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Text and photographs protected by Copyright © Adrian Hoskins 2007, and must not be published in part or in whole elsewhere without prior written permission from the author.
Butterflies of Britain & Europe
 
Small Tortoiseshell
Aglais urticae LINNAEUS, 1758
Family - NYMPHALIDAE
subfamily - NYMPHALINAE
Tribe - NYMPHALINI
 
 introduction | habitats | lifecycle | adult behaviour
 
Aglais urticae, female, Somerset, England
 
Introduction
 

The Small Tortoiseshell may have been present on Earth longer than any other butterfly species - a mid-Miocene fossil, estimated to be 15 million years old, has been described as Aglais karaganica. It is so close to the present day Aglais urticae as to be virtually indistinguishable ( ref: Nekrutenko, 1965 ).
 
Small Tortoiseshells vary considerably in colouration - some examples being a unicolorous dull orange, while others are much redder with contrasting yellow patches. In some the black markings are enlarged and merge into each other, while in others they are much reduced. The blue sub-marginal lunules however never seem to vary in size.
 
The race ichnusa, found in mountainous areas of Corsica and Sardinia, has a fiery orange ground colour, and lacks the colon of black spots on the forewing. Some authors regard it as a separate taxon but captive reared adults closely resemble the normal form. It seems probable therefore that ichnusa is merely a habitat generated form of urticae.
 
The butterfly occurs across the whole of Europe and temperate Asia. A very closely related and visually inseparable species Aglais kashmirensis is found in the mountainous regions of Kashmir, Tibet, Sikkim and Bhutan. In the western Himalayas a third species occurs, Aglais ladakensis, which looks like a very heavily marked version of urticae. The only other member of the genus is Aglais milberti, a north American species in which the dark areas in the basal half of the forewings unite, and in which the 2 small black spots in the median area are absent.
 
The Small Tortoiseshell has always been subject to dramatic fluctuations in abundance from year to year. In some seasons it can be very scarce, but historically it has always had the ability to bounce back, and was until recently regarded as one of Britain's most familiar and common butterflies. However there has been a dramatic decline in numbers since the 1980's. The range of the butterfly seems to be contracting towards the west and north, but it still remains widely distributed.
 
The cycles of abundance are linked at least in part to variations in climate, which affect the population dynamics of the butterfly and it's parasitoids. Fortunately the population crashes are not synchronous - in periods when the species is scarce in one part of the country, it can be relatively abundant in another.
 
Recent research suggests that a tiny parasitic fly may* be responsible for the current decline of the Small Tortoiseshell. The fly Sturmia bella ( Tachinidae ) occurs mainly in warmer climates, and first arrived in Britain in 1998. It lays it's eggs on nettle plants. When the Small Tortoiseshell caterpillars eat the nettle leaves, the microscopic eggs are ingested undamaged and pass into the caterpillar's gut. There they hatch, and the resulting grubs bore their way through the soft flesh, consuming non-vital body tissues. When the grubs are almost full grown they eat the vital organs, and then break out through the skin of the dying caterpillar to pupate.

The fly is widespread throughout southern Europe, north Africa and Asia. In Japan it parasitises the caterpillars of a Danaine, Parantica sita. Interesting research by Hirai & Ishii has revealed that P. sita can often survive attack by Sturmia bella - in experiments over 70 percent of infested sita larvae survived and went on to produce perfect adult butterflies. Sadly the Small Tortoiseshell lacks this immunity.

*Although current research appears to indicate that Sturmia bella may be behind the decline of the Small Tortoiseshell, it is notable that other nettle-feeding Vanessids such as the Peacock, Red Admiral and Comma are as yet unaffected, even though presumably their larvae are also ingesting the Sturmia eggs. The Comma in fact has increased it's numbers substantially during the 2005-2008 period. All 3 species are likely to have very similar body chemistry to that of the Small Tortoiseshell, so it is unlikely that they possess a natural immunity.

Small Tortoiseshells usually oviposit on nettles growing in open sunlit places such as farmyards and field edges - areas exposed to high levels of pesticide and other chemical sprays. Commas, Peacocks and Red Admirals however all tend to breed on nettles in woodland glades, gardens and lanes where they are not exposed to pesticides. This disparity may provide a clue as to why only the Small Tortoiseshell is in decline. Initial studies however indicate that they do not appear to be harmed by extensive spraying of pesticides.

Climate change in itself could be a major factor. The viability of each butterfly species is limited by temperature and humidity. This defines it's geographical and altitudinal distribution and is part of the reason why some species are found only in the Arctic while others only in particular elevations in cloudforest in the tropics. It's quite possible that the Small Tortoiseshell simply can't cope with the mild damp winters Britain is now experiencing, but that these may be beneficial to the Comma, and well tolerated by the Red Admiral and Peacock.
 
The resident population is supplemented by the occasional arrival of migrants from the continent. The numbers arriving however are normally very low, and probably have no significant effect on population levels.
 
Habitats
 
The Small Tortoiseshell is a highly mobile species which can occur in almost any habitat, including woodlands, grasslands, heaths, gardens, country lanes, and town centres. It is a strong flyer, with high powers of dispersal, reaching the most remote islands, and the peaks of mountains. In Scotland it has been recorded from altitudes as high as 1200m, and larval nests have been found at 335m. I have watched adults flying at the peak of Ben Lawers in Tayside, and high in the mountains at Langdale Pikes and various other sites in the Lake District.
 
