Large blue butterfly (Maculinea arion)

SizeWingspan: 16 - 20 mm

The large blue butterfly is classified as Near Threatened (NT) on the IUCN Red List (1). Classified as Endangered in the UK (previously Extinct in the UK, re-introduced 1983). Fully protected under Schedule 5 of the Wildlife and Countryside Act, 1981. Listed under Annex IV of the EC Habitats Directive and Appendix II of the Bern Convention.

The large blue butterfly (Maculinea arion) is blue with a black outer margin and black flashes on the upper wing. It is greyish on the underwing with a blue suffusion at the base. The spots vary in size but are generally larger on the female than the male. The caterpillar is pinkish.

Widely distributed throughout Europe except for the extreme north and southern parts of Spain. The large blue butterfly became extinct in the UK in 1979, but has since been re-introduced.

The large ble butterfly favours south-facing hillsides with close-cropped, unfertilised pasture. It requires the caterpillar's food plant, wild thyme, in the presence of a species of ant, Myrmirca sabuleti.

The large blue butterfly emerges in late June or early July and is on the wing for three to four weeks. The females lay their eggs on wild thyme (Thymus serpyllum) and the caterpillars feed on the soft tissue of the flowers. In common with many other blue butterflies the caterpillar then enters a fascinating and, until fairly recently, little understood phase of its life-cycle. After moulting, the caterpillar drops to the ground and waits for a particular species of red ant to find it. This ant, (Myrmica sabuleti), is attracted by a gland on the caterpillar that secretes a sweet liquid. After the ant has fed on this liquid for some four hours the caterpillar inflates the skin behind its head, mimicking the behaviour of an ant grub. The ant, encouraged by the caterpillar's mimicry, takes it underground to its nest and places it amongst the ant colony's own brood. Here, the caterpillar eats the ant grubs. The blue butterfly caterpillar hibernates in the ant's nest. It then pupates just beneath the surface and it emerges as an adult butterfly in June. Whilst in the ant's nest the caterpillar may eat as many as 500 ant grubs.

This chief reason for the decline of this species is loss of its habitat and scrub encroachment through lack of grazing. The life-cycle of the butterfly is complex and was not fully understood until after it had been lost from Britain. The red ant that supports the large blue butterfly requires a habitat which has few tall plants, is well grazed and where the surface temperature of the ground is warm. The loss of this habitat in many areas led to a drastic reduction in ant numbers and this, coupled with a major reduction in the populations of wild thyme, led to the extinction of the butterfly.

A co-ordinated approach toward conserving the large blue butterfly began in 1962 when a joint committee was formed. The committee was successful in finding colonies of butterflies but failed to discover any information concerning the relationship with the ant or the decline in ant numbers. The large blue butterfly was declared extinct in Britain in 1979. A decision was made to attempt re-introduction of the species and the butterfly was given special protection in 1975. In 1983 a re-introduction programme began with the importing of wild stock from Sweden. Further numbers were brought over in 1986 and in 1991 a five year recovery project was launched as a partnership under English Nature's Species Recovery Programme, together with the Institute of Terrestrial Ecology and the British Butterfly Conservation Society. So far suitable habitats have been re-created on all the earmarked sites and the large blue butterfly has been successfully re-introduced to several of them. As a bonus a significant increase in the numbers of another species of butterfly, the small pearl-bordered fritillary, have been recorded.

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  1. IUCN Red List (April, 2011)