The large blue butterfly (Phengaris arion, formerly Maculinea arion) has one of the most extraordinary life cycles of any British insect — and became extinct in the UK in 1979 before a remarkable reintroduction programme achieved the most successful butterfly reintroduction in European history. Its story illustrates the complexity and reward of evidence-based conservation.
The Extraordinary Life Cycle
The large blue has a parasitic life cycle dependent on both wild thyme (Thymus polytrichus) and specific Myrmica red ants. Adults lay eggs on thyme flower buds; early instar caterpillars feed on thyme flowers before dropping to the ground. The caterpillar mimics the pheromones and surface chemistry of Myrmica ant larvae, causing worker ants to carry it into the nest. Inside the nest, the caterpillar feeds on ant larvae for 10 months before pupating and emerging as an adult the following June.
The Critical Role of Myrmica Ants
Not all Myrmica species are equally suitable. In Britain, M. sabuleti is the critical host; M. scabrinodis is a poor host. M. sabuleti requires shorter, warmer turf — maintained by grazing to 2–4cm sward height. Longer, cooler swards favour M. scabrinodis. The UK extinction was caused not by habitat loss per se, but by cessation of grazing allowing sward height to increase beyond the requirements of M. sabuleti — demonstrating the extraordinary ecological precision required for this species.
The Reintroduction Programme
Following the research of Professor Jeremy Thomas, large blue caterpillars from Swedish populations were reintroduced to restored sites in Somerset, Gloucestershire, and elsewhere from 1983. Precise habitat management — grazing to maintain 2–4cm sward — was implemented. Populations have grown and spread, with an estimated 10,000+ adults in the UK by the 2020s. New populations have been established at multiple sites through ongoing reintroductions.
Welfare Considerations
The welfare significance of this recovery lies in the establishment of flourishing butterfly and ant populations over decades. Conservation management that precisely maintains habitat requirements prevents the suffering of population collapse. The programme's success demonstrates that evidence-based, precisely targeted habitat management can reverse even highly specific conservation challenges.