Small White (lat. Pieris rapae)

In appearance it looks like a smaller version of the large white (Pieris brassicae). The upperside is creamy white with black tips on the forewings. Females also have two black spots in the center of the forewings. Its underwings are yellowish with black speckles. It is sometimes mistaken for a moth due to its plain appearance. The wingspan of adults is roughly 32–47 mm (1.3–1.9 in). Pieris rapae has a wingbeat frequency averaging 12.8 flaps per

second.

The species has a natural range across Europe, Asia, and North Africa. It was accidentally introduced to Quebec, Canada, around 1860 and spread rapidly throughout North America. The species has spread to all North American life zones from Lower Austral/Lower Sonoran to Canada. Estimates show that a single female of this species might be the progenitor in a few generations of millions. It is absent or scarce in desert and semidesert regions (except for irrigated

areas). It is not found north of Canadian life zone, nor on Channel Islands off the coast of southern California. By 1898, the small white had spread to Hawaii; by 1929, it had reached New Zealand and the area around Melbourne, Australia, and found its way to Perth as early as 1943. It does not seem to have made it to South America. In Britain, it has two flight periods, April–May and July–August, but is continuously

brooded in North America, being one of the first butterflies to emerge from the chrysalis in the spring and flying until hard freeze in the fall. The species can be found in any open area with diverse plant association. It can be seen usually in towns, but also in natural habitats, mostly in valley bottoms. Although an affinity towards open areas is shown, the small white is found to have entered even small forest clearings in

recent years. The nominate subspecies P. r. rapae is found in Europe, while Asian populations are placed in the subspecies P. r. crucivora. Other subspecies include atomaria, eumorpha, leucosoma, mauretanica, napi, novangliae, and orientalis.

The small white will readily lay eggs on both cultivated and wild members of the cabbage family, such as charlock (Sinapis arvensis) and hedge mustard (Sisymbrium officinale). P. rapae is known to lay eggs singularly on the host plant. The egg is characterized by a yellowish color and 12 longitudinal ridges. The egg production peaks about a week after adulthood in lab and the female can live up to 3 weeks. Females tend to lay

fewer eggs on plants in clumps than on isolated plants. It has been suggested that isothiocyanate compounds in the family Brassicaceae may have been evolved to reduce herbivory by caterpillars of the small white. However, this suggestion is not generally accepted because the small white has later been shown to be immune to the isothiocyanate forming reaction due to a specific biochemical adaptation. In contrast, the small white and relatives seem to have evolved as

a consequence of this biochemical adaptation to the isothiocyanate-forming glucosinolates. Traditionally known in the United States as the imported cabbage worm, now more commonly the cabbage white, the caterpillars are bluish-green, with tiny black spots, a black ring around the spiracles, and a lateral row of yellow dashes, and a yellow middorsal line. Caterpillars rest on the undersides of the leaves, making them less visible to predators. Although the larval instars have not been fully studied,

different instars are easily differentiated simply by comparing sizes, especially the head alone. During the first and second instar the head is entirely black; third instar has the clypeus yellow but the rest of the head black. In the fourth and fifth instar, there is a dark greenish-yellow dot behind each eye but with rest of the head black. However, the color of the caterpillar head does not necessarily indicate specific instar, as the time

of color change is not fixed. In the larval stage, the small white can be a pest on cultivated cabbages, kale, radish, broccoli, and horseradish. The larva is considered a serious pest for commercial growth of cabbage and other Brassicaceae. The pupa of P. rapae is very similar to that of P. napi. It is brown to mottled-gray or yellowish, matching the background color. It has a large head cone, with a vertical abdomen and flared

subdorsal ridge. The two (pupa of P. rapae and P. napi) can be easily distinguished by comparing the proboscis sheath. In P. rapae, the proboscis sheath extends far beyond the antennal sheath while in P. napi, only a very short distance. Like its close relative the large white, the small white is a strong flyer and the British population is increased by continental immigrants in most years. Adults are diurnal and fly throughout the day, except

for early morning and evening. Although there is occasional activity during the later part of the night, it ceases as dawn breaks. Adult P. rapae can move many kilometers in individual flights. Adults have been observed to fly as much as 12 km in one flight. On average, a female flies about 0.7 km per day and moves 0.45 km from where she starts. Males patrol all day around host plants to mate with females.

Studies in Britain showed that birds are a major predator in British town and city environments (such as in gardens) while arthropods had larger influence in rural areas. Bird predators include the house sparrow (Passer domesticus), goldfinch (Carduelis carduelis) and skylark (Alauda arvensis). Caterpillars are cryptic, coloured as green as the host plant leaves and they rest on the undersides of the leaves, thus making them less visible to predators. Unlike the large white, they

are not distasteful to predators like birds. Like many other "white" butterflies, they overwinter as a pupa. Bird predation is usually evident only in late-instar larvae or on overwintering pupae.

Asher, Jim et al.: The Millennium Atlas of Britain and Ireland. Oxford University Press. Evans, W. H. (1932). The Identification of Indian Butterflies (2nd ed.). Mumbai, India: Bombay Natural History Society.

Pieris project A worldwide citizen science project undertaking research on Pieris rapae Pieris rapae on the UF / IFAS Featured Creatures website Cabbage white, Butterflies of Canada