Wingspan 11 - 14 mm.
P. affinitana flies June - August in England and Wales on saltings where its foodplant, Aster tripolium, grows.
Fresh well-marked specimens have a median fascia visible in the dorsal half of the forewing, but the strength of markings can vary greatly. For identification of Phalonidia species, it is advised to examine the genitalia, or rear from larvae on the appropriate foodplant.
The larva lives July-August in Aster tripolium flowers. In September it bores down the flower stalk and stem to the rootstock, where it overwinters until pupation in March in the same position, or in debris close by.