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 acer (Ranunculaceae); Sedum acre (Crassulaceae); Potentilla sp. (Rosaceae); Anthemis arvensis, Achillea millefolium, Senecio sp., Leontodon autumnalis, Taraxacum officinale, Hieracium pilosella (Asteraceae).
Flowers visited in UK (Else & Edwards, 2018): Ulex gallii (Fabaceae); Rubus, Potentilla (Rosaceae); Angelica, Pastinaca sativa, Heracleum, Daucus (Apiaceae); Solidago, Achillea, Tripleurospermum, Cirsium arvense (Asteraceae).