As this is a cleptoparasitic species, all flower visits will be for nectar only.
Westrich (1989) lists the following nectar sources: Stachys recta, Ajuga genevensis, Teucrium montanum, Teucrium chamaedrys, Thymus pulegioides (Lamiaceae).
Gogala illlustrates a male necatring at Teucrium montanum (Lamiaceae) http://www2.pms-lj.si/andrej/stenas.htm
Kasparek (2015)[citing others] lists Teucrium montanum, Teucrium chamaedrys, Stachys recta, Ajuga spp., Ajuga genevensis, Marrubium vulgare, Thymus pulegioides (Lamiaceae).