Answer: Your new little fruit trees should be happy with a deep soaking every 3-4 days during the summer heat. It's important that the soil is moist but not soggy wet so watering thoroughly twice a week should allow enough time for the soil to drain but not completely dry out.
Since you cannot find the pest doing the damage to the leaves, there's not much to recommend in the way of pesticide sprays. Quite often an insect will take a tiny bite out of a newly forming leaf and the hole will expand as the leaf grows. So it is possible that the leaf damage began before you even planted the trees. Fruit tree leaves are tolerant of insect feeding so I don't think it's anything to be concerned about.
Hope you have a bountiful harvest!
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