As this is a cleptoparasitic species, all flower visits will be for nectar only.
Celary (1991) lists the following plants as nectar sources in Poland: Thymus serpyllum (Lamiaceae); Potentilla argentea (Rosaceae); Bellis perennis, Taraxacum officinale, Hieracium sp. (Asteraceae).
Flowers visited in UK (Else & Edwards, 2018): Ranunculus (Ranunculaceae); Medicago lupulina, Hippocrepis (Fabaceae); Potentilla erecta (Rosaceae); Anthriscus, Torilis, Caucalis, Aegopodium, Oenanthe, Angelica, Pastinaca, Heracleum, Daucus (Apiaceae); Euphorbia (Euphorbiaceae); Jasione (Campanulaceae); Solidago, Achillea ptarmica, Tripleurospermum, Carduus, Taraxacum (Asteraceae).
Grace (2010) reports a female at flowers of Smyrnium perfoliatum (Apiaceae) in June in Lesvos