Pieris rapae is a small-to-medium-sized butterfly species of the whites-and-yellows family Pieridae. It is known in Europe as the small white, in North America and the United Kingdom as the cabbage white or cabbage butterfly, on several continents as the small cabbage white, and in New Zealand as the white butterfly. The butterfly is recognizable by its white color with small black dots on its wings, and it can be distinguished from P. brassicae by the latter's larger size and black band at the tip of the forewings.
The caterpillar of this species, often referred to as the "imported cabbageworm", is a pest to crucifer crops such as cabbage, kale, bok choy and broccoli. Pieris rapae is widespread in Europe and Asia; it is believed to have originated in the Eastern Mediterranean region of Europe and to have spread across Eurasia thanks to the diversification of brassicaceous crops and the development of human trade routes. Over the past two centuries, it spread to North Africa (about 1800), North America (1860s), Hawaii (1897), New Zealand (1930), and Australia (1937), as a result of accidental introductions.
Description
thumb|left|Feeding on the [[nectar of Aster amellus]]
alt=Cabbage white butterfly (Pieris rapae), wings closed. Montgomery County, PA.|thumb|Cabbage white butterfly (Pieris rapae), wings closed
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 .
Pieris rapae has a wingbeat frequency averaging 12.8 flaps per second.
Distribution and habitat
thumb|left| Global invasion history of Pieris rapae
thumb|Small whites mating (German/Dutch border region)
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. 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.
Life cycle
thumb|right|Egg
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.
thumb|left|Caterpillar
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.
thumb|right|Pupa
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. Adult P. rapae can move many kilometers in individual flights. Adults have been observed to fly as much as 12 km in one flight. Larvae are shown to feed mostly during the day. They move around the plant mostly spending their time feeding. A feeding bout is immediately followed by a change in position, either to a new leaf or to another part of the same leaf. Within the family Brassicaceae, larvae show no significant difference in feeding behavior; larvae placed on kale show no difference from larvae placed on Brussels sprouts.
Survival rates do not differ depending on nutrition availability of host plant. Elevated plant nutrient levels decrease larval duration and increase larval growth rate. Some flowers, like Brassica rapa, have a UV guide which aids the butterfly in search for nectar where the petals reflect near UV light, whereas the center of the flower absorbs UV light, creating a visible dark center in the flower when seen in UV condition. The search for nectar is also limited by the memory constraint. An adult butterfly shows a flower constancy in foraging, visiting flower species that it has already experienced. The ability to find nectar from the flower increases over time, showing a certain learning curve. Furthermore, the ability to find nectar from the first flower species decreased if the adult butterfly started to feed on nectar from other plant species.
Courtship and reproduction
thumb|right|Copulating pair
The male, when it spots a female, zigzags up, down, below, and in front of her, flying until she lands. The male flutters, catches her closed forewings with his legs, and spreads his wings. This causes her to lean over. He usually flies a short distance with her dangling beneath him. An unreceptive female may fly vertically or spread her wings and raise the abdomen to reject the male.
Most host plants of P. rapae contain mustard oils and females use these oils to locate the plants. Females then lay the eggs singly on host leaves. Flight behavior of an ovipositing female of P. rapae follows the Markov process. Females foraging for nectar will readily abandon a linear path; they will show tight turns concentrating on flower patches. Females searching for host plant, however, will follow a linear route. As a result of directionality, the number of eggs laid per plant declines with increases in host plant density. Gravid females responded most positively to green and blue/green colors for oviposition. The preference was shown for surfaces with maximal reflectance of 550 nm. P. rapae is shown to prefer smooth hard surfaces similar to a surface of an index card over rougher softer textures like blotting paper or felt. P. rapae use their chemoreceptors on their tarsi to search for chemical cues from the host plant. This behavior may have evolved to exploit the original vegetation in the eastern Mediterranean where brassica plants originated. The main issues with the weather are that strong winds can blow eggs from the leaves and strong rains can drown the caterpillars. Larvae are able to respond well to a wide range of temperature condition, which allows them to inhabit various locations in the world. In natural conditions, larvae shows fastest growth at temperatures close to 35 °C. however, in constant temperature conditions in laboratory, larvae shows mortality at 35 °C. 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.