Swollen midrib or vein on underside of leaf ... Resseliella tulipiferae (Osten Sacken)
Swellings may contain one to several larvae... Probably several generations occur per year. Full-grown larvae cut an exit hole and drop to the ground to pupate. Host: L. tulipifera...
”- Raymond J. Gagne: (1989) The plant-feeding gall midges of North America©