Adonis Blue (Polyommatus bellargus)

In this guide

1. Description and identification
2. Life cycle: egg, caterpillar, chrysalis and adult
3. Where the Adonis blue lives
4. Host plants and diet
5. Behavior: flight, courtship and daily routine
6. Similar butterflies and how to tell them apart
7. Quick facts
8. Did you know?
9. Pictures
10. Frequently asked questions

Description and identification

The Adonis blue is a small member of the gossamer-winged butterfly family, the group that contains the blues, coppers and hairstreaks. It is one of several European blues in the genus Polyommatus — some authors place it in Lysandra — and it is the most intensely coloured of them all. The male's upperside is a clear, luminous sky blue that no other British or western European butterfly matches; the female is brown and easily mistaken for half a dozen relatives.

Adult butterfly

• Wingspan: 1.2–1.6 in (30–40 mm). Second-brood adults are usually a little larger and brighter than the spring generation.
• Male upperside: brilliant sky blue with a very narrow black line at the wing edge and a white fringe beyond it.
• Chequered fringes: the single best field mark. The black veins run out through the white fringe of both wings, breaking it into black and white segments. The common blue has a plain, unbroken white fringe.
• Female upperside: deep chocolate brown, usually with a scattering of blue scales at the wing bases, a row of orange crescents inside the margins of both wings and the same chequered fringes as the male.
• Underside: greyish-brown in the male and warmer, darker brown in the female, patterned with black spots ringed in white and a band of orange crescents near the margin, each crescent topped by a small black-and-blue dot.
• Body: small, furry and blue-grey above in males; the antennae are finely ringed black and white.

How to identify an Adonis blue in the field

Habitat does most of the work. This is a butterfly of warm, short, closely grazed turf on chalk and limestone, flying fast and low, rarely more than a foot or two above the ground. A male is unmistakable once the fringes are checked with binoculars: brilliant blue wings with a fringe striped like a tiny zebra crossing. Females are harder, and are best separated from the brown females of the common blue by the chequered fringe and the company they keep. On the same slopes you may also meet the smaller, sooty small blue, the violet-tinged mazarine blue and the tree-loving holly blue, none of which have the Adonis blue's combination of electric colour and striped fringe.

Life cycle: egg, caterpillar, chrysalis and adult

The Adonis blue has two generations a year. Adults fly in May and June, and again, usually in greater numbers, from early August into September. The species spends the winter as a small caterpillar at the base of its food plant.

Eggs

The female lays her eggs singly, almost always on the underside of a young horseshoe vetch leaf growing in very short turf where the ground is warm. Each egg is a pale greenish-white disc, about 0.5 mm across, flattened on top with a small central depression and a finely pitted surface that gives it the texture of a golf ball under a lens. She tests the surrounding turf carefully before laying: vetch growing in tall grass is ignored, because the microclimate there is too cool for the larva. Eggs hatch in one to three weeks in summer.

Caterpillar

The caterpillar is shaped like a woodlouse rather than a worm — short, flattened and slug-like — and is dark green with broken yellow stripes along the back and sides and a covering of fine dark hairs. It reaches about 0.6 in (16 mm). It feeds only by day in warm weather, resting at the base of the plant at other times, and the larvae of the second brood feed briefly in autumn and then hibernate while still small, resuming in spring.

The caterpillar is rarely alone. It carries a honey gland on its back that produces a sugary secretion, and ants — commonly Lasius alienus and Myrmica sabuleti — attend it constantly, drinking the secretion and driving off parasitic wasps and flies in return. Ants often build a loose earth chamber around the larva. The large blue takes this relationship much further, tricking ants into carrying its caterpillar into the nest to eat the brood, but the Adonis blue's version is an honest trade.

Chrysalis

Pupation happens on the ground, in a hollow or a shallow earth chamber at the base of the food plant, and ants frequently bury the chrysalis or attend it where it lies. The pupa is small, rounded and pale, olive-green at first and ageing to light brown or beige. It lasts about three weeks before the adult emerges.

Adult butterfly

Adults live roughly one to two weeks. They emerge in tight, well-synchronised waves, so a colony can go from almost no butterflies to dozens on the wing within a few days, and males typically appear a little before females in each brood.

