Aculus laevis is a gall mite that causes slightly hairy, more or less spherical galls to form on the leaves of sallows. The galls are 1 to 2 mm high and firmly attached to the leaves; the hairs protrude through the opening on underside.  The galls may be on the leaf margin.  

Taxonomy is tentative; this may be a complex of closely related species. 

Iteomyia capreae (a gall midge) is similar; it has a circular red-rimmed openings underneath.  Aculus tetanothrix ([possibly a species aggregate) forms similar galls on willows. 

Aculus iteina forms similar galls on sallows, but may be a species complex.  The galls are toadstool-like, with a stalk.

Gall Adult

Ensure you have identified host correctly, and that your photos include images of host plant leaves as well as galls; photograph the underside as well as upper

On Willows.

Summer.

Believed to be fairly frequent in Britain, but poorly recorded.

Status in Leicestershire and Rutland not known.