Large blue thrives after transplant from Swedish island

Naturalists hope that this summer will prove to be a record year for the emblematic large blue butterfly, which was declared extinct in Britain in 1979 but has experienced a remarkable renaissance after it was re-introduced from Sweden in 1984.

Numbers have increased gradually over the past 25 years thanks to a carefully managed programme based on a thorough scientific understanding of the butterfly's complex life cycle, which relies on the co-existence of a species of red ant in the same grazed grassland habitat of flowering wild thyme.

There are now some 25 sites in south-west England where the Large Blue butterfly is breeding, including the National Trust's Collard Hill in Somerset, which is the only easily accessible place where the public can see this enigmatic species – widely viewed as the most successful example of wildlife re-introduction in the UK.

The Large Blue was re-introduced to Collard Hill in 2000 and after numbers fell to as low as 36 adults and 900 eggs in 2002, they have risen steadily to a record 827 adults and more than 20,600 eggs in 2009. Experts believe that this year could set a new record.

"The last four years at Collard Hill have seen a steady but strong increase in the numbers of Large Blue butterflies recorded. Getting the habitat in the right condition for this very particular butterfly had been crucial, and has been down to getting the grazing right – using cattle and Dartmoor ponies," said Rob Holden, the National Trust's area warden at Collard Hill.

"With a large number of eggs laid during 2009, we're hoping for a bumper year. It will come down to getting the grass at just the right length and much depends on the weather," Mr Holden said.

After hatching, Large Blue caterpillars feed on wild thyme flowers, but after a few days they drop to the ground and are picked up by red ants and taken to their nests, believing the caterpillars to be their own larvae. However, the caterpillars spend the next 10 months feeding carnivorously on the ants' larvae before emerging as adults.

When the re-introduction of the Large Blue was first mooted, scientists had to identify a foreign population that would be genetically programmed to emerge at the same time as the flowers of British wild thyme plants. They found one such group on a Swedish island.

The National Trust will hold an open day at Collard Hill later this month to celebrate the success of the re-introduction project.