As this is a cleptoparasitic species, all flower visits will be for nectar only.
Flowers visited in UK (Else & Edwards, 2018): Ranunculus (Ranunculaceae); Angelica (Apiaceae); Aster, Cirsium vulgare, Cirsium arvense (Asteraceae).
Celary (1991) lists the following plants as nectar sources in Poland: Salix (Salicaceae); Erophila verna (Brassicaceae); Sedum acre (Crassulaceae); Potentilla verna (Rosaceae); Veronica chamaedrys (Scrophulariaceae); Thymus serpyllum (Lamiaceae); Solidago virgaurea, Bellis perennis, Achillea millefolium, Tripleurospermum inodorum, Tussilago farfara, Taraxacum officinale, Hieracium sp. (Asteraceae).
Pawlikowski & Kruszynski (1997) list Potentilla arenaria (Rosaceae) as a nectar source in Poland.
Mavromoustakis (1948) lists Prunus domestica (Rosaceae) and Ballota (Lamiaceae) as nectar sources in Cyprus.