|Pieris rapae Family: Pieridae
Common name(s): small cabbage white butterfly; small white; imported cabbageworm; cabbage white; small cabbage white
Adult size: wingspan: 1.5 – 2.5 in (3.8-6.5 cm)
Early adults emerging from diapaused pupae in early spring in the Pacific Northwest region are typically a little smaller in size with fewer black markings on the upper wings but with an additional edging of black on the underside at the base of the hindwing.
Each female produces 300-400 eggs in their life span.
The egg is 0.04 inches (1.0 mm) long and 0.02 inches (0.5 mm) wide. Laid on end, singly, on the underside of leaves of host plants, the bullet-shaped egg is pale white in colour initially, turning yellowish. Ridges running the length of the egg are visible.
Larva develops through 5 instar stages. It is a velvety green colour with 5 pairs of prolegs and a narrow yellow line running down the centre of its back. (This line is missing in the first instar stage and can be broken up in early instars.) Dashes of narrow yellow lines, or several yellow spots can also be distinguished on both sides.
The chrysalis varies in colour…yellow, green, grey, speckled brown…and is roughly 0.7 – 0.8 inches (1.8-2.0 cm) long. Angled, keel-like ridges are prominently visible. In summer generations, pupation lasts about 11 days. As the chrysalis is the overwintering stage of this insect, the numbers of pupae entering diapause increases as autumn progresses.
In the Pacific Northwest, small white cabbage butterflies are active from early spring through until temperatures dip below 0 °C (32 °F), typically sometime in October or early November.
Special Notes: Originally native to Europe, Asia, and North Africa, this insect has now become well-established throughout much of the temperate regions in the world. It was accidentally introduced in the Montreal area of Canada in the 1860s. From there, it gradually spread throughout North America…although it is scarce or non-existent in semi-desert and desert regions. Nowadays it is classified as an important pest species.
Its life cycle and duration of all stages is governed by temperature. Whereas there are only two or three generations per year in Canada, there are three in the New England area of eastern United States, three to five in California, six to eight generations in the southern states with the exception of Florida where this pest is active year-round.
Remedial Actions: The small cabbage white has several natural predators including ambush bugs, shield bugs, predatory wasps and flies, as well as insect-eating birds. There are also various naturally occurring fungal and virus diseases which are detrimental to this pest, including glanulosis disease (GV)…a disease which acts quickly, inducing diarrhea in insect larvae and typically leading to a mortality rate of over 90%.
There are some crucifer crops which exhibit less susceptibility to damage by foraging cabbageworm larvae. These include: Chinese cabbage, kale, mustard, rutabaga, and turnip. Crucifer crops with glossy, dark green leaves seem to also be less susceptible to cabbageworm damage.
Female small white cabbage butterflies appear to avoid laying their eggs on all red cabbage varieties…although it is has been proven larval survival is indeed favoured by red cabbage.
Covering vegetable varieties which suffer cabbageworm damage with row covers is effective in preventing egg-laying.
List of host plants:
In the Cruciferae family:
In the Capparidaceae family:
In the Tropaeolaceae family:
In the Resedaceae family:
Posted on November 8, 2017