What are these bumps on the leaves of my walnut trees?

These bumps are walnut blister galls. They are caused by eriophyid mites that are feeding inside the bumps on the leaves. They do not harm the tree, and there is no need to control them or spray chemicals.

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Eriophyid Mites: Bud, blister, gall, and rust mites
Insect and mite galls