Holly Blue (Celastrina argiolus) in Ireland

Gormán cuilinn in Gaeilge. The Holly Blue, Celastrina argiolus, is Ireland’s garden and churchyard blue, distinguished from the Common Blue by its habitat and by the pale silvery-white underside.

Identify it in four steps

1. Upperside pale sky-blue with a narrow black border in males; broader dark border in females, especially on the forewing tip.
2. Underside silvery pale blue-white with a scattering of small black dots, no orange marks and no strong pattern, quite unlike the Common Blue.
3. Wingspan 26 to 34 mm; flight is a high, quick flit around Holly and Ivy foliage, typically above head height and often around ornamental garden shrubs.
4. Rarely visits ground-level flowers in the way the Common Blue does; look upward around Holly hedges and mature Ivy on walls.

Habitat in Ireland

The Holly Blue uses two different larval foodplants across its two broods each year: Holly (Ilex aquifolium) buds and flowers for the spring generation and Ivy (Hedera helix) flowers for the summer generation. This dual-foodplant strategy allows it to occupy gardens, churchyards, hedgerows, and urban parks that hold either or both plants.

Populations show striking multi-year cycles driven by the parasitoid wasp Listrodomus nycthemerus, with high years followed by crashes. Irish records have shown a northward and westward range expansion since the 1990s.

Where to see it

• Dublin churchyards and mature suburbs: Holly hedges and Ivy-clad walls hold reliable April and August broods.
• Cork city parks and hedgerows: mature Ivy on walls and shrubbery produces summer records.
• the Vale of Clara, County Wicklow: hedgerow and woodland-edge Holly supports lowland populations.

Related species

• Common Blue (Polyommatus icarus)
• Small Blue (Cupido minimus)
• Green Hairstreak (Callophrys rubi)