Saving the Large Blue Butterfly

A striking cobalt-blue butterfly from the United Kingdom is making headlines as a conservation success story. The large blue butterfly (Maculinea arion) disappeared from Great Britain in 1979 and remained locally extinct until 1983 when scientists imported large blue butterflies from Sweden in an effort to revive the former British population.

The team of scientists that reintroduced the large blue butterfly to Britain has now published a paper detailing 40-years of meticulous observations that they complied on the species. The long-term study, led by Jeremy Thomas of Oxford University, represents a landmark in conservation science—little is known of how to successfully protect rare invertebrates and the findings for the large blue butterfly provide a superb example of invertebrate conservation.

The large blue butterfly is a globally threatened lepidopteran species. Its current range extends across northern Spain, Italy, Greece, and southern Scandinavia. In the UK, the species was estimated to consist of 91 colonies from the late 1790s to the 1840s. But in the 1950s, only 25 popluations remained and in 1972 only two colonies remained. Those two colonies became extinct in 1979.

Maculinea arion has a complex life cycle and requires a very specific type of habitat. The species prefers areas with grasses and wildflowers, especially those that include thyme (Thymus) species. Adult female large blue butterflies lay their eggs on thyme flowers in the summer (June–July). When the eggs hatch, the young caterpillars feed on the flower heads of the thyme plant for about three weeks before dropping to the ground. That's when the Maculinea life cycle takes an odd turn.

The caterpillar secretes chemicals that mimic those of ant larvae, so when a worker ant (either Myrmica sabuleti or Myrmica scabrinodis) comes across the caterpillar, it takes it back to its underground nest where the ant colony cares for the caterpillar as one of its own. Thus the large blue butterfly caterpillar acts as a parasite to the unsuspecting ant host species for 10 months, after which time the large blue butterfly caterpillar enters the pupal stage. Two to three weeks later, the large blue emerges as an adult butterfly.

The key to conserving the large blue butterfly in the UK was understanding the primary cause of its decline. But for a long time, no one really knew why the large blue was disappearing. What Thomas and his colleagues discovered was that seemingly small changes in its habitat were having disastrous consequences for the success of large blue butterfly populations. They noted that when caterpillars were adopted by their primary ant host species, Myrmica sabuleti, they survived far better than when adopted by their secondary host ant host species, Myrmica scabrinodis. Yet when the grass in their habitat was long (greater than 1.4 cm tall) the secondary host ant host species, Myrmica scabrinodis took over most of the habitat while the primary ant host species, Myrmica sabuleti became rare.

This meant that large blue butterfly caterpillars were far less successful and as a result the population of the butterflies declined consistently until the entire UK population went extinct. In 1983, Jeremy Thomas and his colleague put their knowledge of caterpillar success, ant species, and grass height to work. They took what they had learned of the large blue butterfly, its habitat, and its host species and created the ideal habitat for the large blue butterfly. Then they set off to Sweden to captured some large blue butterflies–individuals that would serve as founders of a new colony— and transported them to the UK where they set them free into a number of restored habitat sites. By 2008, the butterfly population had blossomed to include 30 percent more colonies than were present in the 1950s.

Refs: Thomas, J., Simcox, D., & Clarke, R. (2009). Successful Conservation of a Threatened Maculinea Butterfly Science DOI: 10.1126/science.1175726

Photos © David Simcox / Centre for Ecology and Hydrology, UK.