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:Fragaria vesca, Potentilla recta (Rosaceae); Heracleum sphondylium, Angelica sylvestris (Apiaceae); Convolvulus arvensis (Convolvulaceae); Veronica chamaedrys (Scrophulariaceae); Achillea millefolium, Cirsium arvense, Cichorium intybus, Leontodon autumnalis (Asteraceae).
Flowers visited in UK: Angelica, Heracleum (Apiaceae); Jasione montana (Campanulaceae); Cirsium (Asteraceae).