Abstract
In order to save resources, honey bee (Apis mellifera) colonies in the temperate zones stop brood rearing during winter. Brood rearing is resumed in late winter to build up a sufficient worker force that allows to exploit floral resources in upcoming spring. The timing of brood onset in hibernating colonies is crucial and a premature brood onset could lead to an early depletion of energy reservoirs. However, the mechanisms underlying the timing of brood onset and potential risks of mistiming in the course of ongoing climate change are not well understood. To assess the relative importance of ambient temperature and photoperiod as potential regulating factors for brood rearing activity in hibernating colonies, we overwintered 24 honey bee colonies within environmental chambers. The colonies were assigned to two different temperature treatments and three different photoperiod treatments to disentangle the individual and interacting effects of temperature and photoperiod. Tracking in-hive temperature as indicator for brood rearing activity revealed that increasing ambient temperature triggered brood onset. Under cold conditions, photoperiod alone did not affect brood onset, but the light regime altered the impact of higher ambient temperature on brood rearing activity. Further the number of brood rearing colonies increased with elapsed time which suggests the involvement of an internal clock. We conclude that timing of brood onset in late winter is mainly driven by temperature but modulated by photoperiod. Climate warming might change the interplay of these factors and result in mismatches of brood phenology and environmental conditions.
Highlights
The timing of life-history events, such as flowering in plants, insect emergence, and reproduction, with respect to the changing abiotic and biotic conditions of the environment is critical for most organisms (Van Asch & Visser, 2007; Visser, Both & Lambrechts, 2004)
Variability of comb temperature Stability of comb temperature and mean ambient temperature had interacting effects on mean comb temperature measured in the winter cluster
While interacting effects of different abiotic conditions could help to minimize the risk of premature brood onset, our results suggest that increasing winter temperatures and more frequent spells of warm weather due to global climate change could result in advanced timing of brood onset
Summary
The timing of life-history events, such as flowering in plants, insect emergence, and reproduction, with respect to the changing abiotic and biotic conditions of the environment is critical for most organisms (Van Asch & Visser, 2007; Visser, Both & Lambrechts, 2004). Endothermic mammals generally keep their body temperature actively above ambient temperature, but often go into a state of reduced metabolism, i.e., hibernation or daily torpor, to reduce energy expenditure and tend not to reproduce during winter (Kortner & Geiser, 2000). Due to their capability of social thermoregulation, honey bees (Apis mellifera L.) are able to maintain colonies over the whole year (Jones & Oldroyd, 2006), using a strategy analogous to hibernation in mammals. Increased risk of predation and starvation are hazarded in order to reproduce early so that the offspring has sufficient time to develop and build up resource storages or fat-tissue before the winter (Kortner & Geiser, 2000; Meyer, Senulis & Reinartz, 2016)
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