The large blue butterfly was declared extinct in Britain in 1979. Reintroduced from Swedish stock in 1983, it has since increased in recorded abundance by 1,866 per cent, according to figures published this September as part of the UK Butterfly Monitoring Scheme’s fiftieth anniversary report. It is now established at thirty three sites across south west England, and the region is believed to hold the largest concentration of the species found anywhere in the world.
That is a genuinely remarkable number, and I want to come back to what it does and doesn’t actually mean. But the more interesting part of this story, to me, is not the recovery itself. It is the reason the butterfly went extinct in the first place, which turns out to hinge on a single species of red ant that almost nobody outside specialist entomology had ever heard of.
An Extraordinarily Strange Life Cycle
Large blue caterpillars spend the first part of their lives quite ordinarily, feeding on wild thyme in the way most butterfly larvae feed on a host plant. Then something considerably stranger happens. The caterpillar drops to the ground and waits to be found by a red ant, and when one arrives, it produces chemical and behavioural signals that trick the ant into believing the caterpillar is one of its own grubs.
The ant carries the caterpillar back into its underground nest, where it proceeds to spend the following months, in most cases, eating the ant’s own larvae from the inside of the colony that is unwittingly caring for it. It is a genuinely startling piece of natural history once you actually understand what is happening, a butterfly that cannot complete its life cycle at all without being adopted and then, in effect, betrayed by an insect that has no idea it has been deceived.
Why the First Rescue Attempt Failed
For a long time, conservationists knew the large blue depended on red ants in some general sense, but that knowledge alone was not enough to save it. Early conservation efforts through the 1960s and 1970s worked from the assumption that more or less any red ant species would do, and sites were managed accordingly. The butterfly kept declining anyway, and in 1979, despite genuine effort to protect it, it disappeared from Britain entirely.
The breakthrough came from Professor Jeremy Thomas, who spent years working out that the relationship was far more specific than anyone had realised. The large blue does not simply need red ants in general. It needs one particular species, Myrmica sabuleti, and that species itself needs short, well grazed turf at a very specific temperature to thrive. Where grazing had lapsed and turf grew longer, as it had across much of the butterfly’s former range through the mid twentieth century, sabuleti was gradually replaced by a related ant species that would not accept the large blue’s disguise at all. The butterfly had not simply lost habitat in any general sense. It had lost one specific, unglamorous ant, itself dependent on one specific grazing regime, and nobody had understood that chain clearly enough to intervene in time.
What Changed Once the Chain Was Understood
Once Thomas had worked out the actual mechanism, the reintroduction that followed in 1983 was built around restoring exactly the right conditions for Myrmica sabuleti first, through careful, closely managed grazing, before any butterflies were ever released. That is a considerably more precise piece of conservation than “protect the habitat” as a general instruction. It meant managing turf height with real precision, effectively farming the right ant before farming the right butterfly.
The results, over the following four decades, speak for themselves. Rodborough Common in Gloucestershire, prepared for five years before receiving 1,100 large blue larvae in a single release, saw an estimated 750 butterflies emerge in its very first breeding season, on ground that had not supported the species in a hundred and fifty years. Twelve further sites across the Cotswolds have since been created or restored specifically to extend the recovery, led by the Royal Entomological Society with funding from Natural England’s Species Recovery Programme.
The Number Worth Reading Carefully
I want to be honest about the 1,866 per cent figure, because conservation bodies reporting it have themselves been careful to add an important caveat. That percentage reflects growth from a reintroduced population starting close to zero, not growth within a population that had remained continuously present in the wild. A number that dramatic, taken entirely on its own, risks suggesting a completely different kind of story to the one that actually happened. What occurred here was not a wild population quietly recovering unaided. It was a species brought back from genuine extinction in Britain, through an unusually precise piece of scientific detective work, and then built up carefully across four decades of deliberate management.
That distinction does not make the achievement smaller. If anything, I find it makes the story more interesting, since it means the recovery depended entirely on somebody working out exactly why the previous approach had failed, rather than simply trying harder at the same thing that had not worked the first time.
Why This One Stays With Me
What I keep returning to is how much depended on a piece of knowledge that sounds almost absurdly narrow when you first hear it: not “protect the meadow,” but “maintain grazing precisely enough that one particular ant species keeps outcompeting a nearly identical one.” Conservation at that level of specificity is not the kind of thing that makes for an easy headline, and I suspect that is part of why the large blue’s story is not as widely known as it probably deserves to be.
A butterfly that needs to trick a specific ant, in a nest maintained by a specific grazing pattern, on a specific turf height, is a genuinely fragile arrangement when you lay it out in full. That it has gone from zero to more than ten thousand individuals across thirty three sites is not a story about nature simply bouncing back once left alone. It is a story about how much can be achieved once somebody finally understands, in precise and unglamorous detail, exactly what a species actually needs.
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