Anthocharis lanceolata desertolimbus, a subspecies of the Gray Marble named in 1998. Photographed along the Cactus Springs Trail in the Santa Rosa Mountains, March 30, 2011. This one is nectaring on Acmispon rigidus - Desert Lotus.
Another desertolimbus, this one from April 4, 2015, also from Cactus Springs Trail.
The larva of desertolimbus has yellow along the top of the white stripe, an easy way to differentiate it from the much more common Anthocharis sara larvae that also use Boechera perennans.