The body length of the adult Dark-tailed Thistle Aphid (Uroleucon aeneum) apterae is 3.0 to 4.3 mm. The cauda (tail) in this species is black which helps to identify it (see below).

Large Thistle Aphid (Uroleucon cirsii) is similar and also uses Thistle as a host plant, but in U. cirsii the cauda (tail) is dusky yellow not black as in U. aeneum.

A close up photograph is needed showing the cauda (tail).  Please also state the host species.

Where thistles are present.

Summer.

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

Rarely recorded in Leicestershire and Rutland.

https://influentialpoints.com/Gallery/Uroleucon_aeneum_dark-tailed_thistle_aphid.htm