The timing of oviposition and selection of the phenological stage of the host plant can have significant consequences for development and success of offspring, and is particularly important for endangered specialist species with rare habitats, such as Phengaris alcon butterflies. Females of this species oviposit on marsh gentians, Gentiana pneumonanthe. For the first time, we evaluate the survival of eggs deposited by early and late flyers in relation to the phenological stage of marsh gentian flower buds, as well as caterpillar survival and development. An analysis was conducted on 127 gentian shoots, on which 837 eggs were monitored. We observed more frequent oviposition on the apical and youngest buds, with increased egg load by females during the first one-third of the flight period. Offspring survival of about 55 % was observed, with up to 15 caterpillars per bud. Offspring survival was significantly higher from eggs that were oviposited on larger flower buds and on flower buds in an early developmental stage. Also, early flyers’ offspring gave rise to better survival rates and the caterpillar development in flower buds differed significantly according to bud size, with more days required in smaller buds. Finally, the significant differences found across the entire study period illustrate that the understanding of oviposition through time is important to the conservation of this rare European species.