The mite causes an erineum to form on Field Maple (Acer campestre), usually on the leaf underside.  The mites live amongst the hairs of the erineum, which is white to begin with, then pink and then brown; the individual hairs (use a hand lens) are mushroom shaped with rounded bead-like tips.  

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 Field Maple as this species aggregate.   

Field Maple

Common

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