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: Fragaria (Rosaceae); Calluna vulgaris (Ericaceae); Chrysanthemum (Asteraceae).
Flowers visited in UK (Else & Edwards, 2018): Potentilla (Rosaceae); Foeniculum vulgare, Daucus (Apiaceae).