Leaf problems don't get me too excited, they are mostly cosmetic. By having the leaves chewed off repeatedly there is a certain amount of stress on the tree and it will slow its growth. If it looks bad, then perhaps it is time for an insecticide.
I checked the label on Merit and it includes leaf beetles. It's just mixed up and poured on the base of the tree in fall. It moves up the tree slowly and systemicly, so the following spring when the beetle chews on the tree, they die.
We have a customer with an Elm that for years was covered with Elm Leaf Miners. Like you, their trees were all chewed up by mid-summer. The trees didn't look that bad but they noticed there was something chewing and wanted the bugs dead. So we sprayed. The tree was never so full and lush. The first time some high wind came through, the massive new canopy created so much sail area that the tree failed at a main crotch. The next day it was removed.
