may eat too many leaves or take a nap on plants.
Welded wire works for runners in my experience. If you could keep them around fruit trees for the rest of the day that’s beneficial. Predation is a big issue with ducks. But they are super useful.