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Butterflies of Britain & Europe
Small White
Pieris rapae  LINNAEUS, 1758
Family - PIERIDAE
subfamily - PIERINAE
Tribe - PIERINI
Small White Pieris rapae, 2nd brood female, Noar Hill, Hampshire  Adrian Hoskins
Introduction
The Small White is one of the most widely distributed species in the world ( the most widespread of all being the Painted Lady ). It occurs throughout all of Europe, including the sub-arctic areas of Scandinavia, the Outer Hebrides, all of the Mediterranean islands, the Azores, and the Canary Isles.
Beyond Europe it occurs in north Africa, Tunisia, throughout the Middle East, and eastward across temperate Asia to Japan. It was introduced to Quebec in 1860, and to California in 1866. By 1881 it had spread across most of the eastern United States, and is now a common species throughout all of temperate North America, where it is known as the Cabbage White. During the 20th century it was introduced to many more countries, including Bermuda, Australia, New Zealand and Iceland.
In Britain it can be confused in flight with the Large White Pieris brassicae, which is marked almost identically. The latter however is considerably larger, averaging 58mm in wingspan, while rapae averages only 47mm. It can also be confused with the Green-veined White Pieris napi, but appears brighter, slightly larger, and has a more powerful and purposeful flight than that species, usually flying in a constant direction.
In Europe rapae can be mistaken for the Southern Small White P.mannii, in which the black spots in the median area of the forewings are crescent-shaped ( they are squarish in rapae ); and with the Mountain Small White P. ergane, in which the greyish apical patch is squarish ( concave in rapae ).
Habitats
This is a strongly migratory species which can be found in any British habitat with the exception of the highest mountain peaks. However it is far commoner in the vicinity of gardens, allotments and farmland where crucifers ( cabbage, sprouts etc ) are grown.
It is also common in late summer at flowery grassland sites adjoining cruciferous crops, and along the south coast, as arriving migrants.
Small White Pieris rapae, 2nd brood female, Stockbridge Down, Hampshire  Adrian Hoskins
Lifecycle
In Britain there are normally 2 generations, emerging in May and August. In exceptionally warm summers there may be a partial 3rd generation emerging in October. Migrants can arrive at any time, but tend to peak in April and July. Thus it is possible to see this species at any time between early April and November.
The pale yellow, skittle-shaped eggs are laid singly on the underside of the leaves of cabbages and sprouts Brassica, garden nasturtiums Tropaeolum, and less commonly on charlock Sinapis arvensis or other wild crucifers. Often several eggs will be laid on the same plant, or even under the same leaf, but these are laid by different females, or by successive visits by a single female.
The dull green, slightly hairy caterpillars feed diurnally on the leaves of the foodplants. On cabbages the young larvae bury deep into the heart of the plants, inflicting serious damage to the developing leaves. When older they feed openly, nibbling a mass of irregularly shaped holes out of the leaves, but leaving the midrib and tougher veins intact. They are a serious commercial pest of cabbage crops but do less damage than the larvae of the Cabbage moth Mamestra brassicae.
Small White larvae are heavily predated by harvestmen and nocturnal beetles, and by birds including sparrows, tits, warblers and thrushes. Also, in common with most other caterpillars they are attacked by various parasitoids, the main culprit in this case being the wasp Apanteles rubecula.
The chrysalis has several colour forms, ranging from pale green to a dirty brownish white, with dark dots on the abdomen. It is attached either vertically or horizontally to fence-posts, brick walls, or the underside of shed roofs, windowsills and other sturdy materials.
Small White Pieris rapae, 2nd brood female, Stockbridge Down, Hampshire  Adrian Hoskins
Adult behaviour

Early in the day, or in overcast conditions, the butterflies bask with their wings half-open, reflecting solar energy onto the thorax and abdomen, which are covered with dark hair-like scales that assist in rapid heat absorption. In sunny conditions they usually keep their wings shut when resting or nectaring, but copulated pairs usually rest with wings partly open.

Small White Pieris rapae, 2nd brood copulating pair, Hampshire  Adrian Hoskins

Males patrol around cabbages and other crucifers, waiting to intercept females. When the sexes meet, the male flies up and down in front of the female, enticing her to settle, and then settles beside her. He then uses his outspread wings to force the female to lean to one side, and bends his abdomen round to copulate with her. Immediately after joining, the pair fly a short distance to settle on the foliage of a nearby plant. If a male intercepts an unresponsive female, she spreads her wings and raises her abdomen as a rejection signal.

Small White Pieris rapae, 2nd brood female, Martin Down NNR, Hampshire  Adrian Hoskins

The butterflies often congregate in sheltered gardens to oviposit, or to seek nectar at cultivated flowers, particularly favouring buddleia. In the countryside they often gather to nectar at patches of hemp agrimony, marjoram, thistles, knapweeds, yellow rattle, or valerian.

They roost singly, usually on the upper surface of the leaves of bushes or herbaceous plants, even in rainy weather.

 

 

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