The round wart-like structures on the upper surface of your maple are maple bladder galls caused by a small Eriophyid mite, Vasates quadripedes (Shimer). The structures are often first noticed in May when the leaves have become fully expanded. The galls are initially green, but they quickly turn pink to red and eventually black. Leaves may become covered with so many galls that they completely twist out of shape and then drop early. These galls seldom harm the tree, so control measures are not generally needed. The following sites explain the life cycle of these mites.

• Insect and Mite Galls
• Maple Bladder Gall, Spindle Gall, and Gouty Vein Gall