Acalitus rudis is a gall mite that causes galls to form on the leaves of Birch (Betula species). The gall is an erineum between the veins of the leaf. It is usually on the underside of the leaf, with or without a bulge on the other side. The hairs are quite long, white or yellowish when young, later a deep red brown.

Other mites can cause an erineum (hairy patch) to form on birch leaves

Gall Adult

Where the host Birch species are found.

Whenever Birch is in leaf.

This is the commonest erineum on Birch, but it remains difficult to determine the exact status in Britain due to lack of records.

Status in Leicestershire and Rutland not known.