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Maple trees of all types are a mainstay in No. Illinois. Their vibrant fall color, hardiness, disease resistance and fast growth rate make them a landscape favorite. Some species of Maple can develop small red or black bumps on their leaves during the summer months that can be alarming at first glance. These small red and black bumps are caused by an insect called the Maple Bladder Gall.
These galls are caused by tiny eriophyid mites that feed on the new growth of many species of maple trees. These mites are microscopic and will feed on the leaves of the freshly emerging leaves. The mites will start feeding early in the spring, just as the leaves begin to emerge. As the leaves mature, the mites cause the tree to enclose them in an abnormal green / red growth. Once the mites are completely enclosed, they are protected from other insects and pesticides. The mites will mature, lay eggs within the galls and then die. After the eggs hatch, they will leave the gall and begin the cycle over again, starting to feed on the new growth of the tree.
Typically this cycle can only continue until early-summer, since the mites are only able to feed on the new growth of the trees. However, even though the mites can only feed until the trees stop growing for the season, the galls will remain on the leaves of the tree until they fall off. This means that, in some cases, you may not notice gall on your tree until late-summer or fall. This is far past the time that the mites are active and able to be treated for.
Most cases of Maple Bladder Gall are harmless to the tree and are considered an aesthetic issue only. If you have a minor case of Maple Bladder Gall, fertilizing your tree in the spring and fall with an organic, slow-release fertilizer is the best course of action. Wasco Nursery recommends Espoma’s Tree-Tone.
If you have a more severe / persistent case of Maple Bladder Gall, you can spray the leaves with an all-season’s horticultural oil starting in the early spring just as the leaves begin to emerge. Wasco Nursery recommends Bonide’s Organic All-Seasons Horticultural Oil. There is a very short window time when the insect is susceptible to spray applications. For this reason, Wasco Nursery typically does not recommend treatment for Maple Bladder Gall.
