Common spangle gall

Common spangle gall (Neuroterus quercusbaccarum) is a small wasp belonging to the gall wasps (Cynipidae). The common spangle gall causes the +4 mm lens gall on the underside of the leaf. The galles are flat on one side and flat on the other side with an elevation. Often yellow dots can be seen on the top of the leaf where the galls attach to the underside of the leaf. The gall turns from yellow to rusty brown during the season. In autumn, they are often found in large numbers under oaks.
The galls contain the larvae. The galls fall from the leaves before the leaves detach from the tree in autumn. On the ground, the larvae hibernate in the galls to emerge as insects in May.
The common spangle galls do not do much damage, although a tree may weaken a bit if all the leaves are covered with galls.