Answer: The little bumps are probably caused by blister mite feeding. The blister mite galls won't harm your plant to any great degree. In fact, the leaves look kind of interesting. The little mites feed in the spring and inject a fluid which causes the spindles or bumps on the leaves. By the time you see the damage, the mites are long gone. Other than the mere aesthetic damage and possible early leaf drop, no significant damage is done to the health of maple trees. 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. Next year you may not see any damage at all, or you may see similar damage. Insect pest populations usually ebb and flow depending upon weather and predatory insects so just expect that some years you will see this type of damage and other years you won't. Best wishes with your maple trees.
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