Today I am going to talk about insect galls. I’ve gotten several questions from people that have seen something strange looking growing on their plant, and they wanted to know what it is. Galls can make a leaf look alien, but they rarely harm the tree. Let’s talk about some of them, what causes galls, and the damage that they do to a plant.
Insects usually cause galls. They look like an extra growth or tumor on the plant. They can take a variety of colors and shapes. Use of pesticides is not recommended in most cases to treat galls. Most pesticides will be ineffective because the insect or disease causing the gall is inside the plant.
Oak apple galls are somewhat common. These galls form on oak trees. They look like round balls that are attached to the oak leaves. They start out green, but turn brown, and are about half an inch to 2 inches in diameter. Oak apple wasps cause these galls. Oak apple wasps are really small wasps. These small wasps are not dangerous to people and won’t hurt you. Inside the gall is a single wasp larva. Once the larva emerges the gall will turn brown, and you can probably see the hole where it emerged, if the gall is still intact.
Azalea leaf gall is another fairly common gall. This one is not caused by an insect, but is a fungal disease. The disease overwinters inside the plant. Then in the spring and summer, you’ll start to see whitish swollen tissue appearing. That tissue has spores that can be moved to other leaves or plants by rain or wind. Fungicides are not an effective way to treat azalea leaf gall. If you see one of these galls, just pick it off and throw it in the trash, which will reduce its spread. Azalea leaf gall can also infect rhododendrons.
Maples can have a couple of different types of galls. One of those is called maple eyespot gall. A midge causes it. A midge is a really small insect. These insects will lay their eggs on the underside of leaves. The eggs hatch and the larvae feed on the leaf from inside the gall. The hormones that the midge injects into the leaf causes a dark red ring to form that has a yellow inner ring and a dark red center. The baby midge will then drop to the ground and finish growing into an adult. These galls rarely do real damage to the tree. Using an insecticide to treat is not recommended because the midge is protected inside the gall.
One type of gall that is problematic for farmers is phylloxera. This louse like insect will feed on roots, which causes roots to swell and die. One of their favorite plants to feed on is grape. Nowadays grapevines are grafted onto a rootstock that is resistant to phylloxera insects. Phylloxera is native to North America, so our native grapes like muscadine and fox grape are resistant to it. However, European grapes are not. In the mid 1800s, some American vines were shipped to Europe that were infected with phylloxera. The arrival of phylloxera in Europe and other countries around the world nearly destroyed the winemaking industry. For the vineyards that did not die, they had to rip out their vines and replace them with ones that were grafted onto resistant, American rootstock.
If you have questions about galls contact your County Extension Office or email me at Jacob.Williams@uga.edu.