Good Morning,
They do not harm the tree and chemical control is not necessary.  Gall wasps are difficult to treat because the tissue around the grubs is so distorted that even systemic pesticides have a hard time getting in to kill the grub. Even when the trees are treated, the galls will not disappear, so it is hard to tell if a treatment worked or not. To try to break the cycle of stem galls and leaf galls in next year's growth, collect some of the galls and store them in plastic bags outside, in the shade. When the tiny, dark wasps emerge, it is time to spray. If no wasps have emerged after a few weeks, then collect another sample and store it in the shade outside and wait for the wasps. Some oak gall wasps emerge from stem galls in winter. Other gall wasps that form galls on leaves and succulent stems emerge in late spring or early summer. When the gall wasps emerge, spray the tree thoroughly. Sevin (carbaryl) pesticide is labeled for gall wasp control on shade trees and park trees.
Cynipid Wasp Galls: Making A Mess of White Oaks (psu.edu)
Detachable Woolly Leaf Gall Wasp | NC State Extension Publications (ncsu.edu)

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