The mite causes an erineum to form on Sycamore (Acer pseudoplatanus), 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 long and tapering.  On purple-leaved varieties, the erineum is flushed pink. 

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 Sycamore as this species aggregate.   

Sycamore

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