Small White (Pieris rapae)

Description

The Small White is a medium sized white butterfly (wingspan 38-57 mm) very similar in appearance to the large white but smaller of course! It is one of our earliest butterflies to emerge in spring during April. The female has two black spots on its upper forewings and tend to have darker colouration, while the male has a single spot on their forewing. Both sexes have black tips to their forewings and black spots on the underside. The underside of the hindwings are yellow with a dusting of black scaling towards the bases: the second generation tend to have stronger yellow colouration.

Credit: Jim Black

Life Cycle

The adults emerge from early April and set about finding mates. Two broods are generally produced each year. Females are attracted to the mustard oils produced by plants of the brassica family. Unlike the Large White, which lays large clusters of eggs, female Small Whites lay their pale yellow eggs singly on the underside of the leaves of both cultivated and wild brassicas. Commonly used food plants include cabbage and brussels-sprouts (Brassica oleracea), nasturtium (Tropaeolum majus), charlock (Sinapis arvensis), hedge mustard (Sisymbrium officinale) and garlic mustard (Alliaria petiolata). The larva of the small white feed individually on the food plant and hence cause less damage to crops than their larger relatives The green caterpillars have a thin yellow stripe down their back. They grow rapidly before forming a green-brown pupa on the foodplant or nearby vegetation. The pupae of the second generation are the overwintering form and tend to be formed on more stable structures such as a trees, fence posts or walls. .

Flight Times

In south west Scotland, adult Small White can be found on the wing from April through to early October with numbers peaking in late May/early June and then again in August from the second generation.

Habitat & Distribution in SW Scotland

Distribution map 2010

The Small White can be found almost anywhere within the branch area as local populations are augmented by migrants from Europe which have a strong urge to migrate north in the spring. However, the Small White is much less abundant outside urban areas than the Green Veined White which favours the damper grassland and moorland habitats while the Small White prefers drier habitats such as urban areas, parks, gardens, woodland edges and coastal areas. The adults are highly mobile and don't have discrete colonies. In late summer the population is thought to be bolstered by migrants from continental Europe resulting in larger populations in the second generation. The flowers of cruicifers, dandelions, knapweed and thistles are favoured nectar sources. The Small White has benefited from the growing of brassicas in gardens and also the popularity of Nasturtiums as a garden plant which is a favourite foodplant. This is the major reason why this butterfly is more common in urban areas than in the general countryside.

The Small White has also has benefited from the trend to growing oil-seed rape as an arable crop in recent decades as this brassica is another foodplant of the Small White. However, because the females prefer sheltered locations to lay eggs, larvae only tend to be found near the edges of fields. So oil-seed rape can bolster countryside populations in arable areas and this may explain why lowland Wigtownshire, Ayrshire and the central belt have a much higher density of Small White than Argyll and inland parts of Ayrshire and Dumfries & Galloway.

References

• Barbour, D., Moran, S, Mainwood, T & Slater, B 2008. Atlas ofr Butterflies in Highland & Moray. Big Sky.
• Futter, K., Sutcliffe, R., Welham, D., Welham, A., Rostron, A., MacKay, J., Gregory, N., McCleary, J., Tait, T., Black, J. & Kirkland, P. 2006. Butterflies of South West Scotland. Argyll Publishing. (see Atlas page)
• Asher, J., Warren, M, Fox, R., Harding, P., Jeffcoate, G., Jeffcoate, S. 2001. The Millennium Atlas of Butterflies in Britain & Ireland. Oxford University Press.
• Fox, R., Asher, J., Brereton, T., Roy, D., Warren, M. 2006. The State of Butterflies in Britain and Ireland. Pisces Publications.
• Thomson, G. 1982. The Butterflies of Scotland. Croom Helm London.
• Tomlinson, D. & Still, R. 2002. Britain’s Butterflies. WILDGuides.

Written by Scott Shanks & Andrew Masterman