I put a mesh bag over the top bud of the central leader of one of the trees.The bud allready has damage. I will see if it recovers and grows. I also looked tonight with a flash light, and did not see any insects. That doesn't mean there are none, I just did not see any.
Once the trees get big, I'm sure the damage won't be a problem, its just while they are small that the damage is a problem.
I tried to make a repelant with garlic, soap, and vegetable oil. Sprayed it on the buds. Worked for a few days only. And caused burned spots on the leafs.
Also, I have planted many species of trees in my yard. 2 species of cottonwood, 4 oak, Ash, sycamore, eucalyptus, apple, pear, peach, orange, guava, pecan, elm, olive, and more. The cottonwoods are the only trees that have the bees "chewing" the leaf buds, and the only ones to have this type of damage. Some others have had leafs eaten, but it is obviously eaten, and not deformed and distorted.
I put chewing in quotes, because I don't know what else to call it when there mouths are on the plant moving there mandibles open and closed.