An extremely variable butterfly, the western white (Pontia occidentalis) ranges from Alaska to California and east to North Dakota. The upper sides of the white forewings have dark marginal markings, while the upper hindwings are faintly marked on the margins. On the lower side the forewing tips and the hindwings have the veins outlined in green-grey against a while background. All western white markings are more prominent in the spring.
Adult western whites feed on flower nectar, particularly rabbitbrush and aster flowers. The female lays eggs singly on Brassicaceae (mustard and cabbage) host plants in the spring.The caterpillar or larval stage feeds on the host plants. Although western white caterpillars prefer to feed on the buds, flowers and fruit of host plants, they will also eat the leaves of their mustard plant hosts. The western white hibernates or overwinters in the chrysalis stage.
These western white butterflies were photographed earlier this spring along the levee at Big Lake (Shasta County CA).