The host plant (see photos) of the Scarce Large Blue (Phengaris teleius) is the Great Burnet (Sanguisorba officinalis).
The female prefers low, young specimens, where she deposits her eggs in July and August.
Once the caterpillars have hatched, they only eat the plant for three to four weeks, especially the flowers.
Then the caterpillar leaves the host plant to be picked up by ants at the bottom.
These ants bring the caterpillars to their nest.
The caterpillars of ant larvae live in the ant's nest.
The butterflies are homebound and are common near areas where Sanguisorba officinalis is found in high densities.
They settle almost exclusively on Sanguisorba: when disturbed they fly mostly only as far as the nearest cluster of that plant, where they settle on a flower with the wings always closed, their flight being rather slow and flapping.
The wingspan of the butterfly is 34 to 38 millimeters.
The female is a lighter blue than the male.

Ruggeler Riet Nature Reserve, Ruggell, Liechtenstein, 7 August 2020