As this is a cleptoparasitic species, all flower visits will be for nectar only.
Westrich (1989) lists the following plant species as nectar sources: Senecio jacobaea (Asteraceae); Melilotus alba (Fabaceae); Knautia arvensis (Dipsacaceae); Origanum vulgare, Thymus pulegium, Clinopodium vulgare (Lamiaceae).
Celary (1995) lists the following nectar sources in Poland: Melilotus alba (Fabaceae); Clinopodium vulgare, Origanum vulgare, Thymus pulegiodes, Thymus serpyllum (Lamiaceae); Scabiosa sp., Knautia arvensis (Dipsacaceae); Senecio jacobaea, Hieracium pilosella (Asteraceae); Allium sp. (Liliaceae).
Smit (2004) lists Thymus serpyllum (Lamiaceae) as a nectar source.