Quantitative evidence for spatial variation in the biennial life cycle of the mountain butterfly Erebia euryale (Lepidoptera: Nymphalidae) in the Czech Republic

Erebia euryale (Esper, 1805) is a montane-zone representative of a Holarctic butterfly genus the species of which occur mainly in alpine areas. As in many mountain insects, E. euryale frequently undergoes a prolonged biennial larval development, which can be synchronised across mountains, resulting in prominent biennial peaks in adult emergence. However, the extant reports are often contradictory, suggesting that populations are not synchronized. We present the first quantitative assessment of the situation in the Czech Republic, based on five years of monitoring adults along transects in three major mountain systems in this country. We detected a two order magnitude difference in biennial fluctuations in adult abundance, with peaks in even years (i.e. 2010, 2012) in the Sumava Mts (southwest Czech Republic). There were less distinct odd year (i.e. 2009, 2011, 2013) peaks in the Hrubý Jesenik Mts (northeast) and no fluctuations from year to year in the number of adults recorded in the Krkonose Mts (north). Although the mechanisms behind these patterns remain unknown, we hypothesize that rugged terrain desynchronises and flat terrain synchronises the length of develop - ment of E. euryale. Finally, the different periodicity in the fluctuations in individual mountain ranges is hypothesized to be affected by interactions with larval parasitoids or reflect the different postglacial histories of respective populations.