<P> Oak apple or oak gall is the common name for a large, round, vaguely apple - like gall commonly found on many species of oak . Oak apples range in size from 2 to 4 centimetres (1 to 2 in) in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae . The adult female wasp lays single eggs in developing leaf buds . The wasp larvae feed on the gall tissue resulting from their secretions . Considerable confusion exists in the general "literature" between the oak apple and the oak marble gall . 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 . Other galls found on oak trees include the oak artichoke gall and the acorn cup gall, but each of these has its own distinctive form . </P> <P> Some common oak - apple - forming species are the Biorhiza pallida gall wasp in Europe, Amphibolips confluenta in eastern North America, and Atrusca bella in western North America . Oak apples may be brownish, yellowish, greenish, pinkish, or reddish . </P>

What are the balls that grow on oak trees