Relationship between the butterfly Phengaris rebeli and its larval host plant Gentiana cruciata in Lithuanian population

Relationship between the endangered herbaceous plant Gentiana cruciata and its specialist herbivore, the endangered butterfly Alcon Blue, Phengaris rebeli, was studied in Lithuanian population which is the northernmost one over the whole known species range in Europe. The size of P. rebeli population was estimated bas-ing on eggshell counts on the larval host plant. Oviposition behaviour of P. rebeli was investigated during summer season of 2011 on a grassland area in Neris Re-gional Park, Lithuania. Potential host plants were checked and eggshells of P. re-beli were counted. A total of 619 eggshells were counted on 248 out of 536 G. cru-ciata stems checked. Selective choice of both host plants and site for oviposition on host plant was revealed. Approximately 89 % eggs were laid on the organs of the uppermost whorl. Plants selected by females for oviposition almost always had a luxuriant growth and most of them were higher than the surrounding vegetation. Key words: Alcon Blue, Cross Gentian, egg deposition, Neris Regional Par