These incredible galls are produced by wasps. The oak apple is created by a gall wasp laying an egg into the leaf bud of an oak which hatches in the spring. The larvae feeds on the gall tissue and the gall develops through their secretion. The oak apple tends to fall this time of year when the gall wasp is ready to fly.
Oak apples grow on most types of Oak trees whereas the knopper galls are produced on pendunculate and Turkey oaks. These different type of gall wasps lay an egg in the flower bud of a pendunculate oak in late spring and as they develop they distort the shape of the acorn. Through the winter the wasps will emerge from the knopper galls and the females will then find the Male flower buds of a Turkey oak to lay her eggs for the next generation.