Maple pimple gall mite

Maple pimple gall mite (Aceria spp.) is a tiny mite less than a millimeter in size. This mite is responsible for the galls on the leaves of trees of the Acer family, especially the Field maple (Acer Campestre). The mites live in the galls. These two-millimeter-sized galls are initially green and change color from yellow-brown to red as summer progresses. The galls are applied at the vein corners of the leaf or at the beginning of the petioles. The galls are on the top of the leaf and are sometimes hairy. The opening of the galls is on the underside of the leaf.
The tree is not harmed by this mite.