An apple on an oak?? Hardly! Oak-apples are one of many interesting growths on our woody plants. This oak-apple and a hedgehog gall, were both found on white oak. Each is caused by a different species of gall wasp that lays its eggs on oak leaves or twigs.
When the eggs hatch, the larvae stimulate the oak to isolate them in galls — a different gall type for each wasp species. The larvae grow and pupate inside the galls, then emerge to mate and the cycle continues. Oaks are host to a great variety of gall wasps, most of which do no real harm to the tree. They produce an array of beautiful and interesting galls.