Leaf galls on plants are usually the result of mites and other sucking insects that make their homes under the plant tissue. Their feeding activities cause some galls, while chemicals secreted during egg growth in saliva or even excretions, may cause the changes to plant tissue. These changes may not be limited to bumps on leaves. Flowers, fruits, and even roots can develop these changes in tissue. Leaf galls are also sometimes found on stems and trunks. Other causes for galls are fungal and bacterial diseases.
Gall makers must attack at a very specific time in order to be successful. Otherwise, they may not be able to stimulate the plant to produce the tissue which forms the gall. Generally, initiation of leaf galls occurs during the "bud break" stage or as new leaves begin to unfold.
Once the symptom or gall appears, the causative animal is protected within the structure. This means that remedial actions, other than pruning in some cases, are not effective. Preventive action is necessary to attempt to reduce the infestation the following season and these may be of limited value. Fortunately, most galls, especially those on leaves and leaf structures, do not harm the health of the tree.
Oaks are one of the most susceptible, being host to over 500 different wasps, aphids, mites, and midges that cause galls on leaves and twigs.
