West Coast lady it lacks obvious ventral eyespots on the hindwings; there are two large ones on "V. virginiensis" and four small ones on "V. cardui". Like the latter, it also lacks a white dot in the pinkish-orange subapical field of the ventral and dorsal forewings. Its upperwing coloration has the purest orange of the three; the American painted lady is usually quite reddish. A less reliable indicator is the row of black eyespots on the dorsal submarginal hindwing. These are usually of roughly equal size in "V. cardui" and lack blue centers, though the summer morph may have a few tiny ones.
Brachycaudus cardui "B. cardui" are usually attended by ants which feed on the honeydew the aphids produce and drive away predators. Researchers found that in a damp meadow which contained ragwort ("Jacobaea vulgaris") but no ants, although winged "B. cardui" arrived at the site, they failed to establish colonies despite an abundance of the host plant. In an experimental habitat, colonies of "B. cardui" on "J. vulgaris" flourished when ants were present but died out if ants were excluded. Infestation with "B. cardui" causes the margins of affected leaves to roll up. This aphid is a vector of plum pox, a serious