Where the Adonis blue lives

The species is found across central, southern and western Europe, from Spain and Portugal through France, Italy and the Balkans, and eastward through southern Russia, the Caucasus and Transcaucasia to Turkey, Iran and Iraq. Its northern limit runs through southern England, and in the United Kingdom it is confined to chalk and limestone downland in the southern counties, from Dorset and Wiltshire east to Sussex and Kent and along the Isle of Wight and the Cotswolds. It does not occur in the Americas.

Everywhere it needs the same thing: a warm, sheltered, usually south- or west-facing slope of unimproved chalk or limestone grassland, grazed short by sheep or rabbits, with horseshoe vetch growing in turf only an inch or two high. In that short sward the ground temperature on a sunny day can be several degrees higher than in the grass alongside, and the caterpillar depends on that extra warmth. Colonies are small, compact and highly sedentary — most adults spend their whole lives within a few hundred yards of where they emerged — which is why the butterfly recolonises new ground very slowly.

Host plants and diet

Caterpillar host plant. The Adonis blue lays on one plant only: horseshoe vetch, Hippocrepis comosa, a low, yellow-flowered member of the pea family that grows on thin, lime-rich soils. Not all horseshoe vetch will do. Females select plants in very short, warm turf, and a slope covered in vetch that has grown rank is worthless to them. This double requirement — one food plant, and that food plant in one particular condition — is what makes the species so vulnerable, and our guide to butterfly host plants explains why such narrow specialists are usually the first to disappear when land use changes.

Adult diet. Adults drink nectar from the low flowers of the same grassland: horseshoe vetch itself, bird's-foot trefoil, wild marjoram, thyme, wild basil, scabious, knapweed and hawkbits. Males spend a great deal of time on bare ground and damp patches, taking up salts, and they will settle on animal dung and on sweat-damp clothing for the same reason.

Conservation. The Adonis blue declined severely across northern Europe during the twentieth century. Downland was ploughed or improved for agriculture, sheep grazing was withdrawn from the steep slopes that remained, and when myxomatosis destroyed rabbit populations in the 1950s the last close-cropped turf grew rank within a few seasons; many colonies died out. Since the 1980s conservation grazing, scrub clearance and a run of warmer summers have allowed the butterfly to recover on many reserves and to spread back along the chalk, and it is now a flagship species for chalk grassland management in Britain.

Behavior: flight, courtship and daily routine

Flight. The flight is fast, low and restless, a few inches to a couple of feet above the turf, with frequent stops to feed or bask. Males cover the ground tirelessly on sunny mornings; in dull weather or late in the day both sexes drop into the grass and become almost impossible to find.

Courtship. Males patrol constantly over the breeding turf, investigating anything blue or brown that moves, and a newly emerged female is usually found within minutes of drying her wings. Courtship is brief — a short hovering flight and a flutter of wings — and mating pairs sit low in the sward, often on a grass stem, where they can remain joined for an hour or more. Our guide to how butterflies mate and reproduce sets out the general sequence.

Roosting. In the evening adults climb a grass stem and roost head-down, wings closed, where the brown underside makes them almost invisible among dead flower heads. Several butterflies often roost within a few feet of each other on the same patch of slope.

Life with ants. The partnership with ants runs through the whole immature life of the butterfly. Ants find the caterpillar within hours of hatching, follow it as it feeds, escort it back to the base of the plant, and stay with the pupa in its earth chamber. In return they harvest a sugary secretion rich in amino acids. Caterpillars that are attended survive far better than those that are not, because the ants intercept the parasitic wasps and flies that would otherwise lay their eggs in them.

Colony structure. Because adults rarely travel far, an Adonis blue population is a set of separate colonies rather than one continuous spread. Strong colonies occasionally send out individuals that found new ones on neighbouring slopes, but only if suitable short, vetch-rich turf lies within a short distance — the reason conservation work concentrates on linking downland sites rather than managing them in isolation.

Similar butterflies and how to tell them apart

Quick facts

Did you know?

