There are similar butterflies in the Polyommatus genus. This looks a lot like the common blue or the Adonis blue, both of which are fairly common in Europe. The common blue also appears in the US and Canada (with first reported sightings as recent as 2005) and its range is believed to be expanding. The one you posted looks like a male but I'm not sure as to the exact species.
As for their diet, Wikipedia lists the following as recorded larval food plants of the common blue:
- Lathyrus species
- Vicia species
- Vicia cracca
- Oxytropis campestris
- bird's foot trefoil (Lotus corniculatus)
- Oxytropis pyrenaica
- Astragalus aristatus
- Astragalus onobrychis
- Astragalus pinetorum
- black medick (Medicago lupulina)
- Medicago romanica
- Medicago falcata
- common restharrow (Ononis repens)
- wild thyme (Thymus serpyllum)
- lesser trefoil (Trifolium dubium)
- Trifolium pratense
- white clover (Trifolium repens)