Rock Buckwheat or, Round-headed Buckwheat
(Eriogonum sphaerocephalum)

Eriogonum species are used as food plants by the larvae of some Lepidoptera (butterflies and moths). Several of these are monophagous, meaning their caterpillars only feed on this genus, sometimes just on a single taxon of Eriogonum. Wild buckwheat flowers are also an important source of food for these and other Lepidoptera. In some cases, the relationship is so close that Eriogonum and dependent Lepidoptera are in danger of coextinction. (from Wikipedia)