In southern California, this is a butterfly of the deserts, whether at Yaqui Well among the bladderpod (Peritoma arborea), at Joshua Tree National Park using the same plant or perhaps Prince's Plume, or at Scissor's Crossing among the wild mustards. At Cactus Spring Trail, I've seen it oviposit on Sisymbrium orientale, one of the only viable mustards in the heat of mid-May.
Becker's White flies in multiple broods over much of the year. It is closely related to Pontia chloridice, a Eurasian butterfly. In fact, James Scott (1986) lists beckerii as a subspecies of P. chloridice (he also has protodice and sisymbrii under the genus Pieris).
This is Becker's White nectaring on host bladderpod at Yaqui Well in Anza-Borrego Desert State Park, January 17, 2006.
Those that emerge early in the year are more darkly marked. (This date was an early record for California.)
Another early dark Becker's White, this one a female from Sentenac Canyon in Anza-Borrego Desert State Park, March 1, 2009.
A male Becker's White, Yaqui Well, October 11, 2005. Males are more lightly marked than females dorsally.
Another male Becker's White at Yaqui Well on the same day as the male above.
A female from Yaqui Well. February 11, 2008.
Followed this female as she oviposited on the bladderpod plants at Whitewater Preserve in Riverside Co. on April 30, 2017...
...and here is an egg on a flower bud.
I watched a female ovipositing on Sisymbrium orientale along Cactus Spring Trail not far from the parking lot. I took this photo of the flower head, and only noticed the egg when I zoomed in on it after I got home. May 13, 2017.
Caterpillar of Pontia beckerii on the southern side of Joshua Tree National Park. It's on the usual - but not exclusive - host, bladderod. May 20, 2020.