A Technical Report from The Bartlett Tree Research Laboratories
Galls are formed when an external agent, such as an insect or fungi, causes plant tissue to swell. Galls can occur on any part of a plant. Gall formation on individual leaves or petioles are not typically associated with tree injury or death, but twig or trunk galls can lead to tree decline if infestations are severe. The insect family Cynipidae, commonly known as the gall wasps, are agents of gall formation on oaks and many other species of trees. Over 750 species of cynipid gall wasps occur in North America. The focus of this report will be cynipid gall wasps that cause galls on oak twigs and leaves. The larvae of gall-forming cynipid wasps develop inside of the galls, which protect them from parasitism and predation. The inside of the gall also provides an ideal internal temperature and food for developing larvae.