How does the large blue caterpillar use ants?

After several instars the caterpillar drops to the ground and mimics the scent and behavior of a red ant larva (*Myrmica*). Ants "adopt" it and carry it into the nest, where it overwinters and feeds on ant larvae. In spring it pupates inside the nest and emerges as an adult.

Why is the large blue disappearing in Europe?

It requires both thyme (food plant of early caterpillars) and colonies of the red ant *Myrmica sabuleti*. When grazing changes, grass grows too tall and shades thyme — thyme disappears, *Myrmica* disappears, and so does the large blue. In Britain it went extinct in 1979 and was successfully reintroduced.

Butterfly Atlas

Phengaris arion

Large Blue

~1 min

The large blue is a big blue butterfly with an unusual life cycle: the caterpillar parasitizes colonies of red ants. A protected species.

Key facts

Latin name: Phengaris arion
Family: Lycaenidae
Wingspan: 30-44 mm
Flight season: June — August
Host plants: Wild thyme, Oregano
Conservation status: NTNear Threatened

Appearance

The large blue (Phengaris arion) is the largest blue in Russia with a wingspan of 30–44 mm. Males: upper side of the wings bluish gray with a row of black rounded spots on the forewing. Females are somewhat darker.

The underside of the wings is gray-brown with black spots and white rings. A white fringe runs along the wing margins.

Range and habitat

Found in Europe and temperate Asia. In Russia in the European part and Siberia.

Strictly tied to limestone meadow slopes and clearings with thyme and colonies of Myrmica ants. It requires a specific combination of conditions.

Unique life cycle

Early caterpillars feed on thyme flowers. In the fourth instar the caterpillar drops to the ground. Encountered Myrmica sabuleti ants "adopt" it — it releases substances mimicking chemical signals of ant larvae. In the nest the caterpillar overwinters, feeding on ant larvae, and pupates inside the colony.

Conservation

No global IUCN assessment; the 2025 European Red List treats it as NT (Near Threatened), not global VU. Not on the 2020 federal Red Data Book of Russia; protected in regional books and under the Habitats Directive. The main cause of decline is cessation of moderate grazing, leading to overgrowth of thyme clearings.