As this is a cleptoparasitic species, all flower visits will be for nectar only.
Flowers visited; In UK (Else and Edwards, 2018) list: Lotus corniculatus (Fabaceae); Veronica (Plantaginaceae); Carduus, Cirsium vulgare, Hieracium (Asteraceae).
Kasparek (2015)[summarising and citing others] lists Centaurea, Cirsium vulgare, Carduus crispus, Onopordum acanthium, Centaurea jacea, Centaurea scabiosa, Hieracium pilosella, Crepis biennis (Asteraceae); Lotus corniculatus (Fabaceae).
Celary (1989) lists the fiollowing nectar sources in Poland: Medicago sp, Trifolium sp. (Fabaceae); Symphytum sp., Echium vulgare (Boraginaceae); Knautia sp. (Dipsacaceae); Jacobaea vulgaris, Carduus sp., Onopordon acanthium, Centaurea sp., Leontodon sp. and Taraxacum officinale (Asteraceae).
Westrich (1989) lists the following nectar sources in Germany: Cirsium vulgare, Carduus crispus, Onopordum acanthium, Centaurea jacea, Centaurea scabiosa, Hieracium pilosella, Crepis biennis (Asteraceae).
Elfving (1968) cites Leontodon autumnalis, Hieracium pilosella, Arnica latifolia (Asteraceae). Arguib et al (2014) list Centaurea nicaeensis, Carduus sp., and Onopordon (Asteraceae) in Algeria.
In Cyprus, Popov (1944), and Mavromoustakis (1948) reports nectaring at Marrubium sp. (Lamiaceae)