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Butterfly Conservation- Glasgow & South West Scotland Branch

Small Tortoiseshell (Aglais urticae)

Description

The Small Tortoiseshell is a brightly coloured butterfly which is common in gardens. Both sexes are similar with orange-brown upperwings with black & yellow stripes on the upper forewing and blue markings at the outer edges of both wings. When the Small Tortoiseshell rests with its wing closed, the dark brown undersides provide excellent camouflage and the brightly coloured butterfly ‘disappears’. This camouflage is also important for the overwintering adult stage.

Credit: Neil Gregory
Credit: Jim Black
Credit: Jim Black

Life Cycle

Large batches of green eggs are laid on the underside of common nettle (Urtica diocia) during May at the edge of a patch of nettles in a sunny location - in contrast, Peacock lay eggs on nettles at the centre of a patch. The young caterpillars spin a conspicuous communal web in which they feed together for the first couple of instars but caterpillars of later instars don't feed in webs, but range more widely across the nettle patch as solitary feeders. In Scotland, there is just one generation of Small Tortoishell such that the caterpillars reach maturity towards the end of July, and form a metallic chrysalsis on dead vegetation in the vicinity of the nettle patch and emerge as adults during August. These adults feed up on nectar rich floweres such as buddleia during the autumn and then overwinter as adults in places such as caves, hollow trees and buildings like churches and garden sheds. In southern England, there are normally two generations but three can occur in very warm years.

Flight Times

Adult Small Tortoiseshell can be seen in any month as very mild weather winters can rouse a few hibernating individuals and migrants from the continent migrate northwards each year in varying numbers which means adults may be seen in June & July when native populations are in the larval stage. Adults remain active into October as they feed up for overwintering.

Habitat & Distribution in SW Scotland

Distribution map 2010

Although the Small Tortoiseshell is a familiar garden species, it can be seen anywhere including above 900ft on the tops of Munros as this butterfly is a strong migrant. It is one of Britain's most successful butterflies and breeds as far north as Orkney (no known breeding records from Shetland although it is seen there) and Thomson (1982) says it is the only butterfly species to breed on St Kilda. Caterpillars have been seen on nettle as high as 335 m (1100 ft) in Scotland but the rarity of common nettle above 300 m would suggest little breeding occurs at these higher altitudes.

The Small Tortoiseshell is common even in NW Scotland vying with Meadow Brown to be the second commonest species here. Laboratory work has demonstrated that the gregarious larvae raise their body temperatures above the ambient temperature, thereby increasing their development rate and this may explain why the Small Tortoishell has been more abundant in NW Scotland than the Peacock.

The Small Tortoiseshell is very common in most parts of SW Scotland but less so in Argyll except along the coast.

Despite being one of Britain's most abundant butterflies, significant changes in populations have occurred with a decline in northern Britain in the 1980s and a significant increase in eastern Britain up to 1992. The latter population change may be the result of agricultural intensification enriching soils which have encouraged the spread of nettles. There is some evidence that nettles are becoming more common as a result of the use of fertilisers which may explain why the Small Tortoiseshell is associated with urban gardens.

Weather also may contribute to changes in Small Tortoisehell populations as drought in summer such as in 1976 causes numbers to decline as nettles do less well while wetter summers seem to favour larger populations. Weather in the autumn may also be important as fine autumn weather enables the adults to feed for longer periods which increases overwintering success.

Small Tortioseshell populations have been in decline in recent years and this decline is greatest in SE England where populations have dropped 80% since 1990. One theory is that a parasitic wasp, Sturmia bella which has recently colonised the UK may be responsible and help is needed to investigate this by volunteers collecting young caterpillars from the field and breeding them up at home and noting how many succumb to parasitism. It is not known whether this wasp has spread as far north as Scotland so data from all parts of Britain including Scotland are required. You can read more about this on the Butterfly Conservation website.

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.
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  • 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 Andrew Masterman


 
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