This is a large blackish aphid that can appear dark metallic green. The abdominal tergites 2-4 often have small marginal tubercles. The femora have the basal half pale and distal half dark, with a rather sharp transition between them. The tibiae are totally black. The siphunculi are reticulated over the distal 0.16-0.27 of their length and are 1.3-1.8 times the length of the cauda. The cauda is dark and bears 18-30 hairs. The body length of apterae is 3.0-4.5 mm long.

The large knapweed aphid lives on various knapweeds (Centaurea spp.) and a few other species of Asteraceae, but not generally on other thistles.

May to September.

It is found across Europe to the Middle East and much of Asia.

Widespread and common in Britain.

Quite common in Leicestershire and Rutland.

https://influentialpoints.com/Gallery/Uroleucon_jaceae_Large_knapweed_aphid.htm