Large Blue Butterfly (Phengaris arion) (Photo One)

Dear Readers, it’s a sunny Friday here in East Finchley, and so I wanted to share some excellent news with you in the midst of all the current misery. This has been the best year so far for the Large Blue butterfly, a species that was declared extinct in the UK in 1979. It’s a fascinating story, that begins with the life cycle of this enigmatic insect, the details of which were only unravelled by entomologist Jeremy Thomas during the 1970s and 1980s. This account of what happens comes from ‘Butterflies’ by Martin Warren, a fantastic book that I thoroughly recommend.

It all starts normally enough – the Large Blue lays its eggs on wild thyme, and the caterpillar chomps away until it has shed its skin three times. At this point it is only a few millimetres long. The larva is an outrageous cannibal and so only one is likely to survive on each flowerhead. Then the caterpillar falls to the ground and waits until it attracts the attention of a passing ant – it has a special ‘honey gland’ that produces honeydew that is irresistible to them. Once the ant has tasted the honey it will alert other ants, until they are all clustering excitedly around the larva.

Large Blue caterpillar on wild thyme (Photo Two)

After a while, the larva suddenly rears up and inflates its body – this seems to drive the ants into a frenzy, and one of them will pick up the caterpillar by the scruff of its ‘neck’ and and carry it off into the nest. It seems that the caterpillar produces chemicals that persuade the ant that it is, in fact, an ant grub that has escaped from the colony, to the horror of the adult ants. The frenzied activity smears the larva with the scent of the ants, in effect making it ‘smell’ like one of them. These chemicals continue to be produced once the caterpillar is in the colony, and once ensconced its behaviour changes – instead of being a passive producer of honeydew it becomes a rampant predator on the ant grubs. Sometimes it can consume the entire brood, and by the end of the process it will have increased in weight 100-fold. It turns into a chrysalis, and even when it emerges as a butterfly it is covered in droplets of a liquid that the ants seem to relish –  they will surround the interloper even as it inflates its wings, and leaves the nest to start the cycle all over again.

However, not all species of ants will tolerate this underhand behaviour, and if the larva is identified it be killed. The Large Blue caterpillars only survive well with one species of ant, Myrmica sabuleti. This ant thrives only in warm, short-grazed turf, and this knowledge unlocked the key to the reintroduction of the butterfly.

12 new sites across the South West of England were managed at a landscape level to provide suitable habitat for the Large Blue, which requires warm slopes where the plants are short-grazed by animals such as rabbits, cattle and horses. This habitat used to be common across the UK, but has since become vanishingly rare. What is so inspiring about this joint effort (between the National Trust, Somerset and Gloucestershire Wildlife Trusts, the J&F Clark Trust, Natural England and Oxford University, with the restoration being overseen by the Royal Entomological Society’s David Simcox and Sarah Meredith ) is that restoration of the habitat has not only helped the Large Blue, but has also benefitted many other scarce species, including the Shrill Carder Bee (Bombus sylvarum)  (probably the UK’s rarest bumblebee) and the Duke of Burgundy butterfly (Haemaris lucina), whose caterpillars feed only on cowslips and primroses.

Shrill Carder Bee (Bombus sylvarum ) (Photo Three)

Duke of Burgundy (Hamearis lucina) (Photo Four)

Large blue caterpillars were reintroduced to the designated sites from Sweden, and have since set up home very nicely. Since then the insects have thrived, having their best year yet in 2022, with the South West of England now holding the largest aggregation of this species in the world.

There are fears, however, that the drought this year will have reduced the numbers of larvae who have survived – more ant nests will have failed, and stressed ants are apparently more able to spot the trickery of the large blue caterpillars. One way to offset the effects of global warming has been to include damper, cooler microhabitats, which would not be suitable for the ants in ‘normal’ conditions but might be taken up in extra hot years like 2022. It will be interesting to see how things play out over the years to come, but for now I think we can celebrate an initiative which is benefitting a whole plant and insect community. The restoration of whole habitats, with their complex and poorly-understood interactions, must surely be the way forward for conservation.

Pasque flower (Pulsatilla vulgaris) – a species also thriving on the restored grassland.(Photo Five)

Photo Credits

Photo One by PJC&Co, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Photo Two by Marcin Sielezniew from https://www.ukbutterflies.co.uk/species.php?species=arion

Photo Three By Ivar Leidus – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50343985

Photo Four By Charles J. Sharp – Own work, from Sharp Photography, sharpphotography, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49619221

Photo Five by By Bernard DUPONT from FRANCE – Pasque Flower (Pulsatilla vulgaris), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=40724485