The butterfly is perhaps commonest on dairy and cattle farmland, where the application of fertilisers and manure enriches the soil, encouraging the growth of stinging nettles Urtica dioica.
 
Aglais urticae, male nectaring at bramble blossom, Stockbridge Down, Hampshire, England
 
Lifecycle
 
In April, female Small Tortoiseshells are often seen flying around young nettle patches, where they lay their greenish eggs in large untidy heaps of 80-100 on the underside of the leaves. The butterflies choose nettles that grow in warm sheltered situations, e.g. at the bottom of south-facing hillsides, along dykes and riverbanks, near hedgerows, and along disused railway cuttings. The summer brood is less fussy about the situation, and will lay on nettle patches in gardens, in woodland glades and rides, and on exposed hilltops. The butterflies of both broods always favour young nettles, and rarely oviposit on the tougher leaves of old growth.
 
The eggs hatch after about 10 days. Immediately after hatching, the caterpillars spin a communal silk web around the terminal leaves of the nettles. They shelter within the web overnight, or in inclement weather conditions, and feed avidly whenever the sun shines.
 
If disturbed the larvae react in unison, wriggling and jerking, presumably as a defence against parasitoid wasps or flies. Nevertheless a high proportion of larvae turn out to be parasitised.
 
When young, they can easily be mistaken for the caterpillars of the Peacock, but Small Tortoiseshell larvae are paler, and even when quite small it is usually possible to discern pale lines running along their backs.
 
Aglais urticae, 2nd instar larvae on stinging nettle, Dorset, England
 
As they grow, they split up into smaller groups, and spin new webs after each moult, but become solitary when they enter the final instar. By this time they are a dull blackish colour, spiky, with 2 prominent yellow lines running along their backs. The older larvae can often be seen curled in a J-shaped posture, resting on nettle leaves, and if disturbed will coil into a tight circle and drop to the ground.
 
The chrysalis is variable in colour, ranging from grey to olive-brown, often with a coppery or pinkish metallic sheen. It can be found suspended by the cremaster, on woody stems, fence posts, walls, or beneath nettle leaves. The butterflies emerge at dawn, about 12 days after pupation.
 
In normal seasons there are 2 generations of this species, but in Scotland and at high altitudes in northern England there is just a single brood. In the south of England, in exceptionally warm seasons there may be a partial 3rd brood, emerging in October.
 
Adult behaviour
 

Over-wintered adults typically awaken in late March or early April, and remain on the wing until early May. They are usually seen as singletons, The post-hibernation adults nectar mainly at dandelion, daisies and sallow catkins.

 

In spring they often bask on bare ground, on low foliage, or on bushes. On hot days they settle on the soil with wings closed, at which time the sombre "tortoiseshell" pattern on the underside affords them excellent camouflage. When settled the abdomen is usually raised so that minimum contact is made with the hot substrate.

 

Males establish vantage points in the early afternoon, and await passing females. When a female approaches she is intercepted, and the courtship ritual begins with both sexes settling on the ground with wings spread. The male approaches the female from behind, walks onto her hindwings, and vigorously drums her wings with his antennae. The pair then fly a short distance and repeat the process. Any other males which attempt to interfere are briskly chased away by the resident male, who then returns to his female to continue wing-tapping.

 

This bonding process continues for several hours, until just before dusk, when the female accepts the male's advances. At this point she leads him to a sheltered and shady spot, typically beneath a bush or hedge, where copulation takes place. It is likely that the pair remain copulated until the following morning.

 

Aglais urticae, nectaring at knapweed, Martin Down, Hampshire, England

 

The summer brood, which emerges in June, has a wide range of food sources available, and can be seen nectaring at bramble, hawkbit, small scabious and thistles. The second generation, which is always more abundant than the first, emerges in August. In late summer Small Tortoiseshells often visit flowery areas in the countryside, congregating to nectar at hemp agrimony, ragwort and marjoram. They also often visit gardens, where they are attracted to sedum, buddleia and michaelmas daisies.

 

In southern Britain the butterflies fly until late August or early September, when they enter houses, sheds, churches and unheated farm buildings to hibernate. In the north, they tend to enter hibernation in early August, after just a few days on the wing. It is common for Small Tortoiseshells to hibernate communally - I have frequently found them indoors in groups of 3 or 4, and once found a group of 17 hibernating clustered together on a ceiling in a hotel in Tayside.

 
                                                       
Almost every British and European species of butterfly is declining rapidly in numbers, due in most cases to loss or degradation of habitats.
 
You can help to reverse the decline by supporting conservation organisations which purchase and manage habitats as nature reserves, and which lobby government at local, national and international levels, often very successfully, to bring about changes in farming, forestry and urban development policies.
 
Please contact the conservation organisations for advice on how you can help protect British and European butterflies and their habitats. You may be able to offer practical help e.g. by monitoring butterfly populations or helping to manage nature reserves. Donations to these organisations enable them to employ ecologists and biologists. Even if you are unable to provide such help, merely having your name on the membership list can be a powerful tool for conservation organisations wishing to demonstrate the levels of support they have for their policies.

 

 
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