Scientific name: Polyommatus icarus
The Common Blue is the most widespread blue butterfly in Britain and Ireland and is found in a variety of grassy habitats all over the British Isles.
The male has blue wings with a black-brown border and a thin white fringe. The female is brown and similar to Brown Argus, but with a blue dusting near its body and fewer orange spots along the lower edges of its wings. Unlike Adonis and Chalk Hill Blues, the dark veins do not extend into white fringes of wing margins.
The brightly coloured male is conspicuous, but the female is less easily seen, her upperwings varying from almost completely brown in southern England, to predominantly blue in western Ireland and Scotland, but the colour is variable within local populations with some striking examples. The male patrols territories in search of females, while the female flies lower and more slowly as she looks for places to rest, nectar and lay eggs in. They both roost at night and on dull days on grass stems.
The egg stage of the Common Blue's lifecycle is around eight days. The eggs are white and shaped like a flattened sphere, and can easily be found on the upperside leaves of the caterpillar's foodplants (see below), usually Common Bird's-foot-trefoil.
The green caterpillar feeds from the underside of leaves, which causes blotching, betraying its presence. There are five stages in the caterpillar's life. The caterpillar is attractive to ants, as are most of the Blues, but only in its final stage. This lasts around six weeks, unless it overwinters as a caterpillar, which it does at the base of its foodplant in leaf litter.
The chrysalis forms under silk strands on the ground, or at the base of the foodplant. Ants finding its secretions attractive may bury it to protect it from predators. Its transluscent case turns pale brown then finally a dark grey-brown by the end of the two week stage of its life.
The Common Blue remains widespread, although there have been local declines within its range.
Size and Family
- Family: Blues
- Size: Small
- Wing Span Range (male to female): 35mm
- Butterfly Conservation priority: Low
- European status: Not threatened
Common Bird's-foot-trefoil (Lotus corniculatus) is the main foodplant. Other plants used include: Greater Bird's-foot-trefoil (L. pedunculatus), Black Medick (Medicago lupulina), Common Restharrow (Ononis repens), White Clover (Trifolium repens), Lesser Trefoil (T. dubium).
The Common Blue can be in a variety of habitats, usually sunny, sheltered places where the larval foodplant can be found. Examples of habitats include; downland, coastal areas, undercliffs, road verges, quarries, disused railway lines, acid grass and woodland clearings.
It is also found on waste ground, disused pits and quarries, golf courses, and urban habitats such as cemeteries.
- Countries: England, Ireland, Scotland and Wales
- Found throughout Britain and Ireland
- Distribution Trend Since 1970’s = -15%
- Butterflies and farmland
- Farmland Butterflies ID chart
- Butterflies in towns and cities
- Woodlands for Butterflies and Moths