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: Stellaria media (Caryophyllaceae); Angelica sylvestris (Apiaceae); Thymus serpyllum (Lamiaceae); Taraxacum officinale (Asteraceae).
Flowers visited in UK (Else & Edwards, 2018): Angelica, Daucus (Apiaceae); Senecio jacobaea, Achillea millefolium, Tripleurospermum, Cirsium arvense (Asteraceae).