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 repens –
black 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 trefoil
1..
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..