Eriosoma ulmi is a greyish waxy aphid that causes a leaf roll gall to form on the leaves of Elm. One half of the Elm leaf blade crinkles and rolls in to form a tubular pouch. The gall is seen more often than the aphid.

Other Eriosoma species can cause leaf blisters and distortions on elm.  The presence of wax inside the gall should distinguish E ulmi from the very similar E grossulariae

Gall  Adult

State host and provide photos of the gall AND the aphids inside, showing presence of wax.

Gall is found on Elm leaves and is especially noticeable on Wych Elm.

Gall on Elm appears from May onwards and can then be found whilst Elm remains in leaf.

Numerous waxy aphids develop in the pouch galls in spring and early summer. Winged aphids (all females) leave the pouch in June and July and fly to currant bushes where they live and reproduce on the roots, returning to the Elm in autumn.

Common and widespread but under recorded in Britain.

Fairly frequent in Leicestershire and Rutland.

https://influentialpoints.com/Gallery/Eriosoma_ulmi_Elm-currant_aphid.htm