Geographic and Intraspecific Variation of Cold Hardiness in Ants of the Genus Lasius Fabricius, 1804 (Hymenoptera, Formicidae)

As a model species to study the geographic variation of cold hardiness in insects, we use ants Lasius flavus from populations of territories separated by thousands of kilometers (southern Finland, the Moscow region, and Amur oblast) and with contrasting climates, from maritime variants to climates differing in degrees of continentality. We measure supercooling points (SCPs) and the temperature at which 50% of the sample perishes (LT_50%) and evaluate overwintering conditions. The data on L. flavus are compared to similar data obtained on L. niger, аs well as to similar measurements in separate nests of L. alienus, L. psammophilus, and L. fuliginosus, which were published earlier. The workers of L. flavus and L. niger have similar values of cold hardiness (LT_50% from –13 to –15°C, minimum average SCP from –24 to –25°C), although these species inhabit different climatic zones and arrange their overwintering chambers at different depths. Such resistance to long-term exposure to negative temperatures is more than sufficient for overwintering, even in the coldest of the studied areas (village of Arkhara, Amur oblast), where the average minimum temperature in January in the ant overwintering chambers at a depth of 50 cm does not fall below –6°C. The variation in cold hardiness and higher values of its parameters in other studied ant species are due to their insufficient preparedness for overwintering. However, the narrow range of SCP values (from –27 to –29°C) in the most cold-resistant individuals of all studied species strongly suggests that their average SCP and LT_50% values are similar to those of L. flavus and L. niger. The similar (or perhaps identical) cold hardiness of the studied ant species appears to be a stable trait of the genus Lasius, as was demonstrated for another ant genus, Myrmica, and which is dissimilar to the genus Formica, in which different species have individual characteristics of cold-resistance. Excessive for overwintering in the warmer climate (southern Finland and Estonia), such cold hardiness appears to have no adaptive value. It can be considered a byproduct of diapause, which is manifested in preadaptation to withstand negative temperature. However, it was this cold hardiness that allowed the ant species to colonize the vast territory of southern Siberia and the Far East.