Silvery Blue (Glaucopsyche lygdamus)

In this guide

1. Description and identification
2. Life cycle: egg, caterpillar, chrysalis and adult
3. Where the silvery blue lives
4. Host plants and diet
5. Behavior: patrolling, puddling and ant guards
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 silvery blue is a small spring butterfly of the gossamer-winged family (Lycaenidae), in the subfamily Polyommatinae that holds the true blues. It is one of the most widespread blues in North America and one of the easiest to name once it lands, because the underside pattern is stripped down to a single row of spots where most of its relatives carry two or three.

Adult butterfly

• Wingspan: 0.75–1.1 in (19–28 mm). Mountain and northern populations run small, lowland western ones larger.
• Male upperside: bright, slightly metallic silver-blue over the whole wing, bordered by a very narrow black line and a clean white fringe. In direct sun the blue looks almost polished.
• Female upperside: a duller, greyer blue with a broad dusky border that can spread over most of the forewing, so that some females look brown with a blue flush at the wing base.
• Underside: pale grey to grey-brown in both sexes, crossed on each wing by one curved row of round black spots, every spot ringed with white. There are no orange marks, no second row of spots near the margin, and no tails.
• Body: the thorax is covered in long blue-grey hairs, the eyes are dark and hairy, and the antennae are ringed black and white with a black club.

Across the species' huge range the blue varies in tone and the spots vary in size, and around twenty subspecies have been described. The Palos Verdes blue of coastal southern California is one of them, and it is among the rarest butterflies in the United States.

How to identify a silvery blue in the field

Count the rows of spots. A small blue butterfly with a grey underside and exactly one arc of white-ringed black dots on each wing, with nothing between that arc and the wing edge, is a silvery blue. The eastern tailed-blue adds a tail and orange crescents, the azures have faint grey smudges instead of bold ringed spots, and the western blues of the genus Icaricia and Plebejus add a second, submarginal row and often a band of orange. Season helps too: the silvery blue is out early, often with the first spring flowers, and is gone before midsummer.

Life cycle: egg, caterpillar, chrysalis and adult

The silvery blue has a single generation a year everywhere in its range and spends about ten months of that year as a pupa. The four stages follow the pattern set out in our guide to the life cycle of a butterfly.

Eggs

Females lay singly on the flower buds, unopened flower spikes and young leaves of legumes, pushing each egg down between the buds where it is hidden. The egg is a small flattened disc, pale greenish white, sculptured with a fine network of ridges, and it turns duller as the larva develops. A female works over a plant methodically and leaves only one or two eggs on a spike, spreading her output across many plants. Eggs hatch in four to eight days.

Caterpillar

The caterpillar is short, flattened and woodlouse-shaped, with the head withdrawn beneath the first segment. Colour varies with the foodplant and the individual, from bright green to pink, cream or brown, and there is usually a darker line down the middle of the back — often maroon or reddish — with pale oblique dashes along each side. It grows to roughly 0.6 in (15 mm) through four moults.

Larvae eat flower buds, flowers and developing seed pods, which is why the species is tied to the short flowering season of its legumes. Almost every caterpillar carries a retinue of ants. A gland on the seventh abdominal segment produces a sugary secretion, and ants of the genera Formica, Camponotus and Tapinoma take it while driving off the parasitic wasps and flies that attack lycaenid larvae. Experimental work on this butterfly was among the first to show that ant-tended larvae survive significantly better than larvae left unattended, and it is one of the clearest demonstrations of the partnership anywhere in the family.

Chrysalis

When feeding finishes in early summer, the larva drops to the ground and pupates in leaf litter, under loose soil or among moss at the base of the plant, sometimes inside an ant nest. The chrysalis is a plain light brown capsule streaked and mottled with darker brown, about 0.4 in long and attached to nothing. It lies there for the rest of the summer, through autumn and all winter, and the adult emerges with the following spring — a pupal rest of eight to ten months.

Adult butterfly

Emergence tracks the season: February and March in coastal California and the desert Southwest, April and May across the central states and the Midwest, and June or even early July in Alaska, northern Canada and the high mountains. An individual lives about two to three weeks, at the long end of the range described in our guide to how long butterflies live for a small butterfly, and a colony's whole flight period lasts four to six weeks.

