Pieris napi Linnaeus Lepidoptera Pieridae

Natural History

Distribution. This insect is found both in North America and Eurasia, where it has a decidedly northern range. In the eastern United States, it occurs prin cipally in the northernmost states, but in the West its range extends as far south as New Mexico and California. It is widespread in Canada and Alaska, extending north to the edge of the Arctic tundra. Mustard white has declined in abundance in the northeastern United States, a phenomenon commonly attributed to competition with imported cabbageworm. However, adults frequent shaded areas, whereas imported cabbage-worm butterflies prefers sunny fields, so competition may not completely explain the decline in mustard white abundance. It has also been suggested that change in habitat availability could account for this population decline.

Host Plants. Larvae feed on wild and cultivated Cruciferae. Weeds such as Virginia pepperweed, Lepi-dium virginicum; toothworts, Dentaria spp.; and the Brassica and Descurainia mustards are suitable larval hosts. Females oviposit on yellow rocket, Barbarea vulgaris, and field pennycress, Thlaspi arvense, but larvae cannot complete their development on these plants. Vegetables attacked include Brussels sprouts, cabbage, radish, turnip, watercress, and probably others. Adults take nectar from flowers, preferring Geranium spp. if they are available.

Life Cycle and Description. One to three generations are reported; two is most common. In areas with three generations, many members of the second generation enter diapause.

  1. The eggs are pale yellow or white, tinged with green, and bear 13 ridges. The eggs are elongate, tapering to a blunt point distally. They measure about 1.2 mm long and 0.45 mm wide. They are deposited singly, on end, to the underside of leaves and on stems of hosts. The female apparently produces about 300 eggs over the course of her life, which may be of several weeks' duration.
  2. Larvae are covered with short, fine hairs, imparting a velvety appearance. They are green with small black spots, with a green stripe dorsally and yellow spots along each side. Mustard white larvae are easily confused with other pierid butterfly larvae; a key to separate these species is provided in Appendix A.
  3. The chrysalis is green, gray, or tan flecked with black. It bears frontal and lateral projections similar to imported cabbageworm. Light-colored dorsal and lateral stripes may occur on the chrysalis. The mustard white undergoes diapause in the pupal stage.
  4. The wingspan is about 4-5 cm. This insect is white dorsally, and sometimes bears one or two black spots on the front wings. The lower side of the
Adult male mustard white.

hind wing is usually yellow, as is the tip of the forew-ing. The most distinctive feature is the darkening along the wing veins; this is especially evident on the hind wings, and most pronounced on the underside of the hind wings. This insect has two color forms. The summer generation tends toward nearly white, while the spring generation has pronounced bands of gray along the veins. A key for separation of this species from other common cabbage white butterflies is included in Appendix A.

The biology of mustard white was given by Opler and Krizek (1984), and Scott (1986).

Damage

Larvae feed on the foliage, and produce small to large holes depending on larval size. They rarely are numerous enough to warrant cancern.

Management

Mustard white is rarely a serious pest, and should respond to management practices developed for imported cabbageworm.

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