Aculus tetanothrix is a mite that causes rounded, sometimes reddened galls to form on the leaves of willows, particularly White Willow.  Small reddish pustules or pouches on the leaf upperside have slit like opening underneath; they are hairy inside but the hairs do not protrude through opening.  The galls may also form on the leaf margins.

Taxonomy is tentative; this may be a complex of closely related species; Aculus salicisalbae may be the correct name for the species on Salix alba.

Iteomyia capreae (a gall midge) is similar, but on sallows; it has a circular red-rimmed openings underneath.

Aculus laevis and Aceria iteina form similar galls on sallows.

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

Wherever White Willow, Crack willow or hybrids are found.

Summer and autumn.

Thought to be widespread and fairly common where White Willow occurs.

Status in Leicestershire and Rutland not known.