As this is a cleptoparasitic species, all flower visits will be for nectar only.
Celary (1991) lists the folowing nectar sources in Poland: Heracleum sphondylium (Apiaceae); Thymus serpyllum, Mentha (Lamiaceae); Succisa pratensis, Knautia arvensis (Dipsacaceae); Jasione montana (Campanulaceae); Helichrysum arenarium, Solidago serotina, Erigeron, Achillea millefolium, Cirsium arvense, Centaurea rhenana, Hieracium sp. (Asteraceae).
Flowers visited in GB (Else & Edwards, 2018):Angelica, Pastinaca sativa, Daucus (Apiaceae); Achillea, Tripleurospermum, Cirsium arvense (Asteraceae)