I have always been enchanted by butterflies – and in recent years have felt depressed by the evident decline in the numbers I see fluttering around English gardens and countryside. The news from official butterfly conservation bodies has been grim too, with most British species in retreat.
So I was thrilled by the research published today in the journal Science, showing the triumphant reintroduction of the large blue into its old haunts on the chalk downs of south-west England. The last native colony of large blues died out in Devon in 1979 – just as Jeremy Thomas of the Centre for Ecology and Hydrology in Oxfordshire was discovering the extraordinary details of the butterfly’s life cycle.
Females lay their eggs on thyme flowers, which caterpillars eat for three weeks before falling to the ground. Red ants recognise the caterpillar as one of their own, because it secretes a special cocktail of odour chemicals identical to an ant grub, and take it underground into their nest. It lives there for 10 months, feeding on ant grubs before forming a pupa at the top of the nest and finally crawling to the surface as an adult butterfly.
Although naturalists had known the general principles of the parasitic relationship between butterfly and ant for many years, Thomas made the key discovery that only one species of ant, Myrmica sabuleti, would nourish the large blue; other red ants ate its grubs. Unfortunately changing agricultural practices were reducing M sabuleti to unsustainable numbers.
His findings – particularly the need for short, tightly grazed grass – came just too late to save the native large blue. The same species, Maculinea arion, was in steep decline elsewhere in Europe but after scouring the continent for a suitable population Thomas and colleagues found one in Sweden. From there they imported eggs into Somerset grassland where the habitat was specially prepared to suit the butterflies and their ant hosts.
Since the original reintroduction in 1984, large blues have colonised 33 sites in south-west England, from chalk downs to railway embankments. (A dozen institutions, from the National Trust to Network Rail, are collaborating to re-establish the butterfly.) Thomas, who remains the overall mastermind of the project, says its success has followed almost exactly his original mathematical models of the 1980s.
Meanwhile more details have emerged of the astonishing way the large blue has evolved to live with its hosts. Not only do the caterpillars smell exactly like ant grubs but they also make the same clicking sounds in the nest as queen ants.
It sounds like a precarious survival strategy – and so it is when land use changes substantially – but Thomas remarks that in a relatively stable environment an ant’s nest can make a safe predator-free home for the growing caterpillar.
Most large blue sites are still closed to the public but one, the National Trust’s Collard Hill in Somerset, welcomes visitors. If Sunday, June 21, is fine that would be a good day to go, because the Trust is holding a special large blue open day then. As large blue numbers increase, the butterfly could become a vehicle for encouraging tourism in the south-west.