A rare butterfly which was previously extinct in the UK has enjoyed its best summer on record after good weather and conservation work, experts said.
The globally-endangered large blue butterfly had died out in the UK by 1979 but has been reintroduced since the 1980s after experts pieced together the details of its life-cycle in research described as like a “detective story”.
This year the butterfly was recorded at 40 sites across the country in June and July, with two nature reserves in Somerset and one in Gloucestershire together supporting 85% of the UK population.
The population in the south west of England now has the highest numbers of any area of the world, experts said.
Numbers are the highest since modern records began, and are likely to be back where they were before the species started to decline decades ago.
Large blue butterfly numbers have reached around 25,000 adults this year, due to the right weather conditions over the past three years and conservation work, although the drought will see numbers fall back next year.
One of the best sites is the National Trust’s Collard Hill, Somerset, where large blue butterflies were reintroduced in 2000 and the landscape has been managed to create the ideal habitat for the species.
Large blue caterpillars use scent and noises to trick red ants that they are ant grubs, and are carried down into the nest and looked after while they feed on ant grubs before pupating and emerging as butterflies the following year.
Research in the 1970s on the last remaining large blue sites by Professor Jeremy Thomas found while caterpillars were carried down into the nests of various kinds of red ants, they only successfully managed to fool one species.
On many former large blue butterfly sites, this species of ant had largely vanished due to changes in farming practices, which no longer saw the grazing needed to keep plant cover down and ant nests warm, research found.
The research work was “like a detective story, and similarly painstaking at times, but also great fun”, Prof Thomas said.
Grazing regimes were brought in to ensure the plants were kept low in spring and autumn, coupled with letting them grow in the summer which has helped rare flowers and other wildlife thrive.
It allowed the ants to bounce back and set the stage for successful return of the large blue, which was brought back from populations in Sweden.
At Collard Hill, the right habitat was created by planting wild thyme, which the eggs are laid on and caterpillars feed on briefly before they head underground, and introducing ponies and cattle to carefully graze the site.
Ian Clemmett, lead ranger for the National Trust’s Somerset coast and countryside said: “The livestock carefully graze the hill in the autumn and early spring, which isn’t always easy to achieve, punctuated with a fallow period in summer that allows insects to thrive and plants to flower.
“The grazing regime also helped to provide optimal conditions for the red ant Myrmica sabuleti which is vital for the butterfly’s survival.”
Prof Thomas, chairman of the joint committee for the conservation of the large blue butterfly, said: “This rare butterfly is really important because it is more difficult to conserve than other butterflies due to its complex life cycle.
“However, despite this summer’s record numbers, numbers next year will most certainly drop due to the drought which will have damaged the ant nests.
“It is nevertheless the first butterfly in the UK to now have numbers similar to when it was previously at its peak and our approach has now become the model for insect conservation worldwide.”