The larvae feed on Onobrychis species.
Polyommatus thersites inhabits nutrient-poor meadows with the larval host plant. In the past the butterlie occurred also in agricultural meadows, but these are today over-fertilized and mowed too intense than that Onobrychis or even the butterfly still might exist in them.
Polyommatus thersites flies in two or three generations from April to October. I found adults of the second generation in the middle of August in the Susa Valley at 900 meters above sea level on extensive meadows and in early May in Northern Greece those of the first generation in grazed grasslands. The caterpillar overwinters. I observed fully-grown caterpillars using the characteristic feeding scars (cuticle remains) in the end of April in the Valais in a nutrient-poor meadow preferred at gappy places hiding at daytime at the base of Onobrychis clumps. The eggs are usually laid on leaves.
Endangerment: strongly endangered
Polyommatus thersites is very rare north of the Alps and occurs locally e.g. at the northern edge of the Swabian Alb or on the Franconian Alb. The butterfly is threatened by habitat loss (succession, but also strong grazing, overbuilding, intensification and eutrophication).
Polyommatus thersites often flies with Polyommatus damon and always together with P. icarus. The overall distribution ranges from Morocco across southern and parts of Central Europe to the east as far as the Tien Shan.