As this is a cleptoparasitic species, all flower visits will be for nectar only.
Celary (1991) lists the following plant species as nectar sources: Heracleum sphondylium (Apiaceae); Thymus serpyllum (Lamiaceae); Solidago canadensis, Solidago virgaurea (Asteraceae)
Flowers visited in UK (Else & Edwards, 2018): Jacobaea vulgaris, Pulicaria dysenterica, Solidago, Cirsium arvense (Asteraceae)