Thank you for the question.  The upper surface of maple leaves often become covered with small, red, round wart-like structures about 1/16 to 1/8-inch in diameter. These are maple bladder galls caused by a small mite, Vasates quadripedes. The structures are generally first noticed in MN in mid-May. At first the galls are green but they quickly turn pink to red and eventually black. Leaves frequently become so covered with the galls that they completely twist out of shape and may even drop early. Other than the mere aesthetic damage and possible early leaf drop, no significant damage is done to the health of maple trees. If the damage from these leaf galls is excessive then affected trees often send out new leaves to replace the damaged ones. http://www.extension.umn.edu/distribution/horticulture/DG1009.html 

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