Large blue butterfly (Maculinia arion) - management needs

The large blue (Maculinia arion) is a butterfly often found on the European continent in large numbers where it is quite tolerant of different heights in vegetation. The population in Great Britain had declined from nearly ninety sites in the 1700s to thirty in 1950s and four in the 1960s. By 1979 the final colony, on the South Devon coast had become extinct. Habitat loss from ploughing of open calcareous grassland, afforestation, building and quarrying are all implicated in its demise. However at several sites the vegetation seemed suitable, with abundant Thymus polytrichus, the main food plant of the young caterpillars, yet the species had disappeared.

Despite major efforts in the 1960s to rehabilitate their habitats, the few surviving colonies failed. One of the principal reasons appeared to be the growth of rank grassland brought about by a decrease in grazing due to the reduction in livestock grazing and the demise of the rabbit population following myxomatosis in the 1950s. At the time the relationship between the butterfly and the red ant (Myrmica sabuleti) was not understood. [The butterfly larvae (developed from eggs laid on Thymus polytrichus) are carried by the ant to underground nests. Here they feed on ant grubs.] The species of ant is critical and requires close-cropped grassland. Even a small relaxation in grazing pressure can result in its replacement with an unsuitable species Myrmica scabrinodis. Although Thymus polytrichus might be present in abundance, without the host ant, the population cannot survive (Thomas 1989).

References

Thomas, J.A. 1989. Return of the Large Blue Butterfly. British Wildlife, 1/1., 2-13.