The blue butterflies are some of the most stunning insects to be found in the UK. To many, they are the optimal sight on a hot summer’s day spent on a species-rich grassland. Not all are constrained to grasslands though, and they make use of a wide variety of habitats. They belong to a subfamily called the Polyommatinae, with nine resident species in the UK. The name ‘blue’ can be confusing, as some species show strong sexual dimorphism (where males and females look very different), with females being predominantly brown. In some species, neither sex is blue at all.

Most blue butterflies have a close relationship with ants, a phenomenon called myrmecophily. In this subfamily, the larvae produce a sweet secretion from a ‘honey’ gland, which is a food source to ants. The ants are attracted to this and tend to the larvae, who in return are offered a level of protection from potential predators and parasites. As we will discover, this relationship is not essential for all blue butterflies, whereas others are dependent on ants to complete their life cycle – sometimes to the detriment of the ants.

Some species are easy to identify, but others can be difficult to distinguish and rely on closer inspection, particularly of the markings on the undersides. It can help to know which larval hostplants are used for egg-laying, and therefore each species’ preferred habitat, as well as the months in which they’re flying. Some species are single brooded (only one generation each year), while others can have second and even third broods. We will have a look at each species, with tips on where and when to look for them, and key features to confirm identification.

Common blue

As its name suggests, this is our most widespread blue, although there have been recent declines throughout its range. The name does not do justice to the beauty of freshly emerged males, which are a striking blue. Females can vary in appearance, but they are usually brown, with a touch of deep blue near the wing base, although this blue can extend over most of the upperwing. All females have orange spots along the edges of the upper wings.

The common blue’s wide distribution reflects its larval hostplant, predominantly common bird’s-foot-trefoil, which is found in a variety of habitats and can readily colonise disturbed ground. The butterfly can be found almost anywhere this plant grows in sunny, sheltered areas – which highlights the importance of climatic factors influencing butterflies, as not all habitats can be used. This includes downland, old quarries, woodland clearings, road verges, sand dunes, coastal cliffs, and even rural gardens.

It is double-brooded, with two generations a year – although a third brood is possible in the south of England if conditions allow. As a result, common blues can be seen throughout different parts of the UK from May into September, and into October if a third generation is produced.

Holly blue

The first blue butterfly on the wing each year, as early as April, and the most likely to be encountered in gardens. Timings can help with identification as any blue seen in early spring will be a holly blue. At a glance, it could be mistaken for a common blue, but seeing the underside of the wings will allow for easy separation. The holly blue has a pale, silvery underside with fine black spots, and lacks any of the orange spotting shown by the common blue.

The holly blue is widespread and is extending its range north. In northern populations, there is only a single brood each year, but throughout much of its range it is double-brooded, with spring and summer generations. The spring generation predominantly lay their eggs on holly, hence the name, and can be seen into early-June. However, the summer generation lay their eggs on ivy, so ‘ivy blue’ would be just as fitting. Summer generations typically emerge in July and can be seen into late August. A third brood is sometimes produced in southern populations, with sightings in October, and rarely, November. Populations can fluctuate greatly on four-to-six-year cycles, which is thought to be linked with a parasitic wasp that is solely dependent on the holly blue.

They live in a wide variety of habitats where the larval foodplants grow, including woodland rides, hedgerows, gardens, and urban parks. The holly blue can be attracted to gardens if holly or ivy is planted in sunny positions and allowed to fruit, providing food for the caterpillars.

Silver-studded blue

This stunning little butterfly gets its name from the metallic blue-green ‘studs’ found on the underside of the hindwings, just beyond the orange spots. Apart from these studs, males are best identified by the thick black borders on their blue upperwings. Females are more challenging and can resemble a faded brown argus.

They’re found predominantly on heathland and coastal sites, where eggs are laid on heathers and gorse. They can also live on limestone grassland and sand dunes, where larval foodplants include rock-rose, horseshoe vetch, and common bird’s-foot-trefoil. There is one generation per year, and this typically peaks in June and July.

