The great blue butterfly has had its best summer for 150 years in Britain thanks to targeted restoration work, which is also benefiting other rare insects, including the robust beetle and the strident carder bee.
The butterfly, which became extinct in Britain in 1979 but was reintroduced via caterpillars from Sweden four years later, flew in the highest numbers in June this year since records began.
South West England is now home to the world’s largest known concentration of great blues, which are listed as one of Europe’s most endangered insect species.
Up to a third of its UK population is at 12 new sites which a conservation partnership has restored to flower-rich grassland from arable fields, failed conifer plantations, railway embankments or land degraded.
The new prairie under special management for the big blues is also proving to be an ideal breeding ground for rare plants such as the extremely rare pasqueflower and 12 species of orchids including the musk orchid, the autumn cypress and the large butterfly orchid.
Rare insects that have benefited include the hardy oil beetle, rosebush beetle, shrill carder bee, land villa and spotted oxen, as well as eight endangered butterfly species, including the reborn Duke of Burgundy , once one of the most endangered in the UK.
The new grasslands are managed or owned by six partner organizations – the National Trust, Somerset and Gloucestershire Wildlife Trusts, J&F Clark Trust, Natural England and the University of Oxford – with restoration being overseen by David Simcox and Sarah of the Royal Entomological Society. Meredith.
Simcox said, “We are extremely proud that the efforts of the partnership have enabled hundreds of people to see this amazing and enigmatic butterfly flying over some of the most beautiful grassland sites in the country.
“The biggest challenge ahead of us is securing this expansion in a warming climate and developing strategies to mitigate the impacts of extreme weather events.”
The big blue has an extraordinary way of life: its young caterpillars trick ants into thinking they are young ant larvae, and they are taken to ant nests. Here, they are protected underground all winter and grow by devouring the larvae of ants.
The species became extinct in Britain because the abandonment of grazing and rabbit grazing in traditional grasslands left the grass too long and the temperatures too cool for the ant species to survive.
The butterfly’s lifestyle and requirements were worked out by Jeremy Thomas, now Emeritus Professor of Ecology at Oxford, who spearheaded its reintroduction, the ant species thrives again when grasslands are grazed to an appropriate level .
Support from the Prince of Wales’s Charitable Fund has enabled the butterfly to be reintroduced to two new sites in the Cotswolds over the past three years, but the focus is now on conservation ways to ensure the new populations are resilient to global warming.
Severe droughts cause ants to undernourish, and when stressed, they are more likely to spot the cunning of the large blue caterpillars and expel or devour them.
The butterfly population crashed after previous drought years, but Thomas said he hoped the worst of the dry weather hadn’t affected the Somerset and Cotswolds sites.
Restoration sites were deliberately chosen with a variety of microclimates and soil depths so that most would have areas too cool and wet for the ant and butterfly in most years, but provide vital refuges during droughts.
Thomas added, “The unprecedented success of this project is a testament to what large-scale collaboration between conservationists, scientists and volunteers can accomplish. Its greatest legacy is that it demonstrates that we can reverse the decline of globally threatened species once we understand the driving factors.