Undetermined species
Swollen base of the petiole of Q imbricaria, 6 mm long by 2 mm in diameter, remaining attached to the twig during the winter.
”- LH Weld: (1928) Cynipid Galls of the Chicago Area©
Reference: https://www.biodiversitylibrary.org/page/57505981#page/196/mode/1up