This article was published originally on 8/25/2010
The Plant and Insect Diagnostic Clinic has been receiving many inquiries about strange bumps and growths on oak tree twigs and leaves. These growths are known as galls, and alarming as they may seem, they do not injure the plant.
Galls are abnormal growths or swellings of plant tissue often caused by the attack of an insect. Galls on oaks are most often caused by small wasps or midges. Insects that cause galls to form are known as gallmakers. Galls grow to surround the tiny insects that form them and provide them with protection from weather, predators and parasites. The gall also provides a source of food for the insect. The insect develops and grows inside the gall during the summer and emerges as an adult either in the summer or the following spring.
Most insect galls do not seriously affect the health of well-established trees. Occasionally, a heavy gall infestation causes severe leaf or stem deformities and early leaf drop. Galls located on the twigs and branches of tree can detract from the overall appearance of the tree, but again, rarely lead to the death of the tree
There is no way to "cure" the tree of galls once they are there. Leaf galls may not appear again the following season, but twig and stem galls will more than likely remain on the tree.
Commonly seen galls on oak include the following:
- Oak apple galls are attached to the oak leaf as round light-green balls up to 2 inches in diameter and house a single wasp larva. They often have a mottled maroon pattern on the surface.
- Oak bullet galls are common twig galls on our native bur oaks. Bullet galls are hard, round, and pea-to-knuckle-sized, and usually are most apparent by August.
- Jumping oak galls grow on leaves and look a bit like small, round seeds or BBs. The galls fall off the oak, carrying the larva with them. The larvae in the fallen galls are active, and as they jerk around, the small galls can seem to jump on the ground (similar to "Mexican jumping beans."
- Oak pill galls are irregular, hardened swellings up to one-quarter inch in diameter on the upper surface of the leaf.
- Woolly oak leaf gall (photo below) look like a dense wad of wool attached to the leaf midvein. They may be as large as three-fourths of an inch and are often bright pink or yellow in color, fading to brown in the fall.
- Horned oak galls are a stem gall that can be numerous on trees. There are no effective treatments.
Woolly oak leaf gall, one of many common galls on oaks, and common in northern Iowa this year.
Horned oak gall, the 'horns' contain the wasp larvae that trigger the gall formation.
Oak tree heavily infested with horned oak galls.