The largest reintroduction of a rare butterfly that was once extinct in Britain has been hailed a success after the insects bred in the first year.
The globally endangered large blue butterfly was introduced to Rodborough Common in Gloucestershire last year, after five years of landscape preparation.
Large blue butterflies have a remarkable life cycle, which involves the larvae tricking a particular species of red ant into carrying them into their nest where they feed on ant grubs before emerging the next year as butterflies.
The species was declared extinct in Britain in 1979, but was first reintroduced from populations on the continent nearly 40 years ago and has been established at a number of sites across southern England.
Key to successful reintroductions has been creating the right conditions for the Myrmica sabuleti ant species and encouraging growth of wild thyme and marjoram, which the butterfly feeds and lays its eggs on.
The scheme at Minchinhampton and Rodborough Commons is the largest ever reintroduction of large blues in the UK.
Some 1,100 were larvae taken from other West Country locations and released last August on the 867-acre site owned by the National Trust.
An estimated 750 butterflies have successfully emerged at the site over the summer, and the team members monitoring the scheme have recorded large blues mating and eggs laid on thyme and marjoram.
Evidence of the butterflies breeding was recorded within the release but also further away on the commons, indicating they had already expanded their range, the charity said.
It is the first time for 150 years the large blue butterfly, largest and rarest of all nine British blue butterflies, has been recorded at Minchinhampton and Rodborough Commons.
It follows work to prepare the site by partners including the National Trust, Butterfly Conservation, Limestone’s Living Legacies Back from the Brink project, Natural England, Royal Entomological Society (RES) and the Minchinhampton and Rodborough Committees of Commoners.
Small temporary grazing areas were created using electric fences, to allow cows including Luing, Hereford and long-horn cattle to graze slopes to provide the right conditions for the ants, and a programme of scrub control was carried out.
David Simcox, research ecologist and co-author of the commons management plan, said: “The butterfly needs high densities of the heat-loving red ant Myrmica sabuleti, which has a crucial role to play in the lifecycle of the butterfly.
“The grazing cows create the ideal conditions for them by keeping the grass down so sunlight can reach the soil which gently warms it, creating perfect conditions for the ants, which are cold-blooded and therefore need warmth in order to actively scout for food throughout the spring, summer and autumn.”
In summer, the butterfly larvae deceive the ants into thinking they are their own young and carrying them into the nest to complete their life cycle by feeding on the ant grubs until they are ready to pupate and emerge.
The grassland commons are also home to fourteen species of orchid, rare pasqueflowers, Duke of Burgundy butterfly and rock rose pot beetles.
But amid the success of the reintroduction, the National Trust is warning that the coronavirus pandemic has had a major impact on the charity’s finances just as it is focusing on nature and climate emergencies.
Ben McCarthy, head of nature conservation and restoration ecology, said: “Now, more than ever, we need support so that we can continue with our vital nature conservation work to improve the state of nature in the UK and achieve carbon ‘net zero’.”