Common blue butterfly. Image credit John Howlett

Blue butterflies

Matt Hayes, Research Assistant writes:

Matthew Hayes, portrait photograph

Whilst everyone is being asked to stay at home it’s important that we continue to look outside and engage with the natural world. To help with this, we are going to be blogging about wildlife you can see from your window or in your garden.

This set of posts will look at different groups of common butterflies as well as some historical species that have been lost or suffered declines. Read to the end for tips on how to support butterflies where you live.

Family Lycaenidae

Common blue, Polyommatus icarus and holly blue, Celastrina argiolus

Male holly blue, University Museum of Zoology collection. Copyright University of Cambridge
Holly blue male.
University Museum of Zoology collection. Copyright University of Cambridge
Holly blue female, University Museum of Zoology collection. Copyright University of Cambridge
Holly blue female.
University Museum of Zoology collection. Copyright University of Cambridge

This summer, if you see a butterfly in your garden dart by with a flash of blue, it will most likely be one of these two species. As the name suggests, the common blue is indeed found across nearly all of the UK and Ireland. However, in recent decades numbers of the holly blue have rivalled it, especially in England and Wales, and in many locations it is more often seen in gardens than the common blue.

Common Blue, University Museum of Zoology collection, copyright University of Cambridge
Common blue male.
University Museum of Zoology collection, copyright University of Cambridge
Common blue female, University of Zoology Museum collection, copyright University of Cambridge
Common blue female.
University of Zoology Museum collection, copyright University of Cambridge

The two species can look similar on the wing, especially the males, which both have a bright blue colour.

However, the female common blue is mainly brown, with a line of orange markings around the wing edges and a smattering of blue scales towards the body, whereas the holly blue female is blue with black margins edging its wings.

The two species also differ in the markings of their underwings. The holly blue underwing is a light powder blue, with some black specks peppered throughout, whereas the common blue has orange and white markings towards the wing edges.

Common blues mating, credit Ed Turner
Common blues mating, credit Ed Turner
Holly blue underwing, credit Ed Turner
Holly blue underwing, credit Ed Turner

Their behaviour in flight can also help distinguish the two species – common blue butterflies tend to stay closer to the ground, whereas holly blues will more readily fly up and over trees or hedgerows.

Both species usually have two broods in the south of the UK, but the common blue has only one generation a year in Scotland. Both butterflies also emerge at similar times, with the holly blue usually seen from April – June and July – August, whilst the common blue can be seen from May – June and July – September, depending on exact locality and weather conditions.

As the name suggests, the holly blue normally selects holly, or sometimes ivy, as its caterpillar’s foodplant, whilst the common blue usually selects common bird’s foot trefoil, a member of the pea family.

A sighting is worth 6 points in Open your Window Bingo!
Click the image to find out more…

…or spotting a common blue is a huge 9 points!

Species of the Past

Mazarine blue, Cyaniris semiargus

Mazarine blue male, University Museum of Zoology collection, copyright University of Cambridge
Mazarine blue male.
University Museum of Zoology collection, copyright University of Cambridge
Mazarine blue female, University Museum of Zoology collection, copyright University of Cambridge
Mazarine blue female.
University Museum of Zoology collection, copyright University of Cambridge

Had you been looking out of your window more than 100 years ago, the iridescent blue of today’s common butterflies would have been joined by another species with a beautiful purple-blue colouration. However, similar to the common blue, mazarine blue females are brown, lacking the striking blue colouration of the males.

Sadly, now extinct in the UK, it is not certain why the mazarine blue disappeared, but one theory is that changes in hay cutting practices contributed to its decline. Its foodplant is red clover and a combination of sowing red clover in hay meadows and altering the time of year the hay was cut, may have lured the mazarine blue to lay eggs on plants that were cut at harvest time, killing its caterpillars. If the species was still around, you would be able to view it on the wing in June and July.

Gardening for the blues

Larval foodplants:

Holly plant

To support the caterpillars of the common blue you can plant bird’s-foot-trefoil in your garden. It likes close cropped turf so can be seeded in amongst an existing lawn or you can introduce seedlings as plugs.

To help the holly blue you could allow ivy to grow up in a section of your garden or plant a holly bush, which will also provide other species with shelter. Late flowering ivy is a valuable nectar source for butterflies and other insects still on the wing in autumn.

Blues relationship with ants:

Many species of blue butterfly (family Lycaenidae) have amazing lifecycles where their caterpillars interact with ants. For example, this caterpillar of a chalkhill blue is being tended to by a worker ant.

The caterpillar offers up suger and amino acid rich secretions for the ant to feed on and the ant will in turn provide protection. Some species like the large blue butterfly take this relationship even further and the ants adopt the caterpillars as if they are their own young.

This is just one example of the complex interactions between different species and how they rely on one another, which highlights the need to conserve as much as we can. This can be done at home by avoiding the use of pesticides, and planting a range of pollinator friendly plants.


For more detailed information on gardening for butterflies and wildlife in general please see:

Butterfly Conservation (website)

The Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire (website)

For more detailed information on the butterflies of the British Isles please see:

Butterfly Conservation (website)

UK Butterflies (website)

The Butterflies of Britain and Ireland (Book):

Lewington, R & Thomas, J (2014). The Butterflies of Britain & Ireland. New Revised Edition. Oxford: British Wildlife Publishing Ltd.


The Esmée Fairbairn Collections Fund is run by the Museums Association, funding projects that develop collections to achieve social impact.

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