In southern California, this is a butterfly of the deserts, whether at Yaqui Well among the bladderpod, at Joshua Tree National Park where Prince's Plume grows, 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 left in the heat of mid-May. Two weeks earlier, I saw a female ovipositing on bladderpod (Isomeris arborea) at the Whitewater Preserve, and that is the plant - a member of the caper family - most strongly associated with beckerii in my mind.
Becker's White flies in multiple broods much of the year. It is closely related to Pontia chloridice a Eurasian butterfly. In fact, James Scott (1986) lists it as subspecies beckerii under Pieris 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.
Found this tiny caterpillar of Pontia beckerii on Cactus Spring Trail. I believe it was Sisymbrium orientale. April 22, 2019.