Small tortoiseshell butterfly: What is behind the demise of species in Britain?

MOST people will have seen the small tortoiseshell butterfly (Aglais urticae), even if they didn’t realise it. Small tortoiseshell is (or rather was) widespread and frequent throughout Britain and Ireland. Over the years, adult butterflies have been observed during unusually warm Januaries after an early exit from hibernation, while butterflies from the second summer brood can still be seen on the wing in the dying days of December. The species name ‘urticae’ gives the game away about the main food plant for the larvae of this species, which is Urtica dioica; the stinging nettle.

Then there were two, again

However, there used to be another native tortoiseshell butterfly which was also widespread and similarly a frequent visitor to suburban gardens especially in south-east England.

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You have to go back to the 1940s to find a time when large tortoiseshell butterflies were still being seen in number, but within two decades the large tortoiseshell butterfly (Nymphalis polychloros), which like its smaller relation is a member of the family Nymphalidae, was declared extinct in the British Isles. But not any more, after two eminent lepidopterists (Neil Hulme and Matthew Oates) discovered the species breeding on rewilded farmland at the Knepp Estate in West Sussex.

Like the small tortoiseshell, the comma butterfly (seen here on rose foliage) nectars on the powerfully perfumed wild common honeysuckle. However, the comma also hibernates with its wings closed in honeysuckle ‘tangles’ where it is thought to be mimicking dead leaves as an effective camouflage. (Image: FJ)

“This butterfly is still officially extinct but the evidence suggests it is making a comeback,” said Oates. “This is a scrubland species and historically and culturally we’ve hated the word ‘scrub’ and done all we can to eliminate it in the countryside. There is ideal scrub for large tortoisehell at Knepp.”  

However, classic butterfly books written in the 19th century when large tortoiseshell was both widespread and common say the most likely places to see the species are lanes lined with English elm trees and the margins of woodland where English elm was also frequent, although generally not inside woodland. Ulmus (elm) was the main food plant for the large tortoiseshell.

Unlike small tortoiseshell butterflies, which feed almost exclusively on nettles, the larvae of the large tortoiseshell butterfly can feed on a variety of trees including elm, aspen, poplar, willow, sallow, birch and whitebeam. However, the main food plant for the larval stage of the large tortoiseshell, and once also called ‘Elm Butterfly’, is Ulmus sp (elm).

19th-century butterfly collectors who bred the large tortoiseshell said those fed on willow were smaller than normal size.  

Larvae found on the Knepp Estate were observed feeding on wych elm, which some contemporary lepidopterists say is the single most favoured of the Ulmus species as a food plant by larvae of the large tortoiseshell butterfly. That may well be the case now and in the future, after the Dutch elm disease pandemic of the 1970s and 1980s wiped out 25 million English elms and related elm species, but cannot explain why the large tortoiseshell butterfly was most common in south-east England, where English elm and related species (like Wheatley elm) were predominant and wych elm was historically few and far between. Indeed, native tree provenance purists would argue, and with some justification, that south-east England is outside of the native distribution of wych elm.

It is not precisely known why the large tortoiseshell became extinct in Britain some 60 years ago. Theories include parasitism, Dutch elm disease destroying English elm (and closely related elms) as favoured food plants for larvae of the large tortoiseshell, and the loss of scrub species such as sallow. 

Sallow may well have been removed in quantity from new conifer plantings. Indeed, foresters have traditionally regarded great sallow and grey sallow as weed species, whether in conifer or broadleaf woodland plantings. That said, sallow has always been ubiquitous, widespread and common with plenty left on the landscape despite the big push in conifer plantings during the post-war period. 

At first sight the devastating effect of Dutch elm disease on English elm and closely related elms, and in turn on the large tortoiseshell butterfly population, does not stand up to scrutiny. Dutch elm disease killed some 25 million mature elm trees but did not dispense with the species altogether. English elm remained in quantity as sucker growth and as a shrub in hedgerows. However, dig deeper in older texts and you will find comments about the larvae feeding on leaves in the tops of large elm trees. 

According to Matthew Oates, numbers of the large tortoiseshell are rising on the European continent again, and increasing numbers are now crossing the Channel to arrive on the south coast. However, the picture is apparently confused because some maverick butterfly breeders have also been secretly releasing the species. 

Spot the difference

Wych elm (Ulmus glabra) is considered to be the most favoured food plant amongst the elms for larvae of the large tortoiseshell butterfly. Wych elm re-foliating and flowering in spring. (Image: FJ)

So, what are the discerning features of the small tortoiseshell and the large tortoiseshell that enable observers to differentiate between the two? To find out you need to consult texts written in the 19th century when there was an abundance of the two tortoiseshells across the British Isles. 

W.S. Coleman, the author of British Butterflies (First Edition 1860; New Edition 1895), said size alone is not sufficient, with Coleman having seen some large tortoiseshells “absolutely smaller” than some very large small tortoiseshell specimens. The orange colour on the wings on the large tortoiseshell is browner or of a more tawny hue than that of the small tortoiseshell says Coleman. However, he acknowledges the colours of each sometimes vary and as such he settles on several, salient and more specific differences. 

First, the blue crescent-shaped spots which are on both wings for the small tortoiseshell are confined to the hind wings for the large tortoiseshell. Second, light markings between the black spots on the upper edge of the front wing are “yellow”, whereas on the small tortoiseshell the outer one next to the blue and black border “is pure pearly white”. Thirdly, the large tortoiseshell has an extra black spot near the corner of the front wing, which is absent from the small tortoiseshell butterfly.

Coevolvement with native plants

Many of our butterflies are closely associated with exotic plants like buddlea that provide a rich source of nectar. Buddleia has been in the British Isles for well over 100 years having been introduced as a garden plant from China in 1890. But there are many native plants which also provide rich sources of nectar. 

They have clearly been on these islands for a much longer period of time that has allowed close coevolvement with native butterflies. 

Two small tortoiseshell butterflies nectaring on exotic buddleia to give a ‘biplane’ visual effect. (Image: Dr Roderick Robinson)

One such plant is the common honeysuckle (Lonicera periclymenum), a wild deciduous climber common in woodlands and hedgerows. A number of butterflies including small tortoiseshell are known to nectar on the powerfully perfumed flowers, but another native butterfly called the comma (Polygonia c-album) hibernates in the tangled mass of leafless honeysuckle stems during winter. Small tortoiseshell does not hibernate in this way but requires a warm dry place which may include houses. 

Big Butterfly Count 2022

The Butterfly Conservation’s Big Butterfly Count for 2022 got underway in July with all eyes on some of our erstwhile common butterflies which have seen significant declines in recent years. Such as the small tortoiseshell butterfly, which has fallen by 79 per cent since 1976. At the time of writing, figures for this year’s count had not been released, but if my back garden in south Hertfordshire and brimming with buddleias is anything to go by, then the signs are ominous. I have not seen a single one.