In Conservation Success Story, Once-Extinct UK Butterfly Has Best Summer in 150 Years

Butterflies overall are struggling in the UK, but one particularly endangered species is thriving. 

After being driven to extinction in the UK in 1979, the large blue butterfly had its best year in 2022 since record keeping began 150 years ago, the Royal Entomological Society announced Wednesday. The success is thanks to reintroduction efforts that began in 1983. 

“I didn’t have a grey hair on my head when I started,” the society’s David Simcox, who helped lead the reintroduction efforts, told BBC News. “Now it’s all grey.” 

The large blue butterfly was rendered extinct in the first place in part because of its unusual lifecycle, the Royal Entomological Society explained. It spends the first three weeks of its life gobbling up wild marjoram or thyme flowers before doing something truly strange. It disguises itself as a red ant grub by mimicking the other insect’s sounds and smells. Once the parent ant takes it into the nest, it eats the real grubs for 10 months before forming a cocoon and emerging as a butterfly. However, the large blue caterpillar can only trick one particular species of red ant, Myrmica sabuleti, which needs a certain amount of heat to thrive. But changes in the way grazing was managed in the UK meant that wildflowers in meadows grew too tall and created too much shade for that particular species of red ant, which is why the large blue butterfly died out. 

Discovering the exact species of red ant the butterfly needed to trick enabled scientists to reintroduce the butterfly by recreating the ant’s preferred habitat through controlled grazing. Simcox headed the initial reintroduction efforts alongside University of Oxford Emeritus Professor of Ecology Jeremy Thomas, who was the one who discovered what the species needed to thrive. They first reintroduced the butterflies in Devon using individuals from Sweden. Next, the large blues were reintroduced to the Somerset Wildlife Trust’s Green Down in 1992 and the Cotswolds in 2010. Recently, conservationists have created or restored 12 new sites for the species in Southwest England that already support as much as one third of the butterfly’s UK population. As of 2022, the species is breeding in greater numbers and in more locations than any point in the last 150 years. 

“The unprecedented success of this project is testimony to what large scale collaboration between conservationists, scientists and volunteers can achieve,” Thomas said in the press release. “Its greatest legacy is that it demonstrates that we can reverse the decline of globally-threatened species once we understand the driving factors.”

This is good news for the large blue butterfly, to be sure. The species is considered one of Europe’s most endangered insects and England, where it was once extinct, now boasts its largest population, The Guardian pointed out. 

Further, the unique habitat required by the large blues and their red ant hosts has also allowed other rare species of plants and insects to thrive. These include the pasqueflower, cut-leaved self-heal and as many as 12 species of orchids on the plant side, according to the press release. On the insect side, the threatened Shrill carder bee, the threatened Rock-rose pot beetle, the threatened Downland villa beefly and the threatened Rugged oil beetle have all appeared in the 12 new sites, as have eight butterflies on the butterfly Red List for Great Britain.

The success of the large blue butterfly is bucking the trend for the charismatic insects in the UK: half of the species in the country are now considered either threatened or near-threatened. The biggest danger for the country’s butterflies is habitat loss and land-use change, but the climate crisis is also a danger, and this is a danger that the large blue shares despite restoration efforts. 

Extreme droughts mean that red ants get less food and are therefore more likely to detect and expel the large blue caterpillars, The Guardian pointed out, and the butterfly’s numbers have fallen during dry years in the past. The country is currently in the midst of a dry spell, with some areas weathering their driest July since 1935. Luckily, Thomas told The Guardian that the large blues’ current homes were not particularly impacted. Further, the restoration sites were chosen to include areas that are too wet and cool for the two species of insect during regular years but can provide respite during droughts. However, the summer of high heat and dry weather could be a harbinger of risks to come. 

“The greatest challenge ahead is to secure this expansion in a warming climate and to develop strategies to mitigate the impacts of extreme weather events,” Simcox said in the press release.