How A Slice of Battenburg Saved the Large Blue Butterfly by Malcolm Smith

In 1970 the iconic Large Blue butterfly was officially extinct in Britain. But the attentions of one inexhaustible scientist, who tracked the crucial role that ants played in nursemaiding their caterpillars, saved the species. 

By Malcolm Smith 

Take a walk across Somerset’s Collard Hill and you’re unlikely to miss the black spotted, iridescent blue butterflies that flutter about the flower-speckled grassland slopes, tacking their way between straw-coloured rosettes of carline thistles and buttercup yellow rockroses.

Just twenty years ago, there were no Large Blues here. Nor anywhere else in England. Because, by the late Seventies this gorgeous insect – the largest of our seven blue butterflies – was extinct, the end result of a century of unrelenting decline.

The Large Blue’s existence in Britain was first described in 1795 though it had probably been around for millennia. In the late 18th century and maybe until 1840, there were perhaps ninety colonies, mostly in the southwest of England. But less than a century after the Large Blue was “discovered”, it started to die out. By 1950, Large Blues were only present at 25 British sites – maybe a few thousand adults at most. Then the last British site, on Dartmoor, was vacated in 1979.

No-one knew why they had disappeared. A few sites had been lost: planted up with conifers, ploughed up to grow crops or built over. But most of the butterfly’s flower-rich grasslands looked the same as they always had.

Enter Jeremy Thomas, then a young PhD student. When the butterfly was reduced to just two colonies in 1972, Thomas was asked by the then Nature Conservancy if he would interrupt his research on other butterflies to see if he could solve the mysterious decline of the Large Blue.

It took five years but unravel it he did. In one of the finest pieces of biological detective work ever undertaken, Thomas worked out the insect’s incredibly pernickety requirements – armed only with large quantities of Battenberg cake!

“We knew that the Large Blue spent part of its life in red ant nests. But after months of laborious work, locating ant nests by leaving trails of Battenberg cake crumbs across Dartmoor, I discovered that its larvae – the caterpillar – only survived in the nests of one red ant species. And that ant was declining too. But we didn’t know why”, says Jeremy Thomas, today Emeritus Professor of Ecology at Oxford University.

It turned out that alterations in sheep and cow grazing of their flower- rich grasslands and the decline of rabbits due to myxomatosis, left the vegetation slightly too tall and shaded for the heat-loving ants to survive in sufficient numbers. And with them went the Large Blue.

Lying for long hours in the grass, he watched the butterfly’s minute eggs hatch and the caterpillars start to eat their foodplant, the purple flowers of Wild Thyme. The caterpillars shed their skin twice as they grow. Only then do they fall to the ground, to wait for the attentions of a particular red ant.

The tiny caterpillars then trick the ants into carrying them in their jaws into their underground nests by secreting a fluid they love to eat. They even “sing” to the ants to fool them into assuming the caterpillar is a queen ant grub.

Their trickery continues in the nest, the larva mimicking their scent and sounds - while it steadily devours the ants’ own larva. If the ants realised it was an imposter they would kill it.

“It’s a very dangerous lifestyle to have evolved unless there is some really close protection from the ants; hence its mimicry”, says Thomas.

When the caterpillar eventually turns into a pupa it hibernates through winter. When the adult butterfly is ready to emerge in early summer, the ants usually carry it back outside and leave it on the ground. There they encircle it to ward off any predators as its wings unfold. When the butterfly is ready to fly, the ants retreat into their nest, their extraordinary job done for another year. Warmed up, the adult Large Blue flutters off to fulfil its aim in its week of existence: to find a flowering thyme plant and lay its eggs.

While this incredible task is attempted by several red ant species, most are unsuccessful. Thomas identified the only species that the Large Blue can rely upon. Getting the turf height right by grazing it to within a centimetre of the ant’s perfect height was a hard task as growth varied year to year. But it was managed and back came the particular species of red ant in good numbers.

With no Large Blues left, it required eggs obtained from thyme on Swedish sites and released in the hope that the ants would accept them. They did! By the early 1990s, the Large Blue was once again fooling ants at a handful of its old haunts in southwest England. Further re-introductions since, and some natural spread, has returned this stunning butterfly across southern England – most recently in the Cotswolds.

Today, England has the biggest Large Blue population in the butterfly’s range, which stretches east across much of Europe to China. Thanks to Thomas’s detective work and a dedicated and continuing conservation effort, this stunning butterfly is back in style.

It’s not just the Large Blue that has a strange relationship with ants. The butterfly is a member of the lycaenid family, which in the UK includes our blue butterflies and others like the hairstreaks, coppers and the Brown Argus.

Many, not all, of the species within this family have evolved similar symbiotic relationships with ants - also called mutualism as it’s of mutual benefit to both sides. Each butterfly is usually associated with chumming up to a particular ant species in different ways.

Where it all begins

Every caterpillar is made up of segments, all of which develop as the larva sheds its skin as it grows. The phases of the larva’s life between these moults are called an ‘instar’ - butterfly caterpillars have around five instars in their part of the life cycle. Lycaenid butterfly larvae have a specialised ‘honey gland’, known as the Newcomer’s Gland, on their seventh segment - different species develop these at different instars.

This gland exudes a cocktail of sugars and amino acids that ants love which, in turn, offer the larva a level of protection.

But, hang on...

We’ve seen that some Lycaenid caterpillars like the Large Blue can eat the ant’s own larvae. Why on earth would the ant tolerate that? After all, there are other sources of sweet foods out there. The caterpillar is completely dependent on the ants but the ants really don’t need the caterpillars. (Incidentally, not all Lycaenid larva are carnivorous. The Adonis Blue’s, for instance, is a ‘vegetarian’.)

The conundrum attracted the attention of Assistant Professor Hojo Masaru of Kobe University in Japan, who guessed that the larvae possessed some mechanism to keep the ants from leaving. He studied the exudant they produced, and the ants’ behaviour, further.

He and his team discovered that ants who only ate nectar were markedly more active and aggressive when they were denied access to the larvae’s magic juice.


He managed to measure the brain chemicals present in ants that were feeding under different conditions. He found that ants who also fed on both nectar and juice from the larvae slowed down their movements and hung around the caterpillars more. This suggested that the secretions were more than a nutritious reward. Indeed, he found that caterpillar’s juices contained substances that regulated a supply of dopamine to the ants’ brains, manipulating the ants like drugs. That way they remained faithful - like junkies? Suddenly that mutualistic relationship looks a lot more like a parasitic one.