Where the silvery blue lives

Few North American butterflies cover as much ground. The silvery blue ranges from Alaska and the Yukon east across Canada to Quebec, Nova Scotia and the Gaspé, and south through the whole of the West to Baja California, Arizona and New Mexico. In the east it runs down through New England and the Appalachians to Georgia, though it thins out badly in the southeastern lowlands and is largely absent from the central prairies and the Gulf coastal plain.

Habitat is anywhere its legumes grow in the open: mountain meadows and forest clearings, sagebrush and chaparral, coastal dunes and bluffs, prairie remnants, river and stream banks, brushy fields, road verges and railway embankments. It occurs from sea level to above 10,000 ft (3,000 m) in the Rockies and the Sierra Nevada. Because road cuttings and disturbed ground suit vetches and sweet clover, the butterfly often follows human disturbance into places it did not previously occupy.

Colonies are usually compact, centred on a stand of host plants, and adults stay close to them. The species does not migrate, and local populations disappear quickly when a meadow is ploughed, developed or overgrown — the reason the Palos Verdes blue subspecies fell to a handful of individuals on a single California hillside.

Host plants and diet

Caterpillar host plants. The silvery blue is a legume specialist, and its host plants all belong to the pea family: lupines (Lupinus), vetches (Vicia), milkvetches and locoweeds (Astragalus, Oxytropis), sweet clover (Melilotus), peavines (Lathyrus) and deervetch (Lotus). Which plant a colony uses is a local matter — lupine in the mountains, vetch and sweet clover along eastern roadsides, deervetch and locoweed in the dry West — and the switch to introduced sweet clover has let the butterfly spread along verges across much of the continent.

Adult diet. Adults are low-flying flower visitors that take nectar from whatever is blooming early: fiddleneck, wild mustards, dandelion, forget-me-not, wild strawberry, blue-eyed grass, the host legumes themselves and, later in the flight, clover and daisies. Males also crowd onto damp sand, mud and gravel to drink dissolved minerals. Sodium taken up this way is transferred to the female during mating and ends up invested in her eggs.

Behavior: patrolling, puddling and ant guards

Flight. The flight is fast but erratic and rarely more than knee-high, a low bouncing weave among the flower heads with frequent stops. Males patrol continuously over stands of host plants through the warm middle of the day, investigating anything blue that moves, while females fly less and spend their time walking over flower spikes with the abdomen curled to lay.

Puddling. Groups of males gather at wet gravel, seepages, the edges of drying puddles and damp animal droppings, sometimes a dozen or more together with their wings closed. These clusters are among the easiest ways to find the species, and they are almost entirely male.

Approachability. Silvery blues are famously indifferent to people. A feeding or puddling adult will often let an observer close in to a foot or two, and mating pairs, joined tail to tail and sitting still for an hour or more, hardly move at all. This is why the species appears so often in close-up photographs.

Ant guards. The caterpillar's relationship with ants is the defining behaviour of the species. Ants patrol the flower spikes, drink from the larval gland and attack the small wasps and flies that would parasitise the larva; some colonies of ants carry larvae between plants, and pupae are occasionally found inside ant nests. The same trick is run, in more extreme forms, by other members of the family such as the endangered Karner blue.

Overwintering. The butterfly's answer to winter is to sit it out underground as a pupa, a very different strategy from the summer azure's stack of overlapping broods, and it commits the whole population to a single narrow window of spring weather each year.

Similar butterflies and how to tell them apart

Quick facts

Did you know?

• A silvery blue population now stands in for an extinct butterfly. The Xerces blue, a close relative from the San Francisco dunes, vanished in the 1940s and was the first American butterfly known to be driven extinct by urban development. Silvery blues from a nearby coastal population have since been moved into the restored dunes of the Presidio to fill the ecological gap it left.
• The Palos Verdes blue, a silvery blue subspecies, was declared extinct in the 1980s and then rediscovered in 1994 on a single fuel depot in Los Angeles County. It survives today through captive breeding and habitat restoration.
• Ants do the babysitting. Work on this species showed that caterpillars attended by ants survive markedly better than unattended ones, making it a textbook case of the sugar-for-protection deal that runs through the whole family.
• Puddling parties are stag parties. The clusters of blues on wet gravel are almost all male, collecting the sodium they will hand to a female as a nuptial gift.
• Introduced weeds helped it spread. Sweet clover, brought to North America as a forage crop, is now a major host along road verges and has carried the butterfly into farmland it could not previously use.
• It is a one-shot species. With a single brood a year and a pupa that sits underground for eight to ten months, a colony gets one chance a year, so a badly timed drought, mowing or fire can wipe it out.

