Somerset Wildlife Trust

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Best Year for Britain's Rarest Butterfly Since 1930's

 30th Aug 2016


Large Blue


Thanks to meticulous conservation management, south-west England now supports the largest concentration of large blues known in the world.

Somerset Wildlife Trust’s Green Down and Gloucestershire Wildlife Trust and Royal Entomological Society’s Daneway Banks saw over 10,000 adult large blues in 2016, which equates to roughly 60% of the UK population according to Professor Jeremy Thomas, chair of the Joint Committee for the Restoration of the Large Blue Butterfly. Together they laid more than a quarter of a million eggs on the abundant thyme and marjoram flowers.

Large blue stats 1

This is no mean feat, for the large blue is the only UK butterfly species that is sufficiently threatened worldwide to be listed in the IUCN’s global Red Data Book, and our only one designated as an ‘Endangered Species’ across Europe. The success of the large blue reintroduction is due to the combined efforts of the Large Blue Project. As well as the Wildlife Trusts in Somerset and Gloucestershire, partners include Natural England, Butterfly Conservation, the University of Oxford and the National Trust.


Gloucestershire Wildlife Trust’s CEO, Roger Mortlock, says: “This is fantastic news for this globally endangered butterfly whose extraordinary life cycle makes its conservation very challenging. Scrub clearance and careful grazing of wildflower-rich grasslands is key to ensuring a future for this beautiful insect. This special management also helps a huge diversity of wild plants and other insects to thrive.”


Professor Jeremy Thomas (Chair of the Royal Entomological Society’s Conservation committee, Emeritus Professor of Ecology at the University of Oxford, and chair of the Joint Committee for the Restoration of the Large Blue Butterfly) said:


"The success of this project is testimony to what large scale collaboration between conservationists, scientists and volunteers can achieve. Its greatest legacy is that it demonstrates that we can reverse the decline of globally-threatened species once we understand the driving factors."


The large blue has a bizarre life-cycle, having fed for three weeks on the flowerbuds of wild thyme or marjoram, the caterpillar produces scents and songs that trick red ants into believing it is one of their own grubs, and is carried underground into the ants nest and placed with the ant brood. The caterpillar spends the next 10 months feeding on the grubs before pupating in the nest the following year and then emerging to crawl above ground as a butterfly.


Despite over 50 years of effort to halt its decline, the large blue butterfly was pronounced extinct in Britain in 1979. Its reintroduction in 1984 was based on the discovery that large blue caterpillars can only survive in the nest of one particular species of red ant, Myrmica sabuleti. Changes in countryside management were responsible for the extinction. Alterations in grazing left grassland too tall and shady for the heat-loving red ant.


Reserves Manager for Somerset Wildlife Trust, Mark Green said: “The amazing numbers of Large Blues recorded this year show what can be achieved through close partnership working and landscape scale conservation land management, underpinned by sound science. Large blue numbers had declined significantly two years ago, due to unfavourable weather conditions. But, thanks to the project partners creating and maintaining a number of well-connected core sites, the butterfly has now bounced back to record numbers. I feel proud to play a part in this highly successful project, which gives me hope that we can reverse the declines of other vulnerable species.”


Today optimum habitat has been restored to more than 50 former sites. The finest of these are Green Down Nature Reserve in Somerset and Daneway Banks Nature Reserve in the Cotswolds, both Wildlife Trust sites. Thanks to good gazing management coupled with favourable weather, their already massive large blue populations increased by 74% and 64% respectively compared with 2015. The National Trust’s Collard Hill site in Somerset also boasted good numbers, and remains the most accessible place to see large blues thanks to its extensive car park and on-site warden.

Large Blue stats Aug-16

A spin-off of our managing grasslands to support the large blue is that it has simultaneously improved conditions for a diversity of other wildlife.

At Daneway, scarce orchids including fly, frog and musk have returned after an absence of many years, and the exceedingly rare cut-leaved germander and cut-leaved self-heal are now flourishing. Among insects, the Downland Villa beefly – not recorded in the UK for 50 years prior to 2000 – bred in great abundance along the tracks and scrapings of Daneway in 2014-16. 


At Green Down, cut-leaved self-heal also flowered abundantly in 2016, and meadow brown and marbled white butterflies had their highest and 2nd highest recorded numbers respectively in decades of recording, again belying predictions for low butterfly numbers elsewhere.