Answer: This may not be any comfort, but trees can be almost completely defoliated and they will maintain their health. As for what's eating your leaves, I imagine caterpillars are the culprits and you're not seeing them because they are feeding either early in the morning or in the late afternoon and then marching away to some secluded place. Malathion works well on contact, but since you're not seeing the pests and are not applying the chemical directly on the pests, you're probably just waisting your time and your money. I would continue to monitor the tree, checking it at various times of the day and inspecting both the upper and under sideds of the leaves. You may very well catch them in the act. In the meantime, aside from aestetics, I don't think the damage will hurt your tree.
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