Climatic, biogeographic and ecological factors influence the spatial patterns of odd‐ and even‐year dominance in biennial European butterflies and burnet moths

Some insect species, particularly in far northern or high‐mountain habitats, require 2 years or more to complete their larval development (biennial species). This can produce still poorly understood spatial patterns of odd‐ or even‐year dominance. Taking the example of Lepidoptera, this contribution investigates the climatic, biogeographic and ecological factors influencing the spatial annual patterns of biennial European species across large areas and multiple genera, mapping their distribution through the proportion of odd‐year observations, spatial autocorrelation and links to available mitochondrial barcode sequence data. Over 250,000 observations of 22 species (7 annual, 11 biennial, 4 flexible) from the Alps and 157,873 observations of 5 biennial species from Scandinavia are evaluated in detail, alongside approximately 360,000 further observations from remaining Europe. At a continental scale, biennial species occur in both high and low mountain ranges, with different species often displaying similar or identical patterns. In the Alps, eastern populations are predominantly odd‐dominant, hinting at stronger interglacial connectivity, while smaller‐scale western patterns likely reflect distinct glacial microrefugia. Similarly, large‐scale east–west differences in Scandinavia suggest post‐glacial repopulation from separate refugia. The observed patterns probably reflect the establishment of odd‐ or even‐dominant populations in different glacial refugia, shaped by various post‐glacial recolonisation routes. Altitude‐related variation in development time, occasional three‐year development under harsh conditions, climate disturbances, parasitoids and contact‐zone diffusion probably locally modified these patterns. Current distributions might thus reveal multiple temporal layers: early post‐glacial immigration, later smaller‐scale movements and very recent local influences.