Aceria macrocheluserinea

This gall mite causes a felt like galling of the underside of the leaves of Field Maple. It appears white at first before turning red and finally brown. The dense hairs shelter numerous mites. The gall is seen more easily than the mite.

Other mites can cause an erineum  (hairy patch) to form on Field Maple leaves, so these galls are best recorded as a species aggregate, following the latest advice from the BPGS. Further information in Sam Buckton's paper, linked below

Gall Adult

Anywhere that Field Maple grows.

The gall can be seen in Late summer until the leaves of the host plant fall.

Thought to be widespread and fairly frequent in Britain, as species aggregate.

Thought to be fairly frequent in Leicestershire and Rutland, as species aggregate.

https://www.ynu.org.uk/images/insects/Acer_erinea_key.pdf