A declining species in most of its range, but appears to be abundant in Vermont. The decline was thought to be caused by competition with the introduced Cabbage White, but is more likely due to the abundance of introduced mustard plants. Females will oviposit on introduced Wintercress (Barbarea vulgaris) and Garlic Mustard (Alliaria petiolata), yet larvae cannot complete development on these plants. Cabbage White larvae are able to thrive on them. Introduced wasp parasites that were imported to combat Cabbage Whites in agricultural fields have also been implicated in the decline of Mustard Whites.
Summer form is pure white above and below; spring form has black-tipped upper forewing. Underside of hindwing and apex of forewing have veins edged with yellow-green or gray-green.
Resident – uncommon
Two to three generations with differing spring and summer phenotypes. Extreme dates: 25 April 2007 in Bristol (B. Collins), 25 August 2005 in Guildhall (K. McFarland), and 11 September in Grand Isle (S. Griggs).
Distribution and Habitat
North American Range- east from the Dakotas and Canadian prairie provinces across the Great Lakes area to New England and Nova Scotia.
During the Massachusetts Butterfly Atlas it was only found at higher elevations west of the Connecticut River valley. This pattern appears to continue northward into southern Vermont where VBS records generally followed the Green Mountains northward into central Vermont where they appeared to be more widespread, except for the lowlands of the Champlain Valley. This elevation and latitude effect may be due to location of forest cover, climatic effects on range of introduced plants and parasites or other factors and needs further study.