The Reintroduction of the Large Blue – to truly understand the complexity of ecology
The Large Blue Butterfly was extinct in Britain for 30 years until its reintroduced in 2000 from Sweden.
The large blue caterpillar drops to the ground after hatching and tricks ants into taking it into their nest by secreting a seductive fluid and even "singing" to the ants so they believe it is a queen ant grub. In the comfort of the nest, the parasitic caterpillar devours ant grubs all winter, pupating and emerging from the ground as a butterfly in June. The butterfly is only able to trick one species of ant, Myrmica sabuleti. If it entered the nests of other ant species, the caterpillar was likely to be attacked and killed.
The Myrmica sabuleti ant requires warm ground, only created by short, well-grazed grass, to survive. A near-imperceptible difference of just a centimetre in grass length could change the soil temperature by 2C-3C, causing the ant — and the large blue with it — to perish. The widespread decline in grazing sheep and cattle and myxomatosis, which decimated wild rabbits after the 1950s, caused traditional grassland to grow too tall and cool for ant and butterfly.
Now reintroduced to six other sites, the large blue has spread to a total of 28 colonies including the Polden Hills, Somerset; it can even be spotted from the train: some of its largest populations fly on railway embankments close to Castle Cary, Somerset, which are managed by Network Rail to encourage the butterfly.