This is a very common species, introduced from Europe in the 19th century.

This is a white-winged butterfly with a brown patch on the forward tip of its wings. Also, the male has a single spot, and the female two, in the middle of each forewing.

Larvae/caterpillars prefer to feed on cabbage, nasturtium, watercress, various capers, and mustards.

References:

Butterflies of North America, J.P. Brock and K. Kaufman, 2003, Houghton Mifflin Co.