Common blue   Polyommatus icarus 
Family: Lycaenidae
Where are they found (in Britain and Ireland)
Throughout all of Britain and Ireland except Shetland and mountainous parts of Wales and Scotland.
 
What is their preferred habitat outside gardens?
Common blues fly in a wide range of grassy habitats, including heaths and downs, woodland glades and edges, roadside verges, wastelands and even coastal dunes.
 
Resident or migrant?
Resident
 
When to see them?
There are two broods in the south of England, where the adults can be seen from the middle of May until June and then again from the second half of July until September. A third brood may even appear in long, hot summers, with the butterflies flying until October. In the north of England, Scotland and Northern Ireland there is usually only a single brood emerging in June and peaking in July.
 
What happens in winter?
Common blues overwinter as caterpillars in leaf litter at the base of their foodplants.
 
What do the early stages look like?
The eggs are laid singly near the base of the upperpart of the foodplant, and are very small white pill-shaped structures.  The first instar caterpillars are tiny (around 1mm length) and a pale-yellow colour. They become bigger and greener with each instar, ending up as bright green caterpillars about 13mm in length. There are 5 instars and the total caterpillar stage of the lifecycle lasts about six weeks. Those that overwinter grow much more slowly and change to an olive colour while hibernating.  The pupa is formed on the ground under the foodplant, and sometimes buried by ants under the ground surface.
 
We cannot show photos of the early stages of this species, but there are excellent images at the UK butterflies website.
 
What do the caterpillars eat?
The main foodplant is common bird’s-foot trefoil Lotus corniculatus. Other foodplants include clovers – both red  Trifolium pratense and white Trifolium repensblack medick Medicago lupulina, and the two species of restharrow. Greater bird's-foot-trefoil Lotus pedunculatus, which is a plant more commonly grown in gardens, is also used, but not so often as common bird’s-foot trefoil1..
 
Flowers they take nectar from
The butterflies feed from a wide range of wildflowers including bugle, clovers, common bird's-foot-trefoil, common fleabane, knapweeds, ragworts, thistles, vetches, wild marjoram and wild thyme2..
 
How are they doing?
The population seems roughly stable at the moment. The 2022 State of UK butterflies report  shows a 17% decrease in common blue numbers from 1976, which is the same as in the 2015 report. However, the distribution has declined further between the two reports, from a 17% decrease to a 37% decrease3..
 
 
What do they look like?
Males have bright, distinctive, violet-blue wings with a black edge and white wing margins.
 
Females are less bright: their wings are mainly brown with a variable amount of blue, often just in the central part of the wings and the body. Females also have a black edge to the wings (although this is less easily noticed than on the males) and white wing margins plus a line of orange and black spots on the outer edges of the wings.
 
The undersides of the wings are similar in both sexes – a pale brown background with black and white spots in the centre of the wings and orange, black and white spots around the edges.
 
Wingspan 29 - 36mm
Other interesting facts
In the British Isles there are two subspecies of common blues. Polyommatus icarus ssp. icarus is found in England, Wales and Scotland whereas Ireland has a subspecies Polyommatus icarus ssp. mariscolore. This is rather larger than the typical ssp. icarus form, and the females have much more blue, and also much brighter orange spots in the wing margin.
 
There is some controversy over the exact distribution of these two subspecies, some authors claiming ssp. mariscolore is also found in Orkney and NW Scotland.
 
References
 
1.  Biological Record Centre database
 
2.  UK butterflies website 
 
3.  State of UK Butterflies 2022 report
 
 
Page written by Judy Skinner, reviewed and compiled by Steve Head
 
Female ssp. mariscolore
Male
Female
Common blue   Polyommatus icarus  
Family: Lycaenidae
Male
Female
What do they look like?
Males have bright, distinctive, violet-blue wings with a black edge and white wing margins.
 
Females are less bright: their wings are mainly brown with a variable amount of blue, often just in the central part of the wings and the body. Females also have a black edge to the wings (although this is less easily noticed than on the males) and white wing margins plus a line of orange and black spots on the outer edges of the wings.
 
 
The undersides of the wings are similar in both sexes – a pale brown background with black and white spots in the centre of the wings and orange, black and white spots around the edges.
Wingspan 29 - 36mm
 
Where are they found (in Britain and Ireland)
Throughout all of Britain and Ireland except Shetland and mountainous parts of Wales and Scotland.
 
What is their preferred habitat outside gardens?
Common blues fly in a wide range of grassy habitats, including heaths and downs, woodland glades and edges, roadside verges, wastelands and even coastal dunes.
 
Resident or migrant?
Resident
 
When to see them?
There are two broods in the south of England, where the adults can be seen from the middle of May until June and then again from the second half of July until September. A third brood may even appear in long, hot summers, with the butterflies flying until October. In the north of England, Scotland and Northern Ireland there is usually only a single brood emerging in June and peaking in July.
 
What happens in winter?
Common blues overwinter as caterpillars in leaf litter at the base of their foodplants.
 
What do the early stages look like?
The eggs are laid singly near the base of the upperpart of the foodplant, and are very small white pill-shaped structures.  The first instar caterpillars are tiny (around 1mm length) and a pale-yellow colour. They become bigger and greener with each instar, ending up as bright green caterpillars about 13mm in length. There are 5 instars and the total caterpillar stage of the lifecycle lasts about six weeks. Those that overwinter grow much more slowly and change to an olive colour while hibernating.  The pupa is formed on the ground under the foodplant, and sometimes buried by ants under the ground surface.
 
We cannot show photos of the early stages of this species, but there are excellent images at the UK butterflies website.
 
What do the caterpillars eat?
The main foodplant is common bird’s-foot trefoil Lotus corniculatus. Other foodplants include clovers – both red  Trifolium pratense and white Trifolium repensblack medick Medicago lupulina, and the two species of restharrow. Greater bird's-foot-trefoil Lotus pedunculatus, which is a plant more commonly grown in gardens, is also used, but not so often as common bird’s-foot trefoil1..
 
Flowers they take nectar from
The butterflies feed from a wide range of wildflowers including bugle, clovers, common bird's-foot-trefoil, common fleabane, knapweeds, ragworts, thistles, vetches, wild marjoram and wild thyme2..
 
How are they doing?
The population seems roughly stable at the moment. The 2022 State of UK butterflies report  shows a 17% decrease in common blue numbers from 1976, which is the same as in the 2015 report. However, the distribution has declined further between the two reports, from a 17% decrease to a 37% decrease3..
 
 
 
Other interesting facts
In the British Isles there are two subspecies of common blues. Polyommatus icarus ssp. icarus is found in England, Wales and Scotland whereas Ireland has a subspecies Polyommatus icarus ssp. mariscolore. This is rather larger than the typical ssp. icarus form, and the females have much more blue, and also much brighter orange spots in the wing margin.
 
There is some controversy over the exact distribution of these two subspecies, some authors claiming ssp. mariscolore is also found in Orkney and NW Scotland.
 
References
 
1.  Biological Record Centre database
 
2.  UK butterflies website 
 
3.  State of UK Butterflies 2022 report
 
 
Page written by Judy Skinner, reviewed and compiled by Steve Head
 
Female ssp. mariscolore
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