England Put 1,100 Caterpillars In a Dead Hill — 1 Year Later, Extinct Butterfly Faces the Impossible

Added: 2026-05-28

[00:00:00]In August 2019, the National Trust released, 1100 large blue caterpillars onto a Cotswald hillside in Glstershare onto a slope where the species had not flown for over 150 years. One year later, scientists walking the same hillside counted something they were not expecting.

[00:00:18]Around 750, adult butterflies had emerged from the soil. They were mating openly in the grass. They were laying eggs on the wild time and some of them were already turning up away from the release zone on slopes the project had never prepared in patterns the model had not forecasted.

[00:00:37]A species that had been declared extinct in Britain in 1979 was now flying in greater numbers on that one hillside than anywhere else in the world. And the strangest part is not that it worked. It is what scientists are still learning season after season about exactly how it worked and what one returning butterfly is doing to an entire English landscape.

[00:01:00]This is the full story from the chain reaction that wiped this insect off the British map to the 40-year scientific operation to bring it back to what is now happening across 12 new sites in southwest England that is rewriting expectations for what insect conservation can achieve. Most videos about butterfly reintroductions show you the release footage and call it a success. This one follows the science 40 years deep into the failed protection schemes of the 1930s, the ecological chain reaction that triggered extinction, the single Swedish island that saved the species, and the evidence that is only now emerging about what a returning insect can do to an entire grassland ecosystem.

[00:01:47]Every claim in this video is sourced from the University of Oxford's Department of Biology, the UK Center for Ecology and Hydrarology, the National Trust, the Royal Entomological Society, and four decades of monitoring data from over 30 restored grassland sites. This is not speculation. This is what the records actually show. But before you can understand what is happening on those Cotswald hillsides now, you need to understand what Britain lost in 1979 and why bringing it back required solving one of the strangest echological puzzles in European wildlife science.

[00:02:26]The large blue is not just another insect. It is the largest and rarest of Britain's nine native blue butterflies.

[00:02:34]An adult has a wingspan of about 5 cm, roughly 2 in, with deep violet blue upper wings marked by a distinctive row of black spots that no other British species carries. It flies low, fast, and only for a few weeks each summer. But its size and color are not what make it remarkable. What makes the large blue irreplaceable is its life cycle. A life cycle so strange that for over a hundred years the greatest entomologists in Europe could not figure it out. The adult female lays her eggs on the flower buds of wild thyme or margarm. The caterpillar hatches, feeds on the flowers and grows for about 3 weeks.

[00:03:18]Then something extraordinary happens.

[00:03:21]The caterpillar stops eating time. It drops to the ground and it waits.

[00:03:28]What it is waiting for is a red ant.

[00:03:31]Specifically, one single species of red ant called murmur sabueti. When a worker from this species finds the caterpillar, the caterpillar performs what scientists describe as a chemical and acoustic deception of breathtaking precision. It secretes a fluid that the ants taste. It vibrates at a frequency that mimics a queen ant grub. The worker is completely fooled. It picks up the caterpillar in its jaws and carries it deep underground into the colony into the sealed chambers where the ants raise their own brood.

[00:04:05]What happens next is the part most people find difficult to believe. For the next 10 months, that caterpillar, which the ants are still treating as one of their own young, feeds on the ant larae. It eats the colony's own babies.

[00:04:20]The workers continue to protect it. They guard it through autumn, winter, and spring. In late spring, it pupates inside the nest. In June, an adult butterfly crawls out of the underground colony, climbs to the surface, dries its wings in the sun, and flies away to mate. This is one of the most extreme parasitic relationships known in the insect world, and it works with only one species of ant. Other red ants will pick up a large blue caterpillar, but the caterpillars almost always die in the nests of any other species. The deception is so specific that the entire fate of one of Britain's most spectacular butterflies hangs on the survival of one tiny ant. And on conditions so precise that a few centimeters of grass height can decide whether either species exists at all.

[00:05:12]Hold on to that detail because the centimeters of grass and the temperature of the soil beneath them are what this entire story turns on.

[00:05:22]For most of the 20th century, no one understood what was killing the large blue. Naturalists blamed butterfly collectors. They blamed weather. They blamed habitat loss in vague terms. Nine nature reserves were set aside for the species between 1930 and 1969.

[00:05:42]Fences were built to keep collectors out. None of it worked. The colonies kept disappearing.

[00:05:49]What actually happened was a chain reaction so subtle that no one saw it until it was almost too late. In 1953, a disease called mixomitosis arrived in Britain. It killed up to 99% of the country's rabbits. Britain's pre-dise rabbit population was estimated at over 60 million animals. Wild rabbits were the unseen gardeners of the chalk and limestone hillsides where the large blue lived. They cropped the turf to two or three cm in height. That short turf let the spring and summer sun warm the soil.

[00:06:26]The warm soil kept the red ant murmur sabueti happy and the happy ant raised the caterpillars. When the rabbits died, the grass grew by just a few cm.

