Dr Nick Read

A curious tale of butterflies, ants, wasps and the passage of thyme

The large blue butterfly is the largest and rarest of our blue butterflies.  Clouds of them can be seen fluttering over heathland on a summer evening, but in the eighteenth century the passion of Victorian gentlemen for collecting butterflies nearly drove them into extinction.  Conservationists tried to protect them by fencing areas of heathland and preventing the grazing of sheep, but still numbers declined and by 1979 they had disappeared from Britain. Before that happened, a naturalist, Jeremy Thomas, spent six years living adjacent to one of the last remaining wild blue colonies on Dartmoor, where he recorded meticulously, every aspect of their life cycle.  What he discovered was truly remarkable. 

The butterflies lay their eggs on the leaves of the Wild Thyme.  When they hatch the caterpillars burrows into the buds and eat the developing seeds.  If there is more than one one caterpillar in a bud, it will be devoured.  The surviving larva then falls to the ground,  hides in a crevice in the earth and is discovered by an ant, which is driven into a frenzy, climbing all over it, licking its skin and sipping the sweet secretions from glands at the end of the caterpillars body.  The larva tolerates the ants attentions for up to four hours before curving its body and making it rigid so that it resembles the larva on the ant.  The ant, thinking that’s what it is, then carries it to its nest and deposits it with the other larvae, which the caterpillar proceeds to devour, biting through the soft skin and sucking out the body fluids. The ants might realize that there is an invader in their nest at this time, but the caterpillar produces a pheromone that is identical to the ants and so remains undetected until its skin becomes too tough to be attacked.  The caterpillar feasts in the nest for a year, by which time it has grown to 100 times its original size.  The workers treat it as a queen, fussing over it and licking its skin.  It even emits noises like those of a queen ant.  It then turns into a chrysalis and late the following spring, amid a flurry of queen like noises and frienzied activity from the attendant ants, it emerges as a butterfly and escapes the nest, where it expands its wings and takes off on its mating flight. 

What an amazing life story, but it still didn’t explain why the Large Blue became extinct.  To answer this, Thomas’s painstaking research discovered two crucial facts.  There are several species of red ant on heathland, but only one of them, Mermica sabuleti,  plays host to the caterpillars of the Large Blue.  This is because the Large Blue larva produces a pheromone that mimics that emitted by sabuleti ants, but not any other species.  The second is that the sabuleti are exquisitely sensitive to temperature and humidity and can only survive when the turf has been cropped by sheep and rabbits and thereby exposed to the sun. When early conservationists tried to protect the butterflies habitat by fencing it in and preventing grazing, they inadvertently broke a crucial link in the butterfly’s life cycle.

But here’s another twist to this tale.  Enter the villain, the devilishly attractive black and scarlet ichneumon wasp.  Different species of wasp parisitise each species of blue butterfly and here’s how they do it.  They seem to know, perhaps by some chemical signal, perhaps by the behaviour of the ants, which ants’ nest contains the larva of the butterfly.  The wasp then invades that nest in search of its prize.  Of course the ants detect the invasion and come out to attack, but the wasp sprays them with a pheromone that causes them to turn on each other instead.  Picking her way through the melee, she finds the larva, injects it with her ovipositor. The caterpillar continues to feed and grow and turn into a chrysalis, but when the skin of the pupa splits, it’s not a beautiful blue butterfly that emerges, but a shiny black ichneumon wasp.

The Large Blue was reintroduced into Dartmoor in 1983 and has since spread to heathland throughout the country, but is still not common.  The question is should scientists reintroduce the wasp that parisitises it.  David Attenborough thinks we should preserve these natural systems in all of their complex diversity. I wonder if we should let well alone, but let new complex relationships develop.

My zoology teacher, Dr Ernest Neal, famous for his doctorate on delayed implantation in the Badger, Meles meles,  once wrote a slim volume on the ecology of the Somerset woods.  It was he who introduced me to notions of biodiversity and the specificity of ecological niches.  The long suffering Ernest tramped his troop of recalcitrant youths through damp coppices, along scratchy hedgerows, across sodden and once, in a fit of pique, marooned us on the summit of Steep Holm in the middle of the Bristol Channel dive-bombed by angry Black Headed Gulls. 

But ecology has developed since Ernest.  Species are no longer regarded as simple organisms, but more as a system of interactions between many organisms dominated by a single species.  Think of the complex ecosystem of an ancient oak tree: the warblers and tits that feed in the canopy, the owls that nest in the cavities left by fallen branches, the woodpeckers that peck away the softer bark to get at the burrowing beetles underneath, the complex fungal mycelia that provide the fungi that supply the root hairs, the ants that crawl up and down the trunk, the ivy, the moss and lichen on the trunk.  And think of the complex relationships between us and the animals we keep for food or for pets, the plants we eat or provide shelter, the insects that live on our bodies or in our houses, the complex ecosystem of bacteria that live in our colons that salvage much of the plant food that we eat.  Symbiosis determines the lives of every species on the planet and drives evolution.  We are not alone.   

I listened to the Large Blue tale as a podcast from David Attenborough’s recent series of Life Stories on Radio 4.  Attenborough is such a wonderful story teller. I hope we are able to preserve the enthusiasm for nature that he so uniquely embodies.