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Common blue butterfly

David Chapman / 19 March 2014 ( 04 March 2021 )

Common blue butterflies small blue butterflies often spotted in gardens. Find out how to recognise them, what they eat and their life cycle.

Common blue butterfly
Common blue butterfly. Photograph by David Chapman.

The common blue butterfly (Polyommatus icarus) was named because the upper wings of the male are bright blue, but females are basically brown with a light dusting of blue. It is the most common British blue butterfly species.

Common blues inhabit grassland where there are wildflowers, so if you have an area where you leave the grass to grow for the summer then you may see common blue butterflies. 

The common blue butterfly is a very active insect when it is warm and the sun is shining but because of its small size it can be difficult to pick out. 

The trick to seeing one properly is to watch where they are active during the day and return to have a look at dusk or dawn. When the temperatures are lower the butterflies rest by clinging to grass stems, but even then, with their wings closed, they can be difficult to spot until you get your eye in.

Attracting common blue butterflies into the garden

Common blue butterfly caterpillars feed on low growing leguminous plants, the best species to grow - if you wish to encourage them - include the bird's foot trefoil, clover and restharrow. If you've spotted a very small blue butterfly in your garden it's most likely to be the common blue or a holly blue butterfly. The easiest way to tell them apart is by looking at the underside - the common blue has a brown underside with black, white and orange marks, while the holly blue has a pale blue underwing with black patches.

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Common blue butterfly life cycle

Egg laying time

Common blue eggs are laid singly on top of food plants such as bird’s foot trefoil. They are white flattened spheres with a bumpy surface and hatch after a week or two.

Common blue caterpillars

Common blue larvae are creamy white with fine hairs when they hatch, darkening to a leafy green as they shed their skin.

Common blue chrysalis

Common blues pupate on the ground close to their food source. The cocoon is translucent at first, but it becomes opaque and eventually turns dark grey. Common blues pupate for about two weeks.

Transformation into butterflies

After about two weeks pupating the common blue butterfly emerges. They're a beautiful, small blue (male) or brown and blue (female) species with a wingspan of about 3.5cm. 

Male and female common blue butterflies

Both sexes are brown underneath but with a number of white-ringed black spots across the wing and an arc of orange spots situated in black crescents, known as lunules, around the edge of the wing. The female's underside is a little more vivid than the male's.

On the upper wing males are bright blue while females have a more inky blue wing with orangey yellow markings.

Males are easier to spot, not just because of their vivid blue colour but also their behaviour. They are frequently in flight, especially on sunny days, and patrol their territory. Females are more likely to be seen feeding on nectar or flying low to the ground, searching for a suitable spot to lay an egg.

Common blue butterfly and hibernation

Common blue butterflies overwinter in larval form. The caterpillars drop to the ground and hide away in leaf litter and reawaken the following spring.

Find out more about British butterflies, including the best butterfly-friendly plants and flowers for your garden and how to take stunning butterfly photographs

The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.