The silver maple capitate mite ( Vasates quadripedes ) is a herbivorous mite from the family Eriophyidae . Monophage, haloformer, damages North American maples: silver maple ( Acer saccharinum ), sugar maple ( Acer saccharum ), red maple ( Acer rubrum ). Induces the formation of small capitate galls on the leaves. Comes from North America. In the 20th century, with planting material, it was brought to Europe. Recorded in 1957 in Latvia, currently widespread in Europe on silver maple ( Acer saccharinum ).
Insects that form galls like this are typically only causing cosmetic damage and don't threaten the health of the tree. I wouldn't worry about them unless they were really out of control or you started seeing leaves dying