This aphid causes galls on Poplar and Buttercup species. On Poplar the galls are usually found as red blisters on the leaf. See link below for more details.

Gall Adult

Adult: Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

Areas where Poplar occurs.

Galls are best looked for in summer.

This species usually host alternates from its primary host of Poplar (Populus) species to Buttercup (Ranunculus spp.) although some populations remain all year on Buttercup roots. On Poplar the fundatrix inhabits a small gall of its own but its offspring leave this gall and stimulate development of a larger gall where the outer, upper side of the leaf becomes blistered and reddish. The winged viviparous females that develop in the gall on Poplar leave in late June-July to found waxy colonies at the stem bases and on the runners of Buttercups. Alatae migrate back to the primary host in autumn.

True status in Britain is difficult to determine due to lack of records.

Status in Leicestershire and Rutland not known.

https://influentialpoints.com/Gallery/Thecabius_affinis_Poplar-buttercup_gall_aphid.htm