EN
A population of the endangered butterfly Marsh Fritillary Euphydryas aurinia, using exclusively Gentiana cruciata as a larval food plant, was recorded for the first time in the valley of the Dukšta river (Vilnius administrative district) in Lithuania. Caterpillars were observed both in summer when they lived gregariously in numerous webs spun on gentians, and in spring when they were feeding on new shoots or basking in the sun. The butterfly is considered as dependent on Succissa pratensis in Central and Northern Europe, therefore our finding is unexpected. Other plants including other Gentiana species are reported as locally used only in Southern Germany, Switzerland and Spain. Phengaris‘rebeli‘ a well known butterfly related to G. cruciata occured sympatrically with E. aurinia at the Lithuanian site.