• The butterfly is named after Adonis, the beautiful youth of Greek myth. Its older English name, Clifden blue, comes from Cliveden in Buckinghamshire, where eighteenth-century collectors found it.
• The whole colony can be destroyed by kindness. Fence off a downland slope to protect it and the turf grows tall, the ground cools, and the Adonis blue disappears within a few years — the grazing animals are part of the habitat.
• Ants are so attentive that the caterpillar is rarely seen without an escort. The relationship is voluntary on both sides: the larva can survive without ants, but far fewer reach the chrysalis when it does.
• Males vastly outnumber females in most sightings. They spend the day flying and searching, while females sit low in the turf between bouts of egg-laying and are simply harder to see.
• The second brood is the better one to look for. August and September adults are generally more numerous and more brightly coloured than the May emergence.
• Colonies are so sedentary that neighbouring slopes can hold genetically distinct populations; a fortnight on the wing leaves an adult very little time to disperse.

In summary

The Adonis blue, Polyommatus bellargus, is a small European and western Asian butterfly of warm chalk and limestone grassland, in which the male is brilliant sky blue and the female chocolate brown with orange crescents, and both have black-and-white chequered fringes that separate them from the common blue. It lays flattened greenish-white eggs on horseshoe vetch in very short turf, its slug-shaped green-and-yellow caterpillar is tended by ants and overwinters half-grown, and it pupates in an earth chamber at ground level. Two broods fly in May–June and August–September, colonies are tiny and sedentary, and the species survives only where grazing keeps the downland turf short.

Adonis Blue butterfly pictures

Frequently asked questions about the adonis blue butterfly

How do male and female Adonis blue butterflies differ?

Males are brilliant sky blue above with a very narrow black edge and a white fringe; females are chocolate brown with a row of orange crescents inside the margins and usually some blue dusting at the wing bases. Both sexes share the chequered fringes and the pale brown underside with black spots and orange crescents.

What does an Adonis blue caterpillar look like?

It is short, flattened and slug-shaped rather than worm-like, dark green with broken yellow stripes along the back and sides and a coat of fine dark hairs. It grows to about 0.6 inches (16 mm). Ants are almost always in attendance, feeding on the sugary secretion produced by a gland on the caterpillar's back.

What is the wingspan of an Adonis blue butterfly?

The wingspan is about 1.2 to 1.6 inches (30 to 40 mm), making it a small butterfly even by the standards of the blues. Adults of the second brood, which flies in late summer, average slightly larger and brighter than those of the spring generation, but the sexes are much the same size.

When are Adonis blue butterflies on the wing?

There are two broods a year. The first flies from about mid-May into June and the second, usually the larger, from early August into September. Adults live only one to two weeks, so a colony's numbers rise and fall sharply, and in dull or cool weather the butterflies stay hidden low in the grass.

What do Adonis blue caterpillars eat?

Horseshoe vetch, Hippocrepis comosa, and nothing else. The female lays only on plants growing in very short, warm turf on chalk or limestone, so a slope can be full of horseshoe vetch and still be useless to the butterfly if the grass around it has grown tall. Adults drink nectar from marjoram, thyme, trefoils and scabious.

Why do ants look after Adonis blue caterpillars?

The caterpillar has a honey gland on its back that produces a sugary, amino-acid-rich secretion, and ants such as Lasius alienus and Myrmica sabuleti drink it. In exchange the ants drive off parasitic wasps and flies and often build an earth chamber around the larva and its chrysalis, so attended caterpillars survive far better than unattended ones.

How do you tell an Adonis blue from a common blue?

Check the wing fringe with a close look or binoculars. On the Adonis blue the black wing veins run out through the white fringe and break it into black and white blocks; on the common blue the fringe is plain white. The Adonis male's blue is also more luminous, and it is tied to chalk and limestone grassland.

Where is the Adonis blue butterfly found?

It occurs across central, southern and western Europe, east through southern Russia and the Caucasus to Turkey, Iran and Iraq. Its northern edge is southern England, where it is restricted to chalk and limestone downland from Dorset and Wiltshire to Sussex and Kent. It does not occur in North America.

What colour is the chrysalis of an Adonis blue?

The chrysalis is small and rounded, olive-green when it is newly formed and ageing to light brown or beige. It is not hung from a stem like a swallowtail's: it lies in a hollow or an earth chamber at the base of the food plant, where ants often bury or guard it for the three weeks or so before the adult emerges.

Why is the Adonis blue a conservation priority?

Because it needs one food plant growing in one very specific condition — horseshoe vetch in short, warm, grazed turf — its colonies vanish when downland is ploughed or when grazing stops and the grass grows rank. Numbers fell sharply in the twentieth century and have recovered since the 1980s where conservation grazing has been restored.