Wool Sower Galls on Oak The wool sower gall is a distinct and unusual plant growth induced by the secretions of the grubs of a tiny gall wasp, Callirhytis seminator. If a fresh wool sower gall is held in a plastic bag out of the sun (so it will not get too hot), within one to three weeks the tiny, harmless gall wasps will emerge. The wool sower gall is specific to white oak and only occurs in the spring. Pulling the gall apart exposes small seed-like structures. The gall wasp grubs develop inside these structures. (This gall is also called the oak seed gall.) Fortunately, wool sower galls are hardly ever abundant enough to cause harm to white oaks. Gall wasps invariably have alternation of generations in which one generation develops in one type of gall (leaf gall) and their offspring develop into another type of gall (stem gall). Wasps of each alternate generation are slightly different in size and the galls of each generation are enormously different from the parents. The wool sower gall may be the leaf gall of this species because of its transient nature. Ornamentals and Turf Insect Note No. 5 provides additional information on galls.