The large tortoiseshell – an elusive and enigmatic butterfly that became extinct in Britain in the last century – is a UK resident species once again, with a flurry of early spring sightings.
Britain’s list of native butterflies has increased to 60 with the return of the insect after individuals emerged from hibernation in woodlands in Kent, Sussex, Hampshire, Dorset, Cornwall and the Isle of Wight.
The sightings have led Butterfly Conservation to declare the large tortoiseshell (Nymphalis polychloros) a resident species in the UK – as opposed to a migratory one – for the first time in the charity’s 58-year history.
“The signs are really positive, which is lovely,” said Richard Fox, head of science for Butterfly Conservation. “It is resident and therefore it is another species to add to Britain’s total, which is good news. It’s not well-established enough yet to say it’s definitely back for good and will be widespread across multiple landscapes – we’re still in that zone of uncertainty at the moment, but there are exciting signs.”
Unlike the small tortoiseshell (Aglais urticae), which is found in gardens and whose caterpillars eat nettles, the large tortoiseshell is a tree-dwelling butterfly whose caterpillars feed on trees including elm, willow, aspen and poplar.
It is bigger, as its name suggests, and lacks the white markings found on its more widespread counterpart.
The loss of elms due Dutch elm disease was once considered a factor in the large tortoiseshell’s extinction in the UK by the 1960s, but lepidopterists now believe Britain may have always been at the northerly limit of its natural range and the species’ return is now assisted by global heating.
It last flourished in Britain during a run of fine summers in the 1940s, but in recent years growing numbers have migrated across the North Sea and the Channel as a result of population rises in the Netherlands and France.
Higher temperatures this century have also helped many continental moth species move into or re-establish themselves in Britain, including the spectacular Clifden nonpareil moth.
The large tortoiseshell first reappeared in the UK in significant numbers on the south coast in 2006-07, but many subsequent sightings have been attributed to unauthorised releases by butterfly breeders who are keen to re-establish the charismatic species.
Since 2020, large tortoiseshell caterpillars have been found feeding on trees in the wild – first in Dorset and now more widely – confirming that the butterfly is breeding again in Britain.
The insect hibernates as an adult, emerging in spring to mate and lay eggs. Its offspring take to the wing as adult butterflies in midsummer, raising hopes that this spring’s sightings will create a burgeoning population later in the year.
Butterfly Conservation, which has added the species to its first sightings page, is urging people to log any observations on iRecord, a free citizen science app, which is helping build a picture of the butterfly’s distribution and expanding population.