[00:06:38]The soil cooled by 2 or 3° C. The red ant murmura saboleti began to disappear, replaced quietly by other murmur species that lived in cooler soil. Species that would still pick up large blue caterpillars, but in whose nests the caterpillars almost always died. At the same time, farmers were abandoning marginal grazing on these hillsides for economic reasons. Sheep and cattle were being moved to better pasture.

[00:07:08]The combined disappearance of rabbits and livestock left the turf to grow tall, dense, and shaded. And then, in one of the crulest twists in British conservation history, the well-meaning fences put up to protect the butterfly from collectors actually killed the surviving colonies faster because those fences also kept out the grazing animals that could have saved them.

[00:07:32]The turf grew highest where the protection was strongest. By 1972, only two colonies remained in Britain. 325 adult butterflies, all on Dartmore. A young doctoral student named Jeremy Thomas was asked by the Nature Conservancy to find out what was killing them. For the next 6 years, he spent every summer on the Devon Hills following the fate of more than 1,300 individual eggs. He measured turf heights. He counted ant nests. He mapped soil temperatures. He documented every stage of the butterflyy's life cycle. He discovered the truth that no one had been able to see. The survival of the entire species depended on one species of red ant. And that red ant depended on conditions that were quietly being erased from the British countryside.

[00:08:23]In 1974, unusually wet weather reduced the number of days the females could lay their eggs. In 1975, drought shortened their adult lifespans.

[00:08:34]By 1979, the last colony on Dartmore was gone. The large blue was declared extinct in Britain. Thomas had cracked the code, but he had cracked it too late to save the original butterflies. The question was now whether his discovery could be used to bring them back. What he and his colleagues did next would take 40 years, cost millions, and depend on a small Baltic island. Most people in Britain had never heard of.

[00:09:02]The island of Oland sits off the southeast coast of Sweden. It has limestone grasslands, traditional grazing, and a healthy population of large blue butterflies that had survived the same agricultural pressures that had wiped them out across most of Northern Europe. The Swedish Nature Conservancy gave Britain permission to collect eggs as an experiment to see whether the so-called Baltic race of the species could adapt to British conditions. In 1983, the first batch of eggs was carried back to Britain by David Simcox, a scientist at the Center for Ecology and Hydrarology, who transported them in his camper van. A second collecting trip followed in 1984. Before any caterpillars could be released, the scientists had to rebuild the habitat.

[00:09:50]52 time grasslands were restored to the exact specifications Thomas' research had identified. Turf cropped between 2 to 4 cm in height, scrub cleared, grazing reintroduced.

[00:10:04]They used cattle on some sites, sheep on others, and ponies on a few. Bespoke management plans were written for each location. Soil temperatures were measured. Ant nest densities were counted year after year. On one Dartmore site, the red ant murmura sabueti had been completely absent in 1973 and then covered the majority of the grassland just 10 years later after grazing was restored.

[00:10:33]By 1985, just 2 years after the first eggs arrived from Sweden, large blues were flying freely in southwest England once again. By 2006, the estimated number of adults flying in Britain was 10,000 individuals on 11 sites, the largest number recorded in the British Isles for over 60 years. But the population was still concentrated in just a few landscapes, mainly Midsomerset and the Pen Hills. A single bad year, a single disease, a single change in land management on those sites could wipe out the entire reintroduced population.

[00:11:13]The species needed to expand. It needed new colonies. It needed redundancy.

[00:11:20]And in 2019, the project entered its most ambitious phase on a hillside in the Cotsworlds where the large blue had not been seen since the 1870s.

[00:11:31]Rodbrook Common is a national trust site in Gstershare. 351 hectares or 867 acres of limestone grassland sitting above the town of Straoud. It is a site of special scientific interest and a special area of conservation. It is also the kind of landscape that on paper should have supported large blues for centuries, except that the hillside had been quietly slipping out of the conditions the butterfly required for over a hundred years. 5 years before any caterpillar arrived, the National Trust began preparing the ground, working with Sarah Meredith and David Simcox of the Royal Entomological Society, with the limestones living legacies back from the brink project, with natural England, with butterfly conservation, and with the Mincchin Hampton and Rod committees of commoners. The partnership rebuilt the grazing regime from scratch. Cattle breeds were chosen for their behavior.

[00:12:29]Luing, Heraford, and Longhorn.

[00:12:33]Small temporary grazing areas were created with electric fencing to push the animals onto the slopes they would not naturally graze. Scrub was cleared by hand. Wild thyme and margarm were given the space and the sunlight to flower again. Soil temperatures rose.

[00:12:51]The red ant began to repopulate.

