Common blue

Larval hosts

The larva feeds on plants from the family Leguminosae (bean family). Recorded food plants are Lathyrus species, Vicia species, Vicia cracca, Oxytropis campestris, Lotus corniculatus, Trifolium pratense, Oxytropis pyrenaica, Astragalus aristatus, Astragalus onobrychis, Astragalus pinetorum, Medicago romanica, Medicago falcata, and Trifolium repens.

Subspecies

Life cycle and food plants

The main food plant on most sites is bird's foot trefoil (Lotus corniculatus). Others used include black medick (Medicago lupulina), common restharrow (Ononis repens), white clover (Trifolium repens), wild thyme Thymus serpyllum, and lesser trefoil (Trifolium dubium). Eggs are laid singly on young shoots of the food plant.

The caterpillar is small, pale green with yellow stripes and, as usual with lycid larvae, rather slug like. Hibernation occurs as a half-grown larvae. They are attractive to ants, but not as much as some other species of blues. The chrysalis is olive green/brown and formed on the ground, where it is attended by ants, which will often take it into their nests. The larva creates a substance called honeydew, which the ants eat while the butterfly lives in the ant hill.

Distribution and behaviour

The common blue is Britain's (and probably Europe's) most common and most widespread blue. It is found as far north as Orkney and on most of the Outer Hebrides. A range of grassland habitats are used: meadows, coastal dunes, woodland clearings, and also many man-made habitats, anywhere their food plants are found.

Males are often very obvious as they defend territories against rivals and search out the more reclusive females. In the south of Britain there are two broods a year, flying in May and June and again in August and September. Northern England has one brood, flying between June and September. In a year with a long warm season, there is sometimes a partial third brood in the south flying into October.

Appearance

Common blue males have uppersides in an iridescent lilac blue with a thin black border. Females are brown above with a row of red spots along the edges and usually some blue at the base of the wings; the upperside may be mostly blue, especially in Ireland and Scotland, but it always has red spots. Undersides have a greyish ground colour in the males and a more brownish in the females.

Both sexes have a row of red spots along the edge of the hindwings and extending onto the forewings, though they are generally fainter there, particularly in the males, where they are sometimes missing altogether. There are about a dozen black-centred white spots on the hindwings, nine on the forewings. These usually include one in the middle of the forewing cell, absent in Chapman's and Escher's blues. The white fringe on the outer edge of the wings is not crossed with black lines, as it is in the chalkhill blue and Adonis blues.

North America

Recently, the common blue butterfly was discovered in Mirabel, Quebec, Canada, by Ara Sarafian, an amateur entomologist who observed the butterfly from 2005 to 2008. He contacted the Canadian National Collection of Insects in Ottawa where the butterfly was identified as Polyommatus icarus, a new alien butterfly to Canada and to North America. The butterfly seems to be well established and is extending its range from year to year.

British Columbia

In 2016 pictures were taken of a male butterfly in the forest near Kamloops, British Columbia.