Papilionaceae as Melilotus, Coronilla etc., but apparently especially Vicia cracca.
Cupido alcetas inhabits more mesophilic, but warm edges along ditches or woods and in ruderal terrain, where the males like to perch on increased places.
The adults fly from April to September/early October in 1-3 generations. I met butterflies of the first generation in late May 2009 in the Pfynwald (Valais) in an habitat that was unfortunately soon strongly affected by road construction. I found still small caterpillars in mid-June only on Vicia cracca, although, for example, Lotus, Melilotus, Medicago and Trifolium pratense would have been available. The caterpillar overwinters fully-grown.
Cupido alcetas is strongly endangered at the few Central European locations due to the destruction of habitat (overbuilding, agricultural intensification).
Cupido alcetas occurs quite locally in Northeast Spain, Southern France, Northern Italy and eastern (southern Poland) and Southeastern Europe (Balkans). In Central Europe it is mainly found in the Valais and also in southeast Austria. The total range extends then through Turkey and southern Siberia to the Altai.