[00:12:55]In August 2019, 1100 large blue caterpillars collected from Daneway Banks in Glstershare and three sites in Somerset were carefully released onto Rodbra Common. It was the largest single reintroduction of large blues ever attempted in Britain. The caterpillars dropped into the grass. They waited for the red ants. The ants carried them underground and nobody on the hillside was going to see them again for 10 months. The waiting was almost the hardest part. There was no way to check the colonies, no way to verify the survival rate. The caterpillars were sealed inside the nests of an ant colony doing something that had not been done on that hillside since Queen Victoria sat on the throne. In late May 2020, on the 27th of the month, the first adult large blue was spotted on Rodber Common.

[00:13:49]By August, the count had climbed to around 750 adults.

[00:13:54]They were mating in the open. They were laying eggs on the time, and some were already moving, turning up away from the original release zone on slopes the team had not even prepared. The pattern was unambiguous.

[00:14:07]The colony was not just surviving, it was establishing. But that was only the beginning of what the scientists started to find.

[00:14:16]The first surprise was geographic. By 2022, just 3 years after the Rodborough release, the Large Blue was flying in its greatest numbers since records began on the largest number of sites in British history. 12 new sites had been created or restored across southwest England, some on former arable land, some on failed conifer plantations, some on the embankments of disused railway cutings, and some on degraded downland brought back into grazing. Those 12 sites alone were already supporting up to a third of the entire United Kingdom population of the species. 3 years earlier, the same sites had been supporting just 7%.

[00:14:59]The colonies were linking up. The interconnected populations now stretched across two whole landscapes through Somerset and now into the Cotsworlds.

[00:15:09]The 1,100 caterpillars released on Rodbra in 2019 were no longer an isolated experiment. They were one node in a network and the network was expanding without anyone needing to release another butterfly. The second surprise was ecological.

[00:15:26]When the partnership restored Rodbbor common for the large blue, they did not just bring back one butterfly. They brought back an entire grassland ecosystem. The same conditions that suited Merurmika Sabueti, such as short turf, warm soil, and sunlit thyme and margarm, also suited dozens of other rare and declining species. Other rare butterflies began to recover. Rare bees returned. Rare beetles, rare flies, and rare grassland plants that had not been recorded on these slopes in decades reappeared as the management changed.

[00:16:02]This is what conservation scientists call an umbrella species. A species whose specific requirements, when properly met, end up sheltering many other species that share the same habitat. The large blue was demanding.

[00:16:16]To make a hillside work for the large blue, the partnership had to rebuild a kind of meadowand that had been quietly disappearing across England for 60 years. And when they rebuilt it for the butterfly, the rest of the meadowand came back with it.

[00:16:32]The third surprise was global. The large blue is listed as one of Europe's most endangered insect species. It has gone extinct or nearly extinct across most of Northern Europe for the same reasons that drove it out of Britain. But southwest England, the place where it was officially declared extinct in 1979, now supports the greatest concentration of large blue butterflies known anywhere in the world. The country that lost the species, has become the global stronghold for the species. The Swedish island of Oland, which provided the original eggs, is no longer where the species is doing best. The best place on Earth to find a large blue, is now the limestone hills of Somerset and Glstershare.

[00:17:17]Professor Jeremy Thomas, the young PhD student who first cracked the ant code in the 1970s and who chaired the joint committee for the reestablishment of the large blue butterfly for over 25 years has called the project arguably the most successful insect reintroduction in the world. The numerical model that he and his colleagues built based on those 1300 eggs followed individually across Dartmore in the years before extinction predicted the recovery. so accurately that it stands as one of the very few documented examples in conservation history of an ecological model successfully forecasting the rescue of an insect. The large blue does not exist without the red ant. The red ant does not exist without the right turf height.

[00:18:06]The turf height does not exist without grazing. And the grazing did not exist for over 50 years because the rabbits had died and the farmers had moved on. A century of small decisions, most of them invisible at the time, had erased an insect from an island. And it took 40 years of equally precise, equally patient work to put it back. What is happening on Rodbrook Common is more than a butterfly returning to a hillside. It is the restoration of a process, a relationship, a small, almost unbelievable piece of natural choreography between a flower, an ant, and an insect that pretends to be the wrong thing in order to survive. Every large blue flying over the Cotswwells today is the product of one of the most complete ecological dossas ever assembled on a single species and a partnership of scientists, landowners, charities and commoners that refused to let the answer come too late twice.

[00:19:09]00 caterpillars, one Cotswald hillside.

[00:19:13]150 years of absence ended in a single season, and a species that was declared extinct in Britain in 1979 now flies in greater numbers on more sites than anywhere else on Earth. But Britain is not the only place where a single returning species is rebuilding an ecosystem that had been running without it for over a century.

[00:19:37]In our next video, we travel to a forgotten valley in the north of France, where engineers spent more than a decade tearing down what had taken 60 years to build, and where a river that no one had seen run freely in three generations is now hosting fish that had not swam those waters since before the First World War.

[00:19:59]What is now moving through the Saloon Valley will change how you think about what rivers are supposed to do.