In summary

The silvery blue is a small, early-flying blue found from Alaska to Baja California and east to the Atlantic, with metallic silver-blue males, duskier-bordered females, and a grey underside carrying one clean arc of white-ringed black spots — the mark that settles the identification. It breeds only on legumes, its slug-shaped caterpillars eat flower buds while ants stand guard, and its chrysalis waits out most of the year in the litter below. Look for it in spring over lupine and vetch, or in a puddling cluster of males on damp gravel.

Silvery Blue butterfly pictures

Frequently asked questions about the silvery blue butterfly

What does a silvery blue caterpillar look like?

It is a short, flattened, woodlouse-shaped larva up to about six tenths of an inch long, with the head hidden under the front segment. Colour ranges from green to pink, cream or brown depending on the individual and the foodplant, and most have a darker maroon or reddish line down the back with pale oblique dashes along the sides.

How can you tell male and female silvery blues apart?

By the upperside. Males are bright, almost metallic silver-blue across the whole wing with only a hairline black border and white fringes. Females are a duller greyish blue with a broad dusky border that can cover most of the forewing, so some females look brown with just a blue flush near the wing base.

What does the silvery blue chrysalis look like?

It is a plain light brown capsule about four tenths of an inch long, streaked and mottled with darker brown, and it is not attached to anything. The caterpillar leaves the plant and pupates loose in leaf litter, under soil or moss at the base of the host, and sometimes inside an ant nest.

Where do silvery blue butterflies live?

They range from Alaska and the Yukon across Canada to Nova Scotia, south through the western United States to Baja California and New Mexico, and down the Appalachians to Georgia. Within that range they use meadows, forest clearings, sagebrush, coastal dunes, prairies, stream banks and roadsides, from sea level to above 10,000 feet.

When do adult silvery blues emerge?

There is one brood a year, and emergence follows the spring. Adults appear in February or March in coastal California and the desert Southwest, April and May across the central and eastern states, and June or early July in Alaska, northern Canada and the high mountains. A colony's flight lasts about four to six weeks.

What is the relationship between silvery blue caterpillars and ants?

The caterpillar has a gland on its back that secretes a sugary fluid, and ants of the genera Formica, Camponotus and Tapinoma feed on it while patrolling the plant. In return they attack the parasitic wasps and flies that would kill the larva. Studies on this species showed that ant-attended caterpillars survive significantly better than unattended ones.

What is the scientific name of the silvery blue butterfly?

It is Glaucopsyche lygdamus, described by Edward Doubleday in 1841 and placed in the family Lycaenidae, subfamily Polyommatinae. The genus Glaucopsyche also contains the arrowhead blue and the extinct Xerces blue of the San Francisco dunes. Around twenty subspecies of the silvery blue have been named across North America.

What do silvery blue caterpillars eat?

Only plants in the pea family, and mostly their flowers rather than their leaves. Lupines, vetches, milkvetch and locoweed, sweet clover, peavine and deervetch are all used, with the choice depending on the region. Larvae feed on buds, open flowers and developing seed pods, so their season is tied to the plant's flowering.

How do you tell a silvery blue from other small blue butterflies?

Look at the underside and count the rows of markings. The silvery blue has one curved row of round black spots ringed in white on each wing and nothing between that row and the margin. Tails plus orange crescents mean a tailed-blue; a second submarginal row or an orange band means one of the western blues.

Is the silvery blue butterfly endangered?

The species as a whole is common and secure across a very large range. One subspecies is not: the Palos Verdes blue of coastal Los Angeles County is federally endangered, was thought extinct in the 1980s and survives today only through captive breeding and habitat restoration on a small number of protected sites.