As this is a cleptoparasitic species, all flower visits will be for nectar only.
Flowers visited Eryngium maritimum (Apiaceae) seems to be the only British record.
In Germany, Westrich (1989) lists Geranium sylvaticum (Geraniaceae); Trifolium campestre, Lotus corniculatus (Fabaceae); Chamerion [as Epilobium] angustifolium (Onagraceae); Thymus serpyllum (Lamiaceae) Jasione montana (Campanulaceae); Knautia arvensis, Scabiosa columbaria (Dipsacaceae); Solidago virgaurea and Centaurea jacea (Asteraceae).