Common Blue

Photo by Peter Mayhew 

Common Blue  Polyommatus icarus (Rottemburg 1775)  

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

Name Origin (Etymology): Polyommatus means "many eyes" and icarus  means  'a piece of fallen sky' after the youth who thought he could fly to the sun but fell to his doom

Identification

Common Blue Female by Paul Kipling

Male 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.  However the Holly Blue has a chequered wing margin and is only seen flying around bushes while Common Blue flies low over grassland

                        Holly Blue                                                                                                       Common Blue

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. The Common Blue underside is grey to buff  (Female darker) with patterns of black and white spots and orange crescents whereas the Holly Blue has rows of small black spots on an almost-white silvery background).

Holly Blue   and its silver underside 

Common Blue

 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’  (or figure of eight) 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.


TIP : Common Blues always fly in the open never near shrubs like the Holly blue but also check the fringe or underside 

Habitat

The Common Blue occurs 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,  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 , Birds foot Trefoil (BFT), on which the adults also nectar.  Look for BFT  in drier sites, and Greater Birds foot Trefoil  in wetter sites (such as lowland heaths); also used by the caterpillars as are Black Medick and to some extent Restharrow and Lesser Trefoil.  Nectaring plants include BFT, clovers, vetches, knapweeds, thistles, Wild Marjoram, Wild Thyme, Ragwort, Fleabane, and Bugle.

Males are territorial, and both sexes typically roost perching head-down on grass stems often together with others in a sheltered spot in their territory. 

Nick Ballard

Status: Distribution, Occupancy and Abundance: 

Widespread but  while still common  is much reduced in abundance and continues to decline

Common Blue is one of the most widespread species globally being seen as far north as Shetland,  the whole of Europe, North Africa, the Canary Islands, and east across the Palearctic to Northern China. After being Introduced into North America  is now spreading rapidly. 

Common Blue has declined by about three quarters over the 20th century  with its host plant Birds foot Trefoil  also declining by a half over that same period.  This has much to do with changes in land use and intensification of farming particularly after WW2.  Use of artificial fertilizers and more intensive grazing in recent decades along with Myxomatosis in the 60's allowed grasses to get the upper hand in many grass swards.  In fact we have lost 98% of traditional meadows  resulting in 96%  loss of the Common Blue from those grassland habitat. Now the majority of the population is  concentrated in remaining limestone grasslands as can be seen from the map above where you can pick out the Craven Limestones,  Magnesian Limestone ridge, parts of the Wolds and the Tabular Hills on the edge of the North York moors.  It can still be seen in moderate numbers on brownfields particularly where they have ben reseeded with Birds foot trefoil like the numerous slag heaps converted to country parks of the Aire, Derne, Don and Rother Valleys. However even in these favoured areas there is evidence of much decline as the butterfly faces a new threat of increasing nitrogen deposition from the atmosphere altering the plant balance in favour of grasses and crowding out vetches like BFT.  Worse even where BFT remains very abundant the butterfly is still decling fast is even more worrying. 

 Occupancy is in slow decline  

Very slow declining occupancy hides the fact that populations are declining.  We can see this species favours hot summers like  2014  and 2018 and  The rate of loss has slowed from 80% over 20 years to 45% over the last 10 years  but the extreme heat of 2022 did not yield any really significant increases possibly because the BFT was badly droughted and the bad spring/summer of 2024 it hit an all time low point 

Where and When to Find

Common Blue can turn up anywhere including  gardens and is certainly a far ranging wanderer.  Some of the largest  colonies are on the Craven Limestones like at  Duck Street Quarry, a well known Butterfly reserve. It can also be seen at Fordon Chalk Banks near Bridlington, Ellerburn Bank at Pickering, Kiplingcotes at Market Weighton, Holme Dale at Fridaythorpe, and Brockadale at Pontefract  and Orgreave Common at Catcliffe.

 Lowland sites have two generations occurring  in May-June peaking in the 2nd week of June  and in August-September peaking in Mid August , Only one brood is found  at higher altitudes, with numbers peaking in mid-July, but Late-September-October individuals in warm summers in the south may belong to a partial third generation. 


Life Cycle

Eggs are whitish, resemble 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.

Mating Pair Paul Kipling

Foodplants  

Birds Foot Trefoil is the most usual foodplant for Common Blue and is commonly found in sandy and chalky grassland habitats but also in  meadow  seed mixes used  to produce biodiverse grass on brownfields, embankments etc .  A low-growing creeping plant 10 to 20cm  that flowers June to September. Not unusual to arise in cut grass  lawns roadsides which it makes it different to the medicks that appear in disturbed land. Flowers are yellow or tinged with red (eggs and bacon plant) and the calyx teeth are erect in bud. Broad stipules at the base that give the impression of their being five leaflets.  BFT is often used in so called meadow seed mix  and unfortunately with a non-native subspecies which is more upright and has hollow stems and narrower leaflets so wise to check providence before sowing. 

Greater Birds Foot Trefoil is the most commonly used foodplant in damp grassland marshes and fens . Flowers June to September. A tall-growing plant to 80cm, Flowers yellow occasionally tinged red; the calyx teeth are spreading in bud. The leaves have broad leaflets and stipules Compared to BFT it only has a scattering of long, white hairs plus the stems are hollow.  

Black Medick  is sometimes used as a foodplant  and is very common in most types of grassland plus it is a pioneer plant on disturbed areas. A short-lived perennial which can form a mat on open ground. Flowers from  May till September. Small Pale yellow flowers  carried in dense clusters forming a round ball. Easily identified from other similar species by the notched leaf point

Aberrations

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

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

With contributions from Peter Mayhew    (19/11/20)