The Common Blue Butterfly

Getting to know the Common Blue butterfly, the UK's most populous blue butterfly

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In the afternoon sunshine of a warm August day, butterflies flit along the wild flower strips edging a wheat field. Among the Gatekeepers and Meadow Browns, the Small Tortoiseshells and the Marbled Whites, a flicker of blue catches the eye. This is a male Common Blue butterfly, feeding hungrily on the nectar of an ox-eye daisy. Once he has sated his appetite, energised by the sugar, he will fly along the hedgerow in search of a female to mate with.

The Common Blue, Polyommatus icarus, is, as its name suggests, the most evident and by far the most widespread of Britain’s seven species of Blue butterflies. It can be seen through virtually the whole of Britain, apart from on the high tops of the Welsh mountains and the Scottish Highlands.

It is the only blue found in Scotland and can even be seen on the Hebrides, Orkney and Shetland. Elsewhere, the species is found throughout Europe and in parts of North Africa, the Middle East and Asia, and has recently been introduced to Canada.

Other blue species are very specialised in their habitat, requiring short grass on chalk and limestone soils. Although usually found on grassland, the Common Blue is able to live on a wide range of habitats. These include field margins, hill slopes, wild flower meadows, roadside verges, woodland edges, brownfield sites and rural gardens.

However, the species is most common on chalk and limestone downlands, especially in southern Britain. Large populations thrive on the North and South Downs, and the chalk ridges of Wiltshire, Dorset and Hampshire. Common Blues are also found on or near the coast, on grassy clifftops and dunes.

 At rest, the line of orange spots on the under wing of the Common Blue butterfly can be clearly observed.

At rest, the line of orange spots on the under wing of the Common Blue butterfly can be clearly observed.

Wings of colour

Being so conspicuous, the species was first noted in the 17th century, when it was known as the Little Blue Argus, because of the prominent spots on its underwings like those of the Brown Argus. The current name dates from 1775.

Common Blues are on the wing for two periods of the year, corresponding with the arrivals of their two broods: in May and June, and from July to September. Like most butterflies, they are much more active on warm, sunny days. When the sun disappears behind the clouds, they usually perch on grass stems with their heads down, making them far less obvious than when they are in flight. When the sun does emerge, they will often fully open their wings and bask in the rays, warming up so that they can fly again in search of food. It is a position they also take up towards the end of the day, in order to get the last drops of warmth before dusk. At night, several of these butterflies may roost together, clinging onto the same plant stem.

Common Blues have a wingspan of approximately 1in (3cm), making them relatively small compared with other British butterfly species. As in most blue butterflies, the male and female Common Blue differ considerably in appearance. The male has bright blue upperwings, occasionally showing a mauve or lilac tinge, with a clear white edge and a very thin black line between the blue and white. His underwings are patterned with black spots, circled with white, and there are orange markings towards the edges. He is also usually more active and conspicuous than the female.

The female is much more variable in her appearance. Most, especially in the south, only show a hint of blue at the base of their chocolate-brown upperwings, with orange and black markings along the edges. However, in north-west Scotland and Ireland, a bluer form of the female is more common, which is probably due to a genetic mutation of some kind.

The intense blue colour of this and other blues is formed by the play of light on the scales on the wings, which also contain blue pigment. These are synthesised inside the chrysalis in the week or so before the adult hatches out. The blue pigments are reinforced by the caterpillar’s diet, from compounds called flavonoids found in their food plants.

 The male Common Blue butterfly: the pale fringe bordering its wings separated from the blue by a notable dark line.

The male Common Blue butterfly: the pale fringe bordering its wings separated from the blue by a notable dark line.

Breeding seasons

The life cycle of the Common Blue is made more complicated by the fact that, like many other summer butterflies, the species has two separate broods of adults. Each feeds, mates and lays its eggs at different times of the year. Very occasionally, in the south of Britain, a warm summer may even allow a third generation to be produced in September. To complicate matters even further, in the north of its UK range, from Yorkshire into Scotland, there is usually only a single brood, which flies from June through to September.

Adults mate soon after hatching out, either in late spring or midsummer, depending on which brood, and the female may be pursued by several eager males. After mating, the female lays her eggs on low-growing vegetation. Her preferred plant is bird’s foot trefoil, which is found on unimproved grassland throughout Britain and provides plenty of food for her hungry caterpillars when they hatch. Other food plants for the caterpillar include the restharrows, clovers and various other species of trefoil.

A female is very choosy when it comes to deciding exactly where to lay her eggs. She will fly low over the ground, stopping frequently to examine each plant. Finally, she will deposit her tiny eggs, one at a time, on the youngest, juiciest plants so that when they hatch, her caterpillars will have enough to feed on. The eggs are a greenish-grey colour when they are first laid, but turn white as they dry out.

The rather slug-like caterpillars, which are bright green with a black line along the back and white lines down their sides, hatch out approximately nine days later. However, the timing also depends on the weather conditions. In warm, sunny weather, this may happen as soon as a week after they are laid, but if the summer is cool, cloudy and wet, they may take a full two weeks to hatch out.

 A Common Blue butterfly alights on a stalk in a meadow of wild flowers.

A Common Blue butterfly alights on a stalk in a meadow of wild flowers.

Mutual benefits

Having emerged, the caterpillars immediately begin to feed voraciously on the leaves and shoots of their home plant. Again, their rate of growth, to a maximum length of ½in (1cm), is dependent on the weather and the size and quality of the food plant on which they hatched. Those that hatch out earlier in the year also take longer to develop than the later generation of caterpillars, due to spring’s cooler temperatures. As they develop, they become quite furry and begin to sing; a sound too soft for the human ear.

Caterpillars from the first generation of adults, which hatch out in June, pupate a few weeks after hatching. After the dull olive-brown chrysalis forms, it secretes substances and continues to sing, both of which attract various species of ants. The ants may then take the chrysalis down into their underground nests, where it will produce honeydew, on which they feed. The chrysalis benefits from being protected by the ants, while the ants benefit from the food source, in what scientists call a ‘facultatively mutualistic’ relationship.

The adults from this first brood then hatch out in July, August or September, as the second generation of the year. When ready, the adult butterfly slowly emerges, dries its wings in the warm air, then flies off to seek food and, eventually, a mate.

This second generation of Common Blue caterpillars overwinter as larvae, burrowing deep down into the vegetation close to the ground and turning brown to avoid being spotted by predators. They then begin feeding the following spring and pupate, emerging as the first generation of adults in May or June.

Like so many grassland butterflies, the Common Blue has declined in numbers over the past few decades as a result of intensive farming and the use of chemicals. However, its ability to live in a wide range of habitats means that numbers have not dropped as much as some more specialist species. On a summer’s day, they still provide the hedgerows with a flash of translucent blue as they take to the wing in search of precious nectar.

I’ve watched you now a full half-hour,
Self-poised upon that yellow flower;
And, little Butterfly! indeed
I know not if you sleep or feed:
— William Wordsworth 'To a Butterfly'

Words: Stephen Moss
Pictures: Alamy