As this is a cleptoparasitic species, all flower visits will be for nectar only.
Celary (1990) lists the following nectar sources in Poland: Echium vulgare (Boraginaceae); Stachys sp., Ballota nigra & Thymus sp. (Lamiaceae).
Westrich (2018) lists the following nectar source in Germany: Stachys recta, Stachys officinalis (=Betonica officinalis), Ballota nigra, Thymus pulegioides (Lamiaceae)