In 1983, Britain reintroduced a butterfly declared extinct just four years earlier; the large blue has since surged 1,866% and now survives at 33 sites
A butterfly once declared extinct in Britain is now at the centre of one of the country’s most striking conservation success stories. The large blue, which disappeared from Britain in 1979, has been successfully reintroduced and now occurs at 33 sites in south-west England. New figures from the UK Butterfly Monitoring Scheme show its recorded abundance has risen by 1,866% since 1983. The figures were released by the Joint Nature Conservation Committee (JNCC), alongside analysis from the UK Centre for Ecology & Hydrology (UKCEH), which marks 50 years of national butterfly monitoring. The recovery stands out against a much more worrying picture for Britain’s butterflies. The UK Butterfly Monitoring Scheme has collected more than 44 million records from 782,000 volunteers since monitoring began in 1976. Across 59 native butterfly species monitored over the long term, 33 have declined, while 25 have increased and one has insufficient data. The large blue’s comeback was not simply a matter of releasing butterflies back into the countryside. Its disappearance was linked to a complex relationship between the butterfly, its food plants and a particular species of red ant.
Why the large blue disappeared
The large blue was once found across dozens of sites in England but began disappearing rapidly during the 20th century. By 1950, it occupied only about 25 sites, and by 1972 that number had fallen to just two. It was declared extinct in Britain in 1979. For years, scientists struggled to understand why apparently suitable sites could no longer support the species. Research led by ecologist Jeremy Thomas eventually revealed the hidden link.Large blue caterpillars initially feed on wild thyme. After several weeks, they drop to the ground and are collected by red ants. But the caterpillars can survive only inside nests of one particular species, Myrmica sabuleti, where they spend months feeding on ant grubs before eventually emerging as adult butterflies.The problem was that changes in land management had altered the conditions needed by these ants. Reduced grazing, together with the decline of rabbits, allowed grass and vegetation to grow taller. That changed the temperature at ground level and reduced the areas suitable for Myrmica sabuleti. Without the ants, the butterfly could not complete its life cycle.
The science behind the comeback
Once researchers understood the relationship, conservationists could begin rebuilding the butterfly’s habitat rather than simply focusing on the insect itself. Natural England and UKCEH worked on a reintroduction programme using Swedish large blues after experiments showed that the population could adapt to conditions in Britain. The first successful reintroductions followed in the 1980s, with the project expanding across former large blue landscapes in south-west England.Conservation managers restored the short, warm grassland required by the ants through measures including intensive grazing and the control of scrub and trees. Monitoring was used to establish whether the habitat was becoming suitable before butterflies were introduced. The approach eventually produced populations across several landscapes, turning the large blue into an example of how understanding an entire ecosystem can change the outcome for a threatened species.Today, the species survives at 33 sites in south-west England, according to JNCC. Its 1,866% increase since 1983 is the largest rise recorded for any butterfly in the latest UKBMS statistics, although the figure reflects the fact that the species was reintroduced after its extinction in Britain.
A wider warning from 50 years of records
The large blue’s recovery is also a reminder of what long-term monitoring can reveal. Volunteers participating in the UKBMS have walked more than 1.5 million kilometres while recording butterflies at thousands of sites since 1976. The resulting dataset allows scientists to separate short-term fluctuations from longer changes in butterfly populations.And the broader picture remains concerning. The pearl-bordered fritillary has fallen by 70% since 1976, while the white-letter hairstreak has declined by 80%. The small tortoiseshell has fallen by 87% over the same period. There are other signs that targeted conservation can make a difference. The black hairstreak, for example, has increased by 844% since 1995, while the silver-studded blue has also benefited from intensive conservation work. The latest figures also show why continued monitoring matters. Despite Britain experiencing its sunniest year on record, 2025 ranked only 20th among the 50 years of butterfly monitoring, and no species recorded its best year.For the large blue, however, five decades of research, habitat restoration and monitoring have transformed a story of extinction into one of recovery. Its return shows that when conservation is guided by detailed ecological evidence, restoring a species can mean restoring the intricate relationships that allow it to survive in the first place.
