Hi- This looks like the work of a caterpillar or a beetle, both of which can cause chewing damage. Very often with insects, once you notice the damage, the culprit has already fled the scene and no treatment is warranted. Usually no pests are serious on witchhazels. You may have had a temporary feeder that is now finished and gone. Most do not cause significant harm. https://extension.umd.edu/hgic/topics/caterpillars-trees

For a young tree, the best thing to do is to provide good care -- especially watering during hot, dry periods in the summer. When trees are stressed by poor growing conditions or environmental conditions like drought, they can become more susceptible to pest and disease problems. Refer to our guidelines about watering trees, https://extension.umd.edu/hgic/topics/watering-trees-and-shrubs.

Also, make sure the tree is properly mulched at the base -- no more than 2"-3" deep and avoid putting the mulch right up against the trunk. https://extension.umd.edu/hgic/topics/mulch

Christa