Imported Cabbageworm
Pieris rapae (Linnaeus), Pieridae, LEPIDOPTERA

DESCRIPTION

Adult - Both sexes of this white butterfly have a black area near the tip of each forewing and a small black spot on the front edge of each hind wing. The female has two black spots on each forewing while the male has only one. The female has a wingspan of about 50 mm; the male is slightly smaller.

Egg - The pale yellow, bullet-shaped egg, about 1 mm long, is ribbed lengthwise and crosswise and is attached endwise to the leaf surface.

Larva - The velvet-like green larva has a faint yellow stripe down its back, a row of faint yellow spots on each side, and five pairs of prolegs. When fully grown, it measures about 32 mm long.

Pupa - The sharply-angled pupa, or chrysalis, is gray, green, or brown and about 20 mm long. It is attached to the lower leaf surface by a silken loop.

BIOLOGY

Distribution - The imported cabbageworm has spread throughout the U.S. from coast to coast after introduction into Canada from Europe. In the Western Hemisphere, this butterfly is most common between latitudes 30 degrees N and 60 degrees N. It is equally destructive in the norther and southern areas of its range.

Damage - Imported cabbageworms are commonly found on the undersides of leaves. Like cabbage loopers, young caterpillars feed superficially, leaving the upper surface intact. Larger larvae leave holes in the leaves and are more likely to eat through small veins than are loopers. In contrast to cabbage loopers, imported cabbageworms bore into the center of the head thereby doing more damage to the edible portion of the plant. The presence of masses of wet, greenish-brown excrement deep among leaves is indicative of this pest.

Life History - Imported cabbageworms overwinter as pupae attached to host plant debris. Adults emerge early in spring, as early as March even in the northern states. Soon after mating, females begin depositing eggs singly on cultivated host plants, if available . Often, however, the first generation of cabbageworms is raised on wild hosts. After hatching 4 to 8 days post egg-deposition, larvae feed and develop through five instars in 10 to 14 days. When mature, larvae fasten themselves to lower leaf surfaces by silk bands. During spring and summer, the pupal stage lasts 7 to 12 days before a new generation of butterflies emerges. There are usually 3 or 4 generations each year.

CONTROL

Although cabbageworms are subject to attack by a number of disease organisms and parasites, a combination of cultural and/or chemical control practices are necessary. The use of resistant cabbage varieties, such as Mammoth Red Rock, Chieftan Savoy, and Savoy Perfection Drumhead, provides some protection but not complete control. New plantings should be as far as possible from those of the previous season. At the end of the season, crops should be harvested without delay. Plowing under or destroying plant residues at this time eliminates an important food source for the overwintering generation of cabbageworms.

Insecticide applications should begin when the cabbageworm population reaches a threshold of one worm per plant. Sprays then should be repeated every 5 to 7 days, as needed. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.

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