Large Blue (Pete Withers)

Each month we’ll be finding out more about a species of butterfly or moth.

This month, let’s meet the Large Blue.

Common name: Large Blue

Scientific name: Phengaris (Maculinea) arion

Size: Its wingspan is 38-44mm

When does it fly: June to early-July

Where does it like to live: The Large Blue likes warm grasslands that haven’t been ploughed, reseeded or heavily fertilised, and where the nests of red ants can be found.

Where you can see it: After being declared extinct in the UK in the 1970s, this species is recovering thanks to painstaking scientific research by Royal Entomological Society, University of Oxford and Centre for Ecology and Hydrology, and a long-term reintroduction programme carried out by a partnership of many organisations and individuals, including Butterfly Conservation, and the butterfly can be enjoyed at locations in Somerset and Gloucestershire.

Amazing fact: Large Blue caterpillars spend most of their time underground in the nest of red ants where they also pupate before emerging as adults!

“The Large Blue has a fascinating life cycle and is an incredible example of the complexity of ecosystems. After being declared extinct in the UK in 1979, this butterfly is now back and thriving thanks to dedicated scientists and conservationists.”  – Dr Caroline Bulman, Head of Ecology

The Large Blue, as its name suggests is the largest of our blues, but only slightly but it is our rarest and is identified by the stunning blue, streaked with darker spots and borders of the upper wings. To spot it, you need to visit areas of unimproved grassland where Wild Thyme or Marjoram, the foodplants of its caterpillar, grow and where colonies of a specific red ant live underground. These ants are critical to the fascinating lifecycle of this enigmatic butterfly.

On sunny days from June onwards, the Large Blue adults will emerge. Look out for them close to the ground where they can be found sunbathing or fluttering about in search of plants to nectar on. Large Blue males are not territorial but will zig-zag up and down a slope looking for newly emerged females. 

When the males and females do meet, the males initiate a brief courtship display before mating.

Once mated, the female hides and rests to allow her eggs to mature and then she seeks out Wild Thyme or Marjoram plants to lay her eggs on – sometimes laying up to 50 in total but often depositing only a single egg on each flowerhead.

Large Blue (egg) by Peter Eeles

Once hatched, usually after five to ten days, the young caterpillars feed on the flower buds and take on a pink colour which helps then blend in with their foodplant.

Large Blue (caterpillar) by Jim Asher

The caterpillars moult multiple times as they grow – after their final moult is when things get really interesting. The caterpillars are now ready to infiltrate the nests of the red ant Myrmica sabuleti.

When the ants are out foraging, the caterpillars will drop to the ground and crawl under a leaf, waiting for a passing red ant. The ants, upon finding one of the caterpillars, will drink droplets of liquid from the caterpillar’s honey gland – sometimes for over four hours! Once the ant has had its fill, the caterpillar will rear up and begin mimicking an ant grub – tricking the ant using sound and scent, into picking it up and taking it back to the nest.

Large Blue (caterpillar with ant) - Peter Eeles

The caterpillar lives in the ant nest, near the main ant grub chamber, which it visits to feast upon the ant grubs. It will spend the winter in the deeper chambers of the ant nests before it is ready to pupate in spring. The chrysalis is formed in the ant nest and, after around three weeks, the butterfly emerges, crawling out through the tunnels of the nest and taking its first steps into the sunlight as an adult, ready to begin the fascinating cycle again.

Large Blue

Sadly, the Large Blue became extinct in the UK in 1979 after many years of decline, due to habitat loss and changes to grazing. However, thanks to the dedicated work of scientists and conservationists, the Large Blue has made an incredible return to areas of south-western England. Using donor populations from Sweden, the Large Blue has been re-established to sites that have been managed to create the exacting conditions the butterfly needs and core populations now exist in Somerset and Gloucestershire. However, it is vital that work continues to restore and protect this globally Endangered species.

Find out more about the Large Blue here.