A medium to fast flying white (subfamily Pierinae), one of eight species found in Ontario (18 in Canada). P. oleracea was only separated from P. napi in 1992 using enzyme electrophoresis. Its common name (Mustard White) remains identical, but older field guides may still have it listed under the Latin P. napi or even Artogeia napi oleracea. Identification: A chalky white butterfly. First generation individuals have some dark scales at the forewing base and tip, and a pale yellow underwing with prominent, dark green lined veins that show through the wings. Summer specimens are almost completely unmarked white. Wingspan: 32-50 mm. Life History: One to four broods depending on latitude. Hibernates as pupa. Caterpillar is green with white or pale yellow lateral line and tiny black spots. Habitat and Range: In Ontario it is widespread and sometimes (as in the Ottawa District) it is locally common. Nationally it is a common species found in all provinces and territories, although in some areas its numbers have declined due to habitat loss. Host Plants: Mustard family (Brassicaceae) plants; especially rock cress (Arabis spp.), and toolworth (Dentaria spp.). Flight Period: Late April to mid-September. Similar Species: In southern Ontario the West Virginia White (P. virginiensis) can be confused with the spring brood of oleracea although the vein shading on the underwing of virginiensis is grey brown rather than the sharp, dark green of oleracea.