Common Blue

Photo by Peter Mayhew

Common Blue Polyommatus icarus (Rottemburg 1775)

Family: Lycaenidae, sub-family Polyommatinae (the Blues)

Description

by J Bone

A female by S Bradshaw

by Paul Kipling

The Common Blue is a widespread small butterfly that flies close to the ground in flowery places.

The male is one of our most beautiful and evocative species, resembling ‘a piece of fallen sky’. It has violet blue uppersides with a white border, and in Yorkshire can only really be confused with the Holly Blue. The Common Blue underside is greyish with patterns of black and white spots and orange crescent marks (whereas the Holly Blue has rows of small black spots on an almost-white background).

The female uppersides are variable, with orange crescent marks on a brown, blue/brown or violet blue background. ‘Blue’ females are very distinctive and look like a different species. The tendency towards blue females is thought to increase from south to north, and is strongest in Ireland and NW Scotland where some authorities give it the subspecies status mariscolore. Most colonies in Yorkshire produce varying proportions of blue or blue/brown females. Brown females can be confused with Brown Argus, which can occupy the same habitat, but Brown Argus are smaller, more active, and have a ‘smarter’ appearance when fresh, with a more distinctive central black spot and orange markings on the upperside forewing. More definitive characteristics, which are harder to see, are the presence of a black ‘colon’ on the underside of the Brown Argus hindwing, which in Common Blue are two more separated spots, and an extra black spot near the wingbase on the underside of the Forewing in Common Blue.

The Common Blue is an extremely variable species with over 200 aberrations described.

Habitat

The Common Blue is normally found in open grassy places, including limestone pastures, hay meadows, urban brownfield sites, riversides, coastal sites and lowland heaths, but it can produce colonies in very compact sites such as roadside verges, quarries, railway tracks, field margins and woodland rides. In Yorkshire it is a fairly common species of the wider countryside, where numbers have probably declined, and urban (especially old industrial) areas. It is generally absent from moor-tops, shady woodland, arable fields, improved pastures, and ‘tidy’ urban areas. However, wandering individuals can turn up almost anywhere including gardens.

Common Blues tend to occur in discrete colonies containing the caterpillar foodplants, on which the adults also nectar. Look for Birdsfoot Trefoil Lotus corniculatus in drier sites, and Greater Birdsfoot Trefoil Lotus pedunculatus in wetter sites (such as lowland heaths); also used by the caterpillars are Restharrow, Black Medick and Lesser Trefoil. Nectaring plants include clovers and vetches, knapweeds and thistles, Wild Marjoram, Wild Thyme, Ragwort, fleabanes, and Bugle.

Males are territorial, and both sexes typically roost perching head-down on grass stems.

Distribution

Found all over Yorkshire where suitable habitat occurs; in 2019 it was recorded in 262 2x2km gridsquares out of 1510 visited, and 4120 in Yorkshire, making it the 16th most widely recorded species in the county, a ‘mid-table’ position.

The Common Blue is generally less abundant in the north (VC62 and VC65) and at higher altitude, where there is typically only one generation and counts are most commonly single figures. Lowland and southern populations (VCs 61, 63 and 64) can produce two or three generations, and numbers can be high at some sites (e.g. 691 at Duck Street Quarry, Greenhow in 2019). There is likely to have been significant loss of colonies due to agricultural intensification, the tidying of urban spaces, and the shading of woodland rides.

As with most other butterfly species, numbers of Common Blue recorded in Yorkshire fell significantly in the cool wet summer of 2012 (4584 recorded), and by 2019 (6081 recorded) had still not recovered to previous, much higher figures (e.g. 12,853 in 2009).

In the UK it is the most widespread of the ‘blues’ being found as far north as Shetland.

Photo above right by Nick Ballard

Life Cycle

Normally one brood is found in the north of Yorkshire and at higher altitudes, with numbers peaking in mid-July, but in lowland and southern Yorkshire sites two generations occur, with a May-June and an August-September generation. Late-September-October individuals in warm summers in the south may belong to a partial third generation.

In 2019, the first Yorkshire sighting was on 2nd May at Goathland (VC62); in some years April sightings occur (e.g. 24th April 2018 in Hull and Beverley, VC61).

In 2019, the last Yorkshire sighting was on 2nd October in Barnsley (VC63). Later sightings occur in some years in the south (e.g. 21st October 2018 in Rotherham, VC63).

Eggs are whitish, very small (0.6mm across), and resemble frosted doughnuts or flattened golfballs (having an uneven surface with a central depression). They are generally laid on the upper surface of young leaflets of the foodplants.

The greenish larva hatches after 1-2 weeks, and feeds by day on the undersurface of the leaf, leaving blotch like patterns which give away its presence. It has a darker line down the back, and white lines along the sides, and is fringed with white hairs. Larvae overwinter at the base of the foodplant, in their third instar. Last (5th) instar larvae are 12mm long, have a honey gland and are attractive to ants.

Pupae are found in the ground or at the base of the foodplant, and are 10mm long, surrounded by loose silk. They are often greenish, with creamy wingcases and covered in bristles. The pupal stage lasts two weeks.

Photo above right by Caroline Walker

Status

The species is not considered threatened in Yorkshire, nationally or internationally, but like many wider countryside species has possibly declined in abundance in Yorkshire in recent decades.

Aberrations

A large number of named colour forms of the female upperside and male or female underside. Feast your eyes at:

http://www.britishbutterflyaberrations.co.uk/species/polyommatus-lysandra-icarus/aberrations

P Mayhew 19/11/20