I've often wondered what these red bumps on a field maple tree are. They develop each year. So tonight, I did some research to see if I could find out what causes red dots to appear on Field Maple leaves.
It turns out they are maple bladder galls or gall mites.
They cover the upper surface of the leaves and are caused by a small mite, vasates quadripedes.
The tree produces extra cells that form the growth which encloses the mite. The mite continues to feed and lays eggs inside.
As the new mites mature they move to other leaves to repeat the process.
The leaf galls usually only cause aesthetic damage, and sometimes early lead drop.
Preventative treatments may be a possible option, but if you can live with the red growths you may choose to let nature take its course.
From experience, the tree seems to be perfectly healthy and the damage is only superficial.