It infests trees in the genus Prunus in the spring and autumn, and mostly plants in the aster family in the summer.

The viviparous wingless females of B. cardui have an oval or pear-shaped body and grow to a length of from 1.8 to 2.5 mm (0.07 to 0.10 in).

The abdomen has a dark, shining patch on the dorsal surface, a fact which distinguishes this species from the otherwise similar plum leaf curl aphid (Brachycaudus helichrysi).

[3] Colonies of 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.