As this is a cleptoparasitic species, all flower visits will be for nectar only.
Celary (1991) lists the following nectar sources in Poland: Vicia cracca (Fabaceae); Knautia arvensis (Dipsacaceae).
Flowers visited in GB (Else & Edwards, 2018): Daucus (Apiaceae); Euphorbia (Euphorbiaceae) Ranunculus bulbosus (Ranunculaceae).