Galls are found on numerous tree species. Most of the time gall formation doesn't harm a tree. Insects cause the greatest number of tree galls, but mites, fungi and bacteria can also cause them. Tree galls, such as the small, seed-like jumping oak gall that is found on white oak leaves, form a protective area for development on the host. Many times, galls are unsightly, which initiates a response to exercise some type of pest or disease control.
Galls formed from the activities of larval wasps create a refuge for the developing insects and mites. Other galls formed from fungi and bacteria also create a place for the reproduction of those organisms. Often, gall-forming organisms require specific host trees. Whereas many galls do not produce a benefit to the host tree, some organisms such as gall-forming bacteria create a benefit through activities like nitrogen fixation in the roots.
Most of the noticeable galls found in trees are caused by small wasps found in the family Cynipidae. Female wasps will lay eggs on or in tree tissues such as buds. After hatching, the larvae begin feeding in the tissues and their saliva cause a growth reaction in the tissue cells. Mites cause reactions in much the same way.
Examples of galls commonly seen on trees such as oak and maple include wool sower galls, jumping oak galls, and maple spindle galls. Wool sower galls are white, round, fuzzy growths that occur on white oak twigs in the springtime. Jumping oak galls are small, seed-like growths that appear on the underside of white oak leaves during the summer. Maple spindle galls are short, erect growths found on the upper side of leaves such as on red and sugar maples.
Control is usually unnecessary for galls. Galls often do not damage a tree, but create a less-than-pleasing appearance than what people are used to seeing. When control practices are performed, they must be used in accordance with the specific cause of the gall. Spray insecticides and miticides to newly opening leaves or apply systemic insecticides at the appropriate time of year to reduce the number of galls. Also, control must be practiced prior to gall formation, because if a gall has formed, a spray will not make it go away.
Although tree galls are often seen as unsightly problems, many have beneficial uses for other organisms. Iron gall ink, which was a very important ink for centuries in Europe as well as in North America, was made from galls found on oak trees. Additionally, tree galls have been used in herbal medicine. Other insects will sometime scavenge galls as a food source after the causal organism emerges or even invade before it emerges.