Have you ever picked up a leaf that was dotted with bumps or had long protrusions dangling from it? Chances are these are leaf galls.
The appearance of leaf galls is a jarring sight. The bumps can be hard or just hollow protrusions. Your first thought might be that it is a disease or insects have laid eggs or burrowed into the leaf, the way leafminers do. The good news is that the bumps are generally not caused by a disease. They are usually insect damage. However, by the time you see those bumps, the insect has long since moved on.
What Leaf Galls Are
Leaf galls are a disturbing sight but are not usually as serious as they appear. These bumps and deformities are generally the result of feeding insects or some other foreign organism such as bacteria, fungi, mites, nematodes, and even viruses. Whatever the original cause, these organisms are usually not still on the leaf. The gall itself is the plant’s response to the irritation. It’s not unlike the bump you get when an insect feeds on you, except the leaf gall is not going to go away.
Despite appearances, the insect is not living in the gall, and it has left behind no eggs in the gall itself. It is very likely that once you notice the galls, the insects have moved on. Before they do, they can do a lot of cosmetic damage to many plants and in particular trees. Galls can also form on stems and flowers. However, leaf galls seem to be the most prominent and get the most notice.
It is almost always the fresh new, leafy growth that is attacked by the insects and other organisms which results in the presence of galls. Mature leaves are rarely affected. However many common trees are susceptible to leaf galls, especially when first leafing out in the spring.
Damage will be more considerable following a mild winter since more insects have survived and are hungry. That is true of any insect problem in your yard or garden. While the damage from galls won’t usually kill a tree, it can weaken the tree and may cause early leaf drop. A healthy tree will send out new growth and recover, but the loss of leaves and photosynthesis will weaken a stressed tree.
What You Can Do About Leaf Galls
As unsightly as they are, the best thing to do is just let them be. Since the damage occurred before the gall formed, treatment is rarely recommended.
If you have a severe recurring problem with the same insect affecting your tree year after year, you can take action to control that insect population in your yard and thereby lessen the severity of the damage to your tree. Contact your local extension office for specific guidelines and recommendations in your area.
Dormant oil is a good general solution for controlling leaf eating insects that feed on trees. This is applied as a spray in early spring, when trees are first leafing out, and the insects will be visiting to chew on the tender new leaves. But if you're patient, nature may take care of the problem for you. Gall-making insects tend to attract predators that stay in the area as long as there is food for them to eat.
The Bottom Line on Leaf Galls
Although leaf galls do not always pose a serious problem for trees, you should not ignore the presence of galls entirely. The formation of leaf galls requires a good deal of energy and nutrients from the tree, as it tries to defend and heal itself. At the start of the growing season, the tree needs all its strength to leaf out, flower and grow. So the formation of galls can stress and weaken the tree itself, as it is coming out of dormancy in the spring. This can happen when there is an unusually high concentration of galls on the plant or when the plant is attacked, and galls are produced several years in a row. If this is the case, you should consider finding out what organism is causing the galls and treat for it the following spring, to prevent further stress and damage.
Galls on Trees. Missouri Botanical Garden
Plant Galls. The Morton Arboretum