Maple Bladder Galls

The following questions were sent to the P&PDL diagnosticians at Purdue University:

Question 1: I have a small silver maple in my yard. I have recently noticed that many of the leaves are covered with small, irregular raised areas on their surface that are red. The irregularities are roughly 1/16 of an inch in diameter and stand about 1/16 of on inch in relief from the leaf's surface. Is this indicative of a threatening condition for my tree? If so, how do I control it?

Question 2: I have small red spheres (size of pin-heads) on the leaves of my maple tree. What is this? will it hurt the tree? how do I get rid of it? (note that if I were to spray with hose could only get half to 2/3 up the tree.

Answer: These are maple bladder galls. The silver or soft maple trees are often attacked by tiny mites that cause small, wart-like growths on the foliage. These growths are first red, then turn green, and finally black. They occur singly or in clusters and may be so abundant that the leaves become crinkled, deformed and drop early. Once formed, the galls cannot be removed from the leaves because they are composed of plant tissue and are actually part of the leaf. Many homeowners become alarmed when they discover infestations of the maple bladder gall, fearing that their trees might die unless control measures are taken. This is not likely. The galls never cause permanent injury and have little effect on tree health and vigor. The galls do, however, detract from the normal beauty of the foliage.

The maple bladder gall mites overwinter in cracks and crevices of the bark. As the buds swell in the early spring, they migrate out on the bud scales. This is when mites are most susceptible to dormant applications of oil spray. When buds open, the mites feed on the newly developing leaves. In response to this feeding, hollow galls are formed. The mites then live, feed, and mate inside. In fall, mites move back to the bark to hide over the winter.