An oak apple is a mutation of an oak leaf caused by chemicals injected by the larva of certain kinds of gall wasp. They are so called because the gall, which can measure up to 5 cm in diameter but is normally only around 2 cm, looks a little like an apple. Considerable confusion exists in the general 'literature' between the oak apple and oak marble galls. The oak marble is frequently called the oak apple due to the superficial resemblance and the preponderance of the oak marble gall in the wild.
European oak apples are caused by the Biorhiza pallida gall wasp and American oak apples by Amphibolips confluenta. Oak apples may be brownish or reddish.
The wasp larva that lives inside oak apples are a good source of bait for fishing, and also are useful as a survival food