Common Blue (Polyommatus icarus) in Ireland

Gormán coiteann in Gaeilge. The Common Blue, Polyommatus icarus, is Ireland’s most widespread blue and the only blue most Irish observers will meet on machair, meadow, and calcareous grassland.

Identify it in four steps

1. Males upperside bright violet-blue with a fine black border and white fringe.
2. Females upperside brown with a variable dusting of blue scales at the base and a row of orange lunules along the outer margin (Irish females often show more blue than British ones).
3. Underside pale grey-buff with two rows of small ringed black spots and an orange band inside the margin on both wings.
4. Wingspan 28 to 36 mm; low, quick flight through the grass sward, males patrolling patches of Bird’s-foot Trefoil.

Habitat in Ireland

The Common Blue depends on Bird’s-foot Trefoil (Lotus corniculatus) as its principal larval foodplant, with occasional use of related legumes including Black Medick (Medicago lupulina) and White Clover (Trifolium repens). It occupies sunny, short-turf grassland: dune slack, machair, calcareous limestone grassland, cliff-top sward, and old hay meadow.

The species declines quickly where trefoil is lost to overgrazing, agricultural improvement, or scrub encroachment. Two broods in Ireland in most years, with a May-June flight and a stronger July-September second brood.

Where to see it

• the Burren, County Clare: limestone grassland and green roads with abundant Bird’s-foot Trefoil support strong populations.
• Inch Strand and machair, County Kerry: coastal dune slacks with short turf and dense trefoil.
• Murlough National Nature Reserve, County Down (Northern Ireland): dune grassland stronghold on the eastern seaboard.

Related species

• Holly Blue (Celastrina argiolus)
• Small Blue (Cupido minimus)
• Small Copper (Lycaena phlaeas)