The Nodding Onion (Allium cernuum) is a most attractive wild onion. Its tepal colouration varies from white through all shades of pink from very pale to a dark purplish pink, which is most common. It is native to a broad swath of both Canada and the United States as well as northeastern Mexico. It is also reportedly naturalized in parts of the British Isles, where it is sometimes called Ladies' Leeks.
However, the claim that Allium cernuum is a larval host plant for an always unspecified hairstreak butterfly is suspect, though it does seem to be listed often by people who must only have repeatedly copied the information.
If it is a larval food plant for some species of Hairstreak (Lycaenidae), I would like to see more information about which butterfly species is involved and where and when this was documented. Perhaps, the original claim was about this Allium being used as a nectar source, though that too is only speculation.
The British Natural History Museum Lepidopteran Larval Host Plant database only lists two Lepidopteran known to use Allium cernuum, both Noctuid moths (Noctuidae): Schinia snowi (syn: Schinia rosea), and the Yellow-striped Armyworm (Sporoptera ornithogalli). Furthermore, only one Lycaenid butterfly (Lycaenidae) is reported to use any species of Allium as a larval food plant.
That is the Pomegranate Playboy Hairstreak (Deudorix livia) native to Africa.