Polylectic. Else (in litt. 2015) has shown that the females will collect the great majority of their pollen at Centaurea nigra and Centaurea scabiosa (Asteraceae) with much lesser but variable amounts of the following: Apiaceae, Asteraceae fenestrate (dandelions, hawkweeds, hawksbeards etc.) Scabiosa (Dipsacaceae), Campanulaceae, Campanula-type & Succisa pratensis (Dipsacaceae). Few grains of Lamiaceae (Mentha type).
Pesenko (2005) reports male visitation to flowers of Chondrilla (Asteraceae) in Kyrghyzstan.
S. Falk (2011) states that (in Sussex) the species is always associated with extensive stands of Centaurea scabiosa (Asteraceae) which is the main female forage plant once this is in flower, though females emerge several weeks before it flowers, and males have been observed swarming around Oreganum vulgare (Lamiaceae) flowers.
Else (in litt. 2015) states that nearly all the females of H. eurygnathus that I and others have encountered in the field were visiting Centaurea scabiosa (Asteraceae) in early summer (June/July), whereas those seen later (August) were mostly on C. nigra (Asteraceae) (most C. scabiosa having finished flowering by then). He also reports seeing a female H. eurygnathus at a Scabiosa columbaria (Dipsacaceae) flower recently but this seemed to be a nectar source as the visit occupied only a few moments.
Specimen data from Lithuania (Budrys, in litt) lists in addition Taraxacum officinalis agg. (Asteraceae), Chelidonium majus (Papaveraceae) and Veronica sp. (Plantaginaceae).