As this is a cleptoparasitic species, all flower visits will be for nectar only.
Westrich (2018) lists the following nectar sources in Germany: Senecio erucifolius, Hieracium umbellatum, Centaurea jacea, Centaurea scabiosa, Solidago canadensis, Symphyotrichum novae-angliae (Asteraceae); Diplotaxis tenuifolia (Brassicaceae); Scabiosa columbaria (Dipsacaceae); Medicago sativa, Trifolium repens (Fabaceae); Origanum vulgare (Lamiaceae); Malva alcea, Malva neglecta, Malva sylvestris (Malvaceae); Rubus caesius (Rosaceae).
David Genoud (FR) has observed the species nectaring at flowers of Succisa pratensis (Dipsacaceae)(pers. comm.); Géraud de Premorel (FR) reports visitation to Pulicaria dysenterica (Asteraceae), Dani Rath (FR) at Galinsoga parviflora (Asteraceae); Hervé Laville (FR) at Thymus (Lamiaceae), Alain C (FR) Satureja sp. (Lamiaceae)