Changes in cold tolerance during the overwintering period in Apis mellifera ligustica

Winter is the honey bee's non-productive season. The colony's main task during the winter is to withstand the cold and to reduce bee losses. Losses during the winter can restrict the colony reproduction in spring, even more the production in the next whole year. Thus, the evaluation of the colony's overwinter ability by studying its cold-hardiness is crucial. The goal of this study was to determine the variation of the cold tolerance during the overwintering period and the major physiological responses to the cold climate. Samples were collected from the late October 2016 to March 2017 in Taian, China. The super-cooling points (SCPs), which ranged from –7.41 to –6.49 °C, were lower than the average daily lowest temperature (–4 °C) in January. The protein content first increased and then decreased during winter. Glycogen is a major energy source, and its content level declined sharply in the middle of overwintering. Glycerol and some low-molecular weight sugars and polyols (mannitol, sorbitol, and trehalose) were found to serve as the primary cryoprotectants for overwintering bees, as their contents accumulated significantly during the winter (P<0.0001). For the first time, this study investigated antifreeze protein, protein lethal (2) essential for life-like (l(2)efl), and vitellogenin protein (Vg) mRNA expression pattern in the overwintering period. The results indicate that seasonal super-cooling capacity is assisted by the accumulation of a complex mixture of winter specific metabolites. Our findings may help in integrated feeding and management of honey bees to increase winter survival.