Despite over 50 years of effort to halt its decline, the Large Blue butterfly was pronounced extinct in Britain in 1979. Today the butterfly can be found on 33 sites in the south-west of England. This is a tribute to a large-scale conservation programme underpinned by innovative science and implemented by a determined and broad partnership
Path to extinction
From the 1790s, when the butterfly was first documented as British by William Lewin, all knowledge about its life history and distribution came from butterfly collectors. This enigmatic species was prized by collectors due to its great beauty and rarity. Frustratingly, they were unable to procure perfect specimens as it proved impossible to rear them in captivity - caterpillars always died when they were about three weeks old.
This conundrum was solved in the early 1900s by three of the great lepidopterists of the day: Purefoy, Frohawk and Chapman. It was discovered that after feeding on the flowers of wild thyme and reaching their fourth instar, caterpillars fell to the ground, where they were found by red ants (Myrmica species). It was confirmed that the ants picked up the caterpillar and took it into their underground nest, where it was placed in the brood chamber. The caterpillar spent the next 10 months feeding on ant grubs before pupating and then emerging as a butterfly the following year.
Owing to the difficulty of maintaining ant nests in captivity, this information failed to produce cabinet specimens or halt the decline of the butterfly. In 1950 the Large Blue occupied only 25 sites, which rapidly declined to two by 1972. Although some sites were destroyed by changes in land use such as ploughing or afforestation, most still supported wild thyme and red ants.
So why did the butterfly become extinct in Britain in 1979?
Searching for a scientific solution:
In 1972, perplexed and concerned by the rapid decline in Large Blues, the Nature Conservancy (now Natural England) asked a young PhD student, Jeremy Thomas, to see if he could discover the cause of the inexplicable demise of the Large Blue.
For the next six years Jeremy spent every summer on Dartmoor, monitoring all aspects of the Large Blue’s life cycle. By following the fate of over 1300 butterfly eggs he was able to show that most deaths took place in the ant nests. He found that there were four species of red ants (Myrmica) living on Dartmoor and while all of them would pick up a Large Blue caterpillar, these caterpillars only survived in the nests of one species, Myrmica sabuleti. The butterfly had evolved to fool this ant into accepting it as a dangerous guest, while all other red ant species quickly recognise and kill the impostor.
Jeremy meticulously measured the precise habitat niches of the four different species of red ant and found that each one prospered in different temperature zones, and that the ground temperature was largely determined by turf height. He then led a detailed survey of all former Large Blue sites and discovered that although many still supported thyme and red ants, Myrmica sabuleti was either absent or present in very low numbers.
The work concluded that changes in agricultural practices had resulted in a massive reduction of livestock grazing and, coupled with the devastating effects of myxomatosis on rabbit populations, turf heights had imperceptibly increased. In order to conserve Large Blues, sites would have to be managed to maintain the precise conditions that enabled Myrmica sabuleti to prosper. Unfortunately this pioneering work came just too late to save the butterfly.