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Red bumps on maple leaves

red bumps on leavesYour tree probably has maple bladder galls or gall mites. Maple bladder galls are small, red, round wart-like structures about 1.5mm to 3 mm in diameter that can cover the upper surface of the foliage on silver and red maple trees. Galls can be present every year, though the abundance varies greatly from year to year and from tree to tree.  These galls are caused by a small mite, Vasates quadripedes.

The structures are generally first noticed in May, about the time the leaves have become fully expanded. At first the galls are green but they quickly turn pink to red and eventually black. When there is a severe  infestation leaves may become so covered with the galls that they completely twist out of shape and may even drop early. Adult mites spend the winter under the bark and other protective places on the trees.

In the early spring, the adults move to developing, unfolding leaves and begin feeding. The leaf responds to the small irritation by rapidly producing extra cells that form the abnormal growth at the feeding site. The gall encloses the mite which continues to feed and lay numerous eggs within the gall. Reproduction is prolific and as the new mites mature, they leave the gall and move to other newly emerging leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity continues until mid-summer when it starts to decline. Adult mites leave the foliage in the fall and move to the over-wintering sites.

Maple leaf galls seldom, if ever, cause permanent injury to a tree.  Most commonly the galls only cause aesthetic damage and possible early leaf drop. Following a mild winter, damage from these leaf galls can be excessive but affected trees often send out new leaves to replace the damaged ones.

What you can do

It is important to keep your tree well watered and healthy so that it can build up its own defenses to fight infestations. Galls can not be "cured" after they have formed. Neither sprays nor systemic insecticides will eliminate the galls nor improve the condition of the trees. Preventive treatments, such as dormant oils applied at the time of bud break in early spring are a possible option. As mentioned, the galls are not significant to the trees, and massive pesticide use with little returned benefit is not justified.

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