Tree Galls

Spring is a great time of year. We can enjoy spending time outdoors with our friends and family and enjoy the views throughout our landscape. However, that view is sometimes interrupted by weird formations that show up on our tree leaves which are called galls. The good thing about galls is that they are not harmful to our trees, they are just displeasing to the eye.

Galls are commonly seen on many of our tree species. A gall is a deformation on the plant usually caused by an insect but they can be caused by fungi, bacteria, and other organisms. Galls can form on the leaves or on the branches but it is developed as a result of the feeding activity of the insect or mite that then lives inside the gall.

One of the most common types of galls is the Hackberry nipple gall which forms on the underside of the leaf of hackberry trees. This is a very common gall, in fact, it occurs so often that it can almost be used as an identification characteristic of the Hackberry tree. In the fall, the psyllids, or tiny black insects, come out of these galls to mate and often become a nuisance insect in our homes. They are so small that they can get through our window screens and enter our homes to fly around and pester our families.

HackberryNippleGall, S. Browning

Photo of Hackberry Nipple Gall by Sarah Browning, Lancaster County Extension

Another common gall that we are seeing right now is the bladder gall on Maples. This particular gall is caused by a mite. The signs we see from this insect feeding on our trees would be small, bright pink bumps on the top side of the leaves.

maple bladder gall

Photo of Maple Bladder Gall

Oaks also get a couple of galls. There is a leaf gall that is fairly common among oak trees called the oak apple gall which is a large, round, tan-colored gall on the leaves. It is a growth filled with a spongy center and contains one wasp larvae in the middle. Bur oaks commonly get a bullet gall which grows all over the branches of bur oak trees. These galls appear as marble-sized, tan, hard structures attached to the branches of the tree. The bullet gall, which is caused by a parasitic wasp, is generally not harmful to the tree, but if the infestation gets very high, it can cause branch dieback.

2016-04-01 10.52.50

Oak Bullet Gall

Cottonwoods get a petiole-leaf gall. This gall is caused by an aphid and is a puffy ball located at the base of the leaf where it meets the petiole, or the stem-like structure that attaches the leaf to the branch of the tree. The petiole-leaf gall will contain many small aphids later in the season that will not harm the tree.

These insect galls are generally not harmful to the tree. They mainly cause aesthetic damage and don’t affect the health or longevity of the tree. There is no way to control the gall insects once the galls appear on the tree and prevention is difficult and not recommended.

Part of the information for this article came from an article written by Mary Jane Frogge, a UNL Extension Associate from Lancaster County Extension.