This is a mite which produces tiny red pustule galls on the upper surface of Common Alder (Alnus glutinosa). They are up to 2mm in diameter, and are green to begin with, becoming red or purple later on. They may cover the whole upper surface of the leaf, and in some cases this may prohibit growth of the leaf.
The mites leave the gall via a small hole beneath the leaf which it had originally entered by in autumn. It sees the winter through in empty cones and bark crevices.
Seen summer until autumn. Common and widespread.
Photographs of Eriophyes laevis, taken June and 2013, local wood and nature reserve. © Pete Hillman 2013. Camera used Nikon Coolpix P500.