Oaks (Quercus spp.) are evergreen or deciduous trees that occur in nearly 400 varieties around the world. A number of oak trees, such as the red oak, white oak and black oak, are native to the United States. Oak trees have a mature height of 60 to 70 feet with a 40- to 50-foot spread. The trees are prone to numerous pest infestations, including insects that cause ball-like growths on the tree.
Damage from a number of insects such as aphids, psyllids, midges and cynipid wasps leads to the formation of round balls referred to as galls on oak trees. Although galls are also the likely result of bacteria, nematodes or fungi, insects remain the predominant reason for gall formation. More than 2,000 gall-producing pests exist in the United States, of which 1,500 are gall gnats. Nearly 80 per cent of gall gnats specifically affect oak trees. Of all the identified insect-related galls, 60 per cent occur on oak trees.
Galls are most frequently seen on the foliage and stems of trees. In many cases, galls are also seen on the trunk, flowers, roots and petioles. Galls vary in size ranging from 2 inches in diameter to barely noticeable, depending on the causal insect. The growth is not always round but also tubular and occurs in every possible colour.
Gall Formation Cause
The causal pests start to lay their eggs in newly growing plant tissue during early spring. As the insects lay eggs, they also secrete growth-regulating chemicals from specialised glands in their bodies. These chemicals react with chemicals found in plant tissues and cause the round balls to start developing. Each gall houses the causal insect inside it that feeds only on the gall tissue. Once mature, the adult insect bores its way out of the gall, leaving small exit holes.
No control options are available that will stop the growth of galls or remove them from the tree after they appear. Though galls look like they are damaging in nature, their growth does not affect plant health even when they appear in large numbers. While some homeowners feel that galls reduce the aesthetic quality of the trees, other specifically seek gall susceptible oak varieties as they find the abnormal growth interesting. Since galls are primarily insect related, keeping trees in good health increases defence against causal pest infestations.