Apple and Plum tree spraying

My dad has a couple of old apple trees and an old plum tree.

Last year most of the harvest was riddled with maggots. He has fitted grease bands a while back but what else does he need to do to protect them from everything that wants to eat them or give birth in them ?

That's apples and plums please. Mostly because we can have loads if they don't become inedible.

Thank you

Rob