As this is a cleptoparasitic species, all flower visits will be for nectar only.
Celary (1991) lists the following species as nectar sources in Poland: Ranunculus acre (Ranunculaceae); Heracleum sphondylium, Angelica sylvestris (Apiaceae); Jasione montana (Campanulaceae); Achillea millefolium, Tripleurospermum inodorum, Taraxacum officinale, Hieracium pilosella (Asteraceae).
Flowers visited In UK (Else & Edwards, 2018): Cirsium arvense and Crepis (Asteraceae). probably also others.