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 2–5 cm in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.[1] 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 galland the Acorn cup gall, but each of these has its own distinctive form .(Text Courtesy of Wikipedia)
