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Kelly’s Field Notes
Common Name: Large blue butterfly
Order: Lepidoptera
Family: Lycaenidae
Genus: Phengaris
Species: arion
Description:
Larvae (caterpillar):
Upon hatching, caterpillars are fuzzy and a mottled red and white in color. They feed on flower buds until ready to drop to the ground and molt into a new shape - a mimic of the saber-thorned knot ant (Myrmica sabuleti) and the wet turf knot ant (Myrmica salina) larvae (maggots) (Defranceschi, 2023). The caterpillars inflate their bodies to resemble maggots and even mimic the noises the ant maggots make. They also secrete chemicals that are attractive to these two species of ants (Defranceschi, 2023). More on what happens next under the Life Cycle section.
Adults:
Adults have a wingspan of about 3.3–3.9 cm (1.3–1.5 in). The dorsal (upper) side of male wings are silvery blue in color while females have dark grey or brown coloration. The ventral (lower) side of both sexes are a pale brown color with black spots. Their fuzzy bodies tend to match their dorsal wing color.
This species was declared extinct in the U.K. in 1979, but was reintroduced by researchers between 1983 and 1985. It is still highly endangered there and in parts of Europe, having experienced >90% losses between 1990 ans 2010 (Saccone, 2024).
Life Cycle:
Eggs are laid on the leaves of wild thyme or marjoram. After they hatch (5-10 days after being laid), the caterpillars feed on the flower buds for about 2-3 weeks. While females can lay 50 eggs at a time they often deposit one per flower bud (ButterflyConservation.org, 2024).
After feeding enough to molt into the ant mimic larva, they wait for the ants to pick them up and take them back to the ant nest. The ants feed caterpillars with sugar water. Though the caterpillars can also eat up to 600 ant larvae during their stay (Defranceschi, 2023). The caterpillar will pupate in the ant nest, but there is still danger. Not all ants may be fooled by the pleasing scent of the pupae and it may be destroyed. Additionally, if the caterpillar has eaten all of the ant larvae it may starve before it reaches the pupal stage. Once the butterfly emerges from the pupa it is no longer scent protected and must leave the nest as quickly as possible before it is killed. It doesn’t even dry its wings or stretch before leaving, haste is too important (Defranceschi, 2023).
Reintroduction of the Large Blue:
The largest-ever reintroduction of the globally endangered large blue butterfly in the UK has been declared a success after the insects bred in their first year at Rodborough and Minchinhampton Commons in Gloucestershire. About 1,100 larvae were released and an estimated 750 butterflies successfully emerged. They are now found in 33 sites in the south-west of England (UK Center for Ecology & Hydrology, 2024).
The decline of butterflies was due to change in agricultural practices, but maybe not for the reasons you are thinking of as you read this! Although four species of red ants (Myrmica) would carry caterpillars into their nests, the larvae survived only with Myrmica sabuleti. Other ant species quickly recognized and killed the caterpillars, as the caterpillar’s various forms of mimicry only worked on the one species.
Ecologist Jeremy Thomas found that each ant species occupied a distinct temperature niche, largely controlled by turf height. Surveys of former large blue butterfly sites showed that while thyme and red ants were often still present, Myrmica sabuleti was rare or absent.
The cause of differences in ant species populations were due to changes in agricultural practices. Mainly, reduced livestock grazing and the loss of rabbits due to myxomatosis. When we lose herbivores, grass grows taller, cooling the soil and making conditions unsuitable for Myrmica sabuleti (UK Center for Ecology & Hydrology, 2024).
The solution was cows. Small temporary grazing areas were created within conservation spaces using electric fences, to allow cows and long-horn cattle to graze slopes. A programme of scrub control was also carried out. Short grasses and fewer shrubs allow sunlight to warm the soil so ants can actively forage and hunt given invertebrate dependence on environmental heat to keep them warm (Beament, 2020). It is also very difficult to maintain ant nest in captivity, so providing suitable habitat to foster growth of host ant populations was crucial to the comeback of the butterfly (UK Center for Ecology & Hydrology, 2024).
Super Powers:
Visual Mimicry - larvae mimic host ants visually and secrete a pleasing scent that may be similar to the ants’ scent.
Auditory Mimicry - larvae emit frequencies that attract host ants and ask them to be fed once inside the nest.
Infiltration - larvae are escorted into host ant nests and taken care of until emergence from pupae.
Chemical Calm - larvae emit chemical cues that deter host ants from attacking them.
Flight
BBC Youtube Link - https://www.youtube.com/watch?v=4JoEWdV7tpQ&t=1s
References:
Beament, Emily. “Endangered Large Blue Butterfly Reintroduced.” The Ecologist, 13 Aug. 2020, theecologist.org/2020/aug/13/endangered-large-blue-butterfly-reintroduced. Accessed 30 Jan. 2026.
Defranceschi, Laura. “The Trojan Horse of the Butterflies.” EURAC Research, Eurac Research Magazine, 11 Jan. 2023, www.eurac.edu/en/magazine/the-trojan-horse-of-the-butterflies. Accessed 29 Jan. 2026.
“Large Blue Butterfly.” UK Centre for Ecology & Hydrology, 2024, www.ceh.ac.uk/our-science/case-studies/case-study-large-blue-butterfly.
“Saccone, Bianca Maria. “Insect Conservation Team Publish New Data on Endangered Large Blue Butterfly - Journal Highlight - Royal Entomological Society.” Royal Entomological Society, 30 Sept. 2024, www.royensoc.co.uk/news/insect-conservation-team-publish-new-data-on-endangered-large-blue-butterfly/.
Species of the Month: Large Blue.” Butterfly-Conservation.org, 17 June 2024, butterfly-conservation.org/news-and-blog/species-of-the-month-large-blue.
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