Sunday, December 12, 2010

Cabbage White Butterfly ( Pieris rapae )

Egg: Eggs are laid singly, usually on the lower surface of outer leaves of plants. The egg measures 0.5 mm in width and 1.0 mm in length, and initially is pale white in color but eventually turns yellowish. The egg is laid on end, with the point of attachment flattened and the distal end tapering to a blunt point. The shape is sometimes described as resembling a bullet.
Larva: The larva is green, velvety in appearance, and bears five pairs of prolegs. There are five instars. Head capsule widths are about 0.4, 0.6, 0.97, 1.5, and 2.2 mm, respectively. Body lengths at maturity of each instar averages 3.2, 8.8, 14.0, 20.2, and 30.1 mm, respectively. The larva requires about 15 days (range 11 to 33 days) to complete its development during August. Average (and range) of development times for each instar at 19°C was observed to be 4.5 (2.5-6), 3.0 (1.5-5), 3.3 (2-5), 4.1 (3-6.5), and 7.8 (5-18) days, respectively. All larval stages except the first instar bear a narrow yellow line running along the center of the back; this stripe is sometimes incomplete on the early instars. A broken yellow line, or series of yellow spots, also occurs on each side.

Pupa: Pupation normally occurs on the food plant, but cabbageworm may pupate in nearby debris. The chrysalis is about 18 to 20 mm in length, and varies in color, usually yellow, gray, green and speckled brown. A sharply angled, keel-like projection is evident dorsally on the thorax, and dorsolaterally on each side of the abdomen. At pupation, the chrysalis is anchored by the tip of the abdomen to the silk pad, and a strand of silk is loosely spun around the thorax. Pupation during the summer generations lasts about 11 days. The chrysalis is the overwintering stage, however, so its duration may be prolonged for months. The proportion of pupae that diapause increases as autumn progresses, so that at the time of the final generation all pupae are in diapause.

Adult: Upon emergence from the chrysalis the butterfly has a wing span of about 4.5 to 6.5 cm. It is white above with black at the tips of the forewings. The front wings are also marked with black dots: two in the central area of each forewing in the female, and one in case of males. When viewed from below, the wings generally are yellowish, and the black spots usually show faintly through the wings. The hind wing of each sex also bears a black spot on the anterior edge. The body of the butterfly is covered with dense hair, which is colored white in females, but darker in males. The adult typically lives about three weeks. The female produces 300 to 400 eggs. The adult is very active during the daylight hours, often moving from the crop to flowering weeds to feed.

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