Nature · Insects & other invertebrates

Large blue

One brood, mid-June to mid-July · wingspan 38–44 mm

Phengaris arion — Lycaenidae (blues, coppers and hairstreaks)

The largest and rarest of Britain's blue butterflies, and the most extraordinary in its life cycle: after three weeks on wild thyme its caterpillar drops to the ground, tricks a red ant into carrying it underground, and spends the next ten months eating ant grubs. It went extinct in Britain in 1979. Research by Jeremy Thomas showed that the whole system depended on one ant, Myrmica sabuleti, which needs turf grazed short enough to stay warm — and once that was understood the butterfly was successfully reintroduced from Swedish stock from 1983.

How to spot them

▸The only British blue with a bold row of oblong black spots across the upper forewing
▸Deeper, more slate-blue than a common blue, and noticeably larger; underside pale grey-brown with black spots and no orange
▸One short flight period in high summer, mid-June to mid-July
▸Found only on warm, short-turf unimproved grassland with wild thyme and Myrmica sabuleti
▸Restricted to a handful of reintroduction sites in south-west England

How to know it

Its caterpillar eats thyme flowers for three weeks, then drops to the ground and gets itself carried into a red ant's nest, where it spends ten months eating ant grubs

Large blue — a social parasite of Myrmica ants; survival is far higher in nests of Myrmica sabuleti.

Declared extinct in Britain in 1979 and brought back from Swedish stock from 1983

Large blue — the reintroduction followed Jeremy Thomas's discovery of its ant dependence.

The only British blue with a row of oblong black spots across the upper forewing

Large blue — the single most reliable field mark.

Conserving it turned out to be a matter of grazing turf short enough to keep the soil warm for one species of ant

Large blue: Myrmica sabuleti needs short, sun-warmed turf; when grazing lapsed the ant went and the butterfly followed.

Its caterpillar secretes ant-mimicking chemicals and makes sounds that imitate a queen, so workers treat it as one of their own

Large blue — chemical and acoustic mimicry inside the nest.

It was moved from the genus Maculinea to a genus whose name comes from the Greek for 'shining'

Large blue — now Phengaris arion, formerly Maculinea arion.

Quick-fire facts

•Phengaris arion, formerly Maculinea arion; earlier still placed in Lycaena
•Butterfly Conservation: it became extinct in Britain in 1979, the last colony being in the West Country
•Larvae feed at first on the flowerheads of wild thyme (Thymus polytrichus); eggs are sometimes laid on wild marjoram, which usually flowers too late on British sites
•From the fourth instar the larva feeds on ant grubs inside a Myrmica nest; survival is highest in Myrmica sabuleti and much poorer in M. scabrinodis
•Jeremy Thomas's work for the Institute of Terrestrial Ecology identified the ant dependence and the need for short, warm turf — too late to save the native population
•Reintroduction used stock from the Swedish island of Öland, beginning in 1983; Butterfly Conservation gives an abundance trend of +1883% for 1983–2019
•Current status: Near Threatened on the GB Red List (2022), Endangered in Europe, and fully protected in Great Britain; a Section 41 species of principal importance in England
•The National Trust's Collard Hill in Somerset is managed as a site where the public may watch it; further reintroductions include Rodborough Common in Gloucestershire in 2019
•A ladybird is a beetle and a large blue is reared by ants: both are staple quiz traps

Drill it — 8 cards

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Sources

Butterfly Conservation — Large Blue
Paul D. Brock — Britain's Insects: A Field Guide to the Insects of Great Britain and Ireland