Horticulture/Pieris rapae

The Small White (Pieris rapae) is a small to mid-sized butterfly of the Yellows and Whites Pieridae family. It is also commonly known as the small cabbage white or just the cabbage white. It is native across Europe, North Africa and Asia but has also been introduced to North America, Australia and New Zealand where they have become pests on cultivated cabbages and other mustard family crops.

Description
In appearance it looks like a smaller version of the Large White. The upper side is creamy white with black tips to the forewings. Females also have two black spots in the center of the forewings. Its underwings are yellowish with black speckles.

Its caterpillars can be a pest on cultivated cabbages, but it will readily lay eggs on wild members of the cabbage family. The eggs are laid singularly on foodplant leaves.

Known in the United States as "Imported Cabbage Worms", the caterpillars are green and well camouflaged. Unlike the Large White they don't store the mustard oils from their foodplants and so are not distasteful to predators like birds.

Symptoms and Signs
Leaves are eaten one side at a time, starting from the edge and often eventually reduced to just a stem and the base of the midrib. Seedlings may become completely defoliated after only a day or two. Greenish-brown frass may collect in the leaf axils.

Ecology
Like its close relative the Large White this is a strong flyer and the British population is increased by continental immigrants in most years. Like other "White" butterflies it hibernates as a pupa. Along with the Large White it is sometimes known as the Cabbage White or Cabbage Butterfly.

It has two flight periods, mid-spring and midsummer.

Host plants
Hosts include members of the following genera, and possibly other mustard-family weeds.
 * ../Armoracia/ (Horseradish)
 * ../Brassica/
 * ../Capsella/ (Sheperd's Purse)
 * ../Lactuca/ (Lettuce)
 * ../Lobularia/ (Sweet Alyssum)
 * ../Raphanus/ (Turnip)
 * ../Sisymbrium/
 * ../Tropaeolum/ (Nasturtium)