They lovel apples, sweet potatoes and carrots my D.I.L's horse gets polo's as a treat, but she does not give him a lot of sugar.
Have just spoken to my D.I.L. and showed her the pictures. She said if anyone can get some garlic powder and mix it with some mashed carrots etc., once the horse gets it through it's system the flies don't like the taste, plus the will not be attracted to the horses droppings to lay their eggs in, again because of the smell of garlic. This has to be done on a long term basis during the fly season. Perhaps, if someone meets the owner they can tell them.
I would be happy to help in any way when we come over.