Common Blue

The Common Blue, Polyommatus icarus

In this butterfly the males have bright blue wings on the upper-sides, while the much more rarely seen females have brown wings on the upper-sides. The males can be seen, flashing a sometimes startling blue, usually flying low on flower-rich grassy areas, particularly reliably on the wildflowers sown on the bank of the reservoir east of Victoria Road, sometimes along a couple of the footpaths neighbouring Common Lane and Carpenters Lane, on College land by the River Bourne when the old permissive path allowed access, in Bourneside Meadow and in the south of the parish towards the River Medway.

This is the only obvious “bright blue” looking butterfly that most people are likely to see within the boundaries of Hadlow Parish. There are other “blues” that can be found on the Chalk Downland hills to the north of the Kent, such as the Adonis Blue and Chalkhill Blue, but these are restricted to those specialist habitats with their associated caterpillar food-plants, and only very rarely appear in this parish. The most easily confused sightings in Hadlow will therefore be light-silvery or duller blue butterflies flying higher, usually around shrubs and trees, the Holly Blue. The undersides of the Holly Blues are a light silver-blue, almost white, while the undersides of both the male and female Common Blues are orange-spotted brown, quite visible at rest once you get your eye in!


The males are generally the most obvious and are the most easily seen. Common Blues are about an inch to an inch and a half in wing-span. They are a quite distinctive violet-blue colour as seen from above, as they tend to fly low over the ground, patrolling in search of females, or nectaring. The blue is quite intense on fresh specimens, but tends to fade over time. They quite often seem to “disappear” as they settle, as they close their wings and the blue shape you have been watching is replaced by the entirely different orange-brown of the underside of the wings.

Here is the violet-blue of the upper-wings of a male – notice the unbroken white borders to the wings. This photo was taken at Leybourne Lakes Country Park:

Common Blue, male, upperside of wings, photographed at Leybourne Lakes Country Park, 23rd August, 2015.

and now the orange-brown of the wings undersides of this male, also at Leybourne Lakes Country Park:

Common Blue, male, underside of wings, photographed at Leybourne Lakes Country Park, 23rd August, 2015.

This is a good view of the diagnostic pattern of spots on the underside of the wings. This photo of a male was taken at the Queendown Warren Kent Wildlife Trust reserve. There is a characteristic spot near the body on the under-side of the fore-wing – this is absent in other species such as the Brown Argus. Notice also how the black spots on the under-side of the hind-wing go around the central mark in a very rough circle:

Photographed at Queendown Warren

Female under-wings are similar to the males, but may generally be a little browner in some lights at least. Female upper-wings are generally a lovely but discreet chocolate-brown in colour. This one however, also at Queendown Warren KWT reserve, is somewhat faded:

Photographed at Queendown Warren, 9th August, 2013.

On occasion you get a “blue-ish” female. Here is a rather tatty female, but showing a degree of blue colouration on the normally brown upperwings,

Photographed at Trosley Country Park.

and sometimes you get a really blue female!

Photographed at Queendown Warren, 30th July, 2017.


Common Blues live in small colonies of perhaps tens (to hundreds in better areas than Hadlow), within which the males are said to hold territories which they patrol to search for females – to me however they often seem to wander fairly randomly, often looking for flower nectar sources. They will also wander away from their home colony areas, either looking for new habitat or just getting lost! The other day one was wandering across my neighbour’s lawn – it was a little longer than usual with some hawkbits flowering in it, as he had been away on holiday>

Good places to find Common Blues in Hadlow are many of the long grassy areas, such as Bourneside Meadow, the sunnier parts of footpaths by the old orchard to the west of the cricket field, the banks of the reservoir to the east of Malt Cottage, accessed by the footpath running east from Victoria Lane, the set-aside area alongside Carpenters Lane to the North of Hope Cottage, and some of the wider roadside verges, particularly in the very hot days of early August. It used to be accessible along the old permissive path on the College ground on the other side of the River Bourne to the College dairy.

The early evenings are also good times to check for them, as they often rest quietly with their wings closed, commonly upside down, on grass stems and flower heads, showing the complex and fascinating patterns on the underside of the wings.

The Common Blue is one of the butterflies that likes to roost communally, as shown by this group at Queendown Warren, all lined up to be side-on to the setting sun! Noticed the solitary bee to the right of the group:

Photographed at Queendown Warren

The Common Blue is the commonest “blue” butterfly throughout most of Britain and Ireland, except for the more mountainous areas of Wales and Scotland. It is rarely found above 300 metres. Although it has got to Orkney, it is somewhat less common as you go north, and there may be only one (extended) summer generation (June – September) in the colder wetter conditions of a shorter summer. Abundances do seem to be higher on calcareous and coastal sites in the southern half of the country, but many other warm dry-ish grassland sites where the food plants grow may be used.

