Common blue (Polyommatus icarus): identification, range and life cycle

The common blue, Polyommatus icarus, is a small butterfly of the family Lycaenidae and one of the most widespread blues in the Old World. The species ranges across Europe and North Africa, including the Canary Islands, and east through the Palearctic to northern China, and it has recently become established in eastern Canada, in Quebec. Across most of that range it is probably Europe’s most common and most widespread blue, the species most likely to turn up in a flowery meadow.

Identification

Wingspan runs to 28–36 mm. The sexes look quite different above. A male is iridescent lilac-blue, bright violet-blue or almost hyacinth-blue with a thin black border, while a female is typically brown or black-brown with a row of reddish-yellow spots and a variable dusting of blue at the base of the wings. The underside is quieter in both sexes and carries the pattern used to confirm the species: a greyish, dust-grey ground colour in males and a more brownish one in females, with a row of red or orange spots along the edge of the hindwing and about a dozen black-centred white spots (ocelli) on the hindwing and nine on the forewing. A useful field mark against similar blues is the fringe: the fringes on the outer edge of the wings are uniform white, not crossed with black lines. The ocelli themselves help with identification too: a forewing typically carries nine such spots, including one sitting in the wing cell.

Range and habitat

This butterfly occupies open flowery and grassy places, warm or cool, wooded or open, from sea level to about 2,700 m, and it favours chalk or limestone grassland, woodland clearings, meadows, heathland and sand dunes. That tolerance of both warm lowland turf and cool upland pasture, combined with its broad plant diet, helps explain its spread across the Palearctic and into eastern Canada. It also turns up on man-made ground such as railway embankments and sheltered spots under cliffs, provided the site stays open and its host legumes keep flowering.

Life cycle and host plants

Caterpillars feed on legumes, among them bird’s-foot trefoil, vetches and vetchlings, clovers, medick and restharrow. The female lays her eggs singly on young shoots of the food plant; each egg is very small, about 0.60 mm, and pale greenish-grey. The caterpillar itself is small, pale green with yellow stripes, with the slug-like shape typical of its family, and it overwinters as a half-grown larva. Pupation happens on the ground, where the olive-green or brown chrysalis is often attended by ants, a relationship common among the blues. The species fits in one to three broods a year: two in southern regions, one further north, with an occasional partial third.

Behaviour and flight period

Southern Britain sees two broods, flying in May–June and again in August–September, with an occasional partial third generation in October; further north there is a single brood on the wing from June to September. Males and females use the habitat differently. A male defends a territory and flies further in search of a fertile female, while a female flies lower, searching mainly for nectar and places to lay her eggs. When the sexes meet, copulation usually follows immediately, without any courtship ritual.

Similar species

• Adonis blue (Polyommatus bellargus): a close relative whose wing fringes are crossed with black lines, while the common blue’s fringe is plain white, the most reliable field mark between them.
• Chalkhill blue (Polyommatus coridon): also has fringes crossed with black lines rather than the common blue’s plain white fringe.