The mite causes an erineum to form on Norway Maple (Acer platanoides), usually on the leaf underside.  The mites live amongst the hairs of the erineum, which is pale to begin with and brown later; the individual hairs (use a hand lens) are mushroom shaped.  There is a corresponding blotch on the upper leaf-surface.

erinea on other species of Acer (maples)

Photograph the erineum and ensure you have correctly identified and noted the host species of Acer. Record all erineum galls on Norway Maple as this species aggregate. 

Norway Maple

rarely recorded

The taxonomy of erineum galls on Acer species is unclear.  See British Plant Galls (Redfern & Shirley, 2023) and Buckton, S.J. (2023)  A provisional key to eriophyoid mite erinea on Acer in Britain