Has the Tortoiseshell Butterfly, Aglais urticae (Lepidoptera, Nymphalidae), Shifted to Synanthropy in Northeastern Asia?

An assumed shift of the tortoiseshell butterfly, Aglais urticae, to synanthropy in extremely cold regions (with temperatures close to –60°C) is tested. The tortoiseshell butterfly winters at the stage of imago; their cold resistance is fundamentally insufficient for surviving the cold across most of their distribution range in natural shelters located above the snow level and in unheated buildings. The resistance of the species to cold from England to northeastern Russia is increased from –22 to –29°C, while the average minimum air temperature in January drops from 3.3 to –55.7°C. The objective of the present work was to find out whether well-built, but unheated houses (village of Seymchan) provide winter indoor temperatures above –30°C, a threshold for the tortoiseshell butterfly. Winter temperatures were measured with loggers in two different houses and compared with the outdoor temperature obtained from weather station data. During severe cold weather, it was 11–19°C warmer indoors than outdoors, but minimum temperatures below –30°C were recorded over half of the winter, and below –35°C, over almost a third of it. Thus, winter (unheated) houses, as well as demi-seasonal ones, and natural shelters such as cracks under the bark, hollows, and other cavities located above the snow cover fail to provide the required temperatures. Unheated well-built houses guarantee successful hibernation for the tortoiseshell butterfly only in the milder climatic conditions of western Siberia and in the northeast European part of Russia. Wintering above the snow level in primitive shelters can only be possible within the temperature range of –31 to –20°C and west of it. The results of this work prove the idea of tortoiseshell butterfly synanthropy to be unsubstantiated and return to the idea of its wintering under the snow cover in shelters such as cavities in litter, under fallen trees, etc. This type of wintering guarantees success, being independent from the climate of northern Eurasia.