The range extends across Europe and temperate Asia as well as into North Africa. It has also recently been accidentally (?) introduced into North America. The habitats used in Europe are even more diverse than the UK, as continental climates appear to permit it to be less choosy about the soil-plant characteristics it requires. It is a very common butterfly in Europe.

Although still widespread and easily found in suitable habitat throughout its UK range, both habitat extent and habitat richness have greatly declined over time with more modern agricultural methods, and overall there are far fewer Common Blues than in the past. Over the last 30 years for which there are data, 90/95 to 00/05) total numbers (measured in UKBMS samples) seem to have decreased by possibly 20%. This decrease is probably also reflected in the populations of the Common Blue in Hadlow.

Life cycle

The larval foodplant is generally claimed to be the Common Birdsfoot Trefoil, Lotus corniculatus, but the Greater Birdsfoot Trefoil, Lotus pedunculatus, and other legumes such as the Lesser Yellow Trefoil, Trifolium dubium and White Clover, Trifolium repens, are apparently also quite acceptable. The caterpillars are green with a green and pale line along their bodies and a tiny black head. They get to about half an inch long when full-grown.

The Birdsfoot Trefoil (and clover) food-plants sometimes contain significant quantities of cyogenic glucosides that are intended by the plant to release cyanide upon cellular damage by predatory herbivores – this works well against unsuspecting (generalist) caterpillars. However specialist herbivores such as the Common Blue larvae contain enzymes that can detoxify the glucosides, and actually use the materials as an extra nitrogen supply, on which they grow faster than if fed upon other plants with no or low cyanogenic glucoside content.

In Hadlow the first adults first appear in the parish about the middle of May, and last until the end of June. There is then a second more populous generation lasting from the end of July until the middle or end of September, and occasionally a small third generation October-ish. The males may perch or patrol in search of females, or be seen flying from one flower to another for nectar. The females are far more difficult to see, creeping about and tapping their feet on plants as they test them for their suitability for egg-laying, placed on the upper surface of young leaves.

Photographed at White Hill Butterfly Reserve, 23rd August 2014
Photographed at White Hill Butterfly Reserve, 23rd August 2014

The round white eggs after a week or so will hatch into the caterpillars that eat away at the underside of the leaflets, “window-paning” them, until they finally moult into pupae. Both the caterpillars and pupae emit scents and squeaks as they move, thought to be designed to encourage ants (rather rarely noted). The second generation of caterpillars over-winter, before finally pupating the following spring.

The first generation in this habitat often nectars on the Creeping Buttercup, Ranunculus repens, or Field Buttercup, Ranunculus acris, flowers common at that time of year, such as in this photo of a first generation male taken at Queendown Warren KWT reserve:

Photographed at Queendown Warren

However the second generation appears to quite favour the flower heads of the Common Fleabane, Pulicaria dysenterica, which are found then. This is a second generation male nectaring on Common Fleabane at Bourneside meadow, a small patch of undisturbed ground by the side of the River Bourne:

Photographed at Bourneside Meadow, Hadlow, 7th August 2014.

In other habitats, like this one in the Derbyshire Peaks, other flowers such as this Marjoram, are used (Note the red Trombidium mite attached near this individual’s head):


Populations tend to be highest in years following warm moist summers, but lower in years following hot dry summers, when reproduction is quite badly affected, even if good numbers of butterflies had been seen in the previous year. A positive winter NAO (North Atlantic Oscillation) will also generally encourage better numbers of flying adults in any particular year, particularly through earlier and better emergence of the first generation, as is characteristic of many butterflies with a spring as well as a summer generation of adults.

There are over 43 named variants on the colour pattern in adults that collectors have recognised. One paper suggests that the Common Blue appears to be quite genetically diverse taken as a whole across Europe, but not varying significantly between individual populations. If verified, this could perhaps suggest that the species survived glaciation rather successfully as a large diverse population in Mediterranean regions (with plenty of internal gene flow) that then recolonised the rest of Europe to the north as almost a single unit over the last 12,000 years. As always opinions appear to differ as to the interpretation of the facts.

Flavonoid pigments from the larval food-plants according to another study appear to be selectively absorbed, retained and stored largely in the adults’ wings, and particularly by females. This gender difference might indicate that there is some role for flavonoids in sexual attraction, as well as the other currently suggested roles of UV-protection and antibiosis. Females rich in flavonoids have been shown to be more attractive to males, eliciting more interest from them. This might mean the wings simply appear brighter, or that the flavonoids indicate the better nutrition of the larva – or both.  It also rather begs the question of when, if ever, as a male Common Blue, it is better to cut your losses on a “less-fit” female Common Blue.

Like many other butterfly species, the Common Blue carries an intra-cellular bacterial infection, Wollbachia, passed on via the egg cytoplasm (the maternal line), that appears to be a very unusual symbiont.