Black and yellow grubs both require snails and birds. I can't remeber which is which but one needs herons and the other needs kingfishers. Also the parasites prefer small fish because they can get eaten by birds (no one knows how the parasites select small fish but they seem to).

Introducing pumpkinseeds or red ears will put a dent in your snail population which is probably the easiest way to reduce the parasite load. Getting rid of the birds is also effective.