Have you noticed odd bumps or growths on your tree leaves this year? At first glance, they can look like a serious disease or insect infestation. Fortunately, appearances can be deceiving. In many cases, these unusual growths are harmless leaf galls. While they may make trees look a little unusual, they rarely threaten the health of established trees.
One of the most common galls found in Nebraska is the maple bladder gall. If it looks like your maple tree has the measles, this tiny mite is probably to blame. Maple bladder galls are small, raised bumps that develop on the upper surface of silver maple leaves and occasionally other maple species.
The culprit is a microscopic eriophyid mite. These tiny mites spend the winter tucked beneath bark or in other protected places on the tree. As new leaves begin to unfold in the spring, the mites emerge and begin feeding on the tender foliage. The tree responds by forming a small, blister-like growth, called a gall, around each feeding mite. Inside the gall, the mite continues to feed, develop, and lay eggs, safely protected from predators and the environment.
The appearance of maple bladder galls changes throughout the growing season. They begin as small green bumps, then gradually turn bright red before eventually becoming dark brown or black. Later in the season, the dried galls fall away, leaving small holes in the leaves. During heavy infestations, leaves may become curled or distorted and may drop early, but the overall health of the tree is usually not affected.
Hackberry trees have their own unique gall known as the hackberry nipple gall. Unlike maple bladder galls, these develop on the underside of the leaf and resemble tiny green pouches or nipple-like bumps. They are caused by the hackberry psyllid, a tiny insect related to cicadas.
Like the maple gall, the hackberry nipple gall is simply the tree's response to insect feeding. If you carefully open one of the galls, you'll often find a tiny developing psyllid inside.
Although the galls themselves cause very little damage, the adult psyllids can become a nuisance in the fall. During September and October, large numbers of adults emerge from the galls and may gather around windows, doors, and screens. Because they are so small, they can even squeeze through window screens. Anyone with a mature hackberry tree nearby has probably encountered these tiny visitors.
The good news is that neither maple bladder galls nor hackberry nipple galls are considered serious pests. Healthy, established trees generally tolerate them with little or no long-term damage. At worst, heavily infested trees may lose a few leaves early or appear a little less attractive for the remainder of the season.
Many homeowners wonder if they should spray their trees. Unfortunately, once the galls become visible, it is too late for treatment. The insects or mites are safely enclosed inside the gall, where insecticides cannot reach them. Preventive treatments are available, but they must be timed precisely around bud break before the galls begin to form. Because these galls rarely affect tree health, chemical treatments are generally not recommended. The cost, timing, and amount of pesticide required usually outweigh any benefit.
Sometimes the best management option is simply to appreciate nature at work. Leaf galls are fascinating examples of the relationship between insects and plants. While they may look alarming at first, they are usually nothing more than cosmetic blemishes.
Not every bump on a leaf is cause for concern. In the case of maple bladder galls and hackberry nipple galls, those strange growths are more interesting than harmful—and they offer a reminder that our landscapes are full of remarkable interactions happening right before our eyes.
Elizabeth Exstrom is a Horticulture Extension Educator with Nebraska Extension in Hall County. For research-based gardening information, contact Elizabeth at elizabeth.exstrom@unl.edu, visit huskerhort.com, or follow HuskerHort on Facebook and Twitter.