Silver-studded blues appear to require the presence of black ants to complete their life cycle. The ants tend to and protect the caterpillars in return for sweet secretions. Caterpillars are readily collected and taken into the ant nest, and there is evidence to suggest that the ants may even transport the caterpillars between the nest and their foodplants – in effect, farming the caterpillars. This greatly benefits the caterpillar, which also pupates within the ant nests.

Four subspecies of silver-studded blue have been identified in the UK; sadly, two have become extinct. Of the two that remain, one is found throughout much of its range in southern England and Wales, but the other is now found only on the Great Orme and a few nearby sites in North Wales.

Small blue

This is the smallest species of butterfly to be found in the UK. Its small size is an immediate clue to its identification, but being blue is not as they can appear almost black. Males have a variable amount of silvery-blue dusting on their dusky upperwings, but females have no blue at. The undersides of both are silvery-blue with black spots, with no orange coloration – allowing for easy separation from brown argus, which is also dark and fairly small. They appear as mini holly blues, but can be told from this species by their small size and dark upperwings.

Eggs are laid solely on kidney vetch, and therefore the small blue is constrained to places this plant is found in warm, sheltered sites. These requirements limit its distribution to chalk and limestone grasslands. This species highlights the delicate relationships between insects and plants; if we lose habitats supporting kidney vetch, we lose the small blue.

Despite being dainty, they are very aggressive, and males will gather in lek sites awaiting passing females. In the right conditions, many hundreds can gather in sheltered areas of longer grass and scrub. There are two generations a year with adults seen in May and June, and a second generation in late-July into August. Scottish populations only have one generation.

Adonis blue

The male Adonis blue is arguably the most beautiful of the blues, with eye-catching electric blue upperwings. This bright blue colour readily identifies the males, but as the colour fades the border of the wings helps separate them from other blues. There are black markings crossing the white fringes, creating a chequered pattern. This is diagnostic in the UK and also helps differentiate the predominantly brown females from similar species.

However, care must be taken with chalkhill blue females, which can share the same habitat and have brown chequered borders to the wings. The key difference requires close inspection of the orange spots near the edges of the upper hindwings; Adonis blue females have blue scales below the orange spots, whereas chalkhill blue females have white scales.

Sometimes called the icon of chalk and limestone grassland, this butterfly is found nowhere else as it is restricted to areas where horseshoe vetch grows, the sole larval foodplant. South facing slopes, with close-cropped grass allowing for abundant horseshoe vetch is prime habitat in southern England. There are two generations a year, the first peaking in June, and the second August into September. 

Adonis blue caterpillars have an intimate relationship with ants, which tend the caterpillars. They may even be buried by the ants during the night, as Adonis blue caterpillars only feed during the day.

Chalkhill blue

As its name suggests, this is a species found on chalk and limestone grassland. This is a noticeably larger butterfly and is the second biggest of the blues, after the large blue. The males are unlike any other blue, with silvery-blue upperwings with a broad dark border. The grassland can seem to shimmer as numerous males fly low in search of emerging females. Females themselves are much less conspicuous, as they spend less time flying and are mostly brown. They look similar to a female Adonis blue – see the entry above for details on how to tell them apart.

As with the Adonis blue, the sole larval foodplant is horseshoe vetch, and so the chalkhill blue is only found on unimproved sites where this plant flourishes in southern England. It can tolerate cooler sites than Adonis blue, although both species can share the same habitat patch. There is only one generation a year, with peak emergence in August.

Chalkhill blue caterpillars also have a close relationship with ants, but they feed mostly at night. They hide during the day, sometimes being buried by the ants. They are often attended by yellow meadow ants, which create conspicuous mounds on grasslands. The relationship is purely symbiotic, with the caterpillars gaining protection and the ants being fed sugary secretions. Butterflies are a fine example of how an entire habitat must be protected due to the intricate relationships between different species. As they’re so sensitive to environmental change, they are also a fantastic indicator species, which can alert us to issues in their environment.