Effects of temperature and photoperiod on the seasonal timing of Western honey bee colonies and an early spring flowering plant.

Temperature and photoperiod are important Zeitgebers for plants and pollinators to synchronize growth and reproduction with suitable environmental conditions and their mutualistic interaction partners. Global warming can disturb this temporal synchronization since interacting species may respond differently to new combinations of photoperiod and temperature under future climates, but experimental studies on the potential phenological responses of plants and pollinators are lacking. We simulated current and future combinations of temperature and photoperiod to assess effects on the overwintering and spring phenology of an early flowering plant species (Crocus sieberi) and the Western honey bee (Apis mellifera). We could show that increased mean temperatures in winter and early spring advanced the flowering phenology of C. sieberi and intensified brood rearing activity of A. mellifera but did not advance their brood rearing activity. Flowering phenology of C. sieberi also relied on photoperiod, while brood rearing activity of A. mellifera did not. The results confirm that increases in temperature can induce changes in phenological responses and suggest that photoperiod can also play a critical role in these responses, with currently unknown consequences for real-world ecosystems in a warming climate.

Honey bee waggle dance communication increases diversity of pollen diets in intensively managed agricultural landscapes.

The benefits of honey bee dance communication for colony performance in different resource environments are still not well understood. Here, we test the hypothesis that directional dance communication enables honey bee colonies to maintain a diverse pollen diet, especially in landscapes with low resource diversity. To test this hypothesis, we placed 24 Apis mellifera L. colonies with either intact or experimentally disrupted dance communication in eight agricultural landscapes that differed in the diversity of flowering plants and in the dominance of mass-flowering crops. Pollen from incoming foragers was collected and identified via DNA metabarcoding. Disrupting dance communication affected the way the diversity of honey bee pollen diets was impacted by the dominance of mass-flowering crops in available flower resources (p = .04). With increasing dominance of mass-flowering crops in resource environments, foragers of colonies with intact communication foraged on an increasing proportion of available plant genera (p = .01). This was not the case for colonies with disrupted dance communication (p = .5). We conclude that the honey bee dance communication benefits pollen foraging on diverse plant resources and thereby contributes to high quality nutrition in environments with low-resource diversity.

Seasonal timing in honey bee colonies: phenology shifts affect honey stores and varroa infestation levels.

Increasingly frequent warm periods during winter, which are associated with climate change, may cause mismatches between the colony phenology of the western honey bee, Apis mellifera L., and their floral resources. Warmer winter periods can also affect colony brood rearing activity and consequently the reproduction of the invasive brood parasite Varroa destructor Anderson and Trueman. Until now little is known about the effects of climate change on biotic interactions in such a multitrophic system comprising flowering plants, a pollinator, and its parasite. We performed a reciprocal translocation experiment with honey bee colonies to simulate climate change-induced phenology shifts. Honey bee brood phenology was highly sensitive to environmental conditions in late winter. Colonies in which phenology was experimentally delayed had smaller worker populations in early spring and reduced amounts of stored honey during the following months. During summer, the varroa load in colonies with non-shifted phenology was three times higher than in colonies with delayed phenology. High varroa loads during summer were negatively correlated with worker population growth. Despite a remarkable resilience of colony development to phenology shifts, our results show that mismatches between the phenology of honey bee colonies and flowering plants can affect the build-up of resource stores. Further, an advanced onset of brood rearing activity after hibernation can reinforce the negative impact of the brood parasite V. destructor. We conclude that trade-offs between synchronisation with earlier flower phenology and prolonged brood phases with build-up of varroa populations might constrain the honey bees' capability to adapt to climate warming.

The influence of temperature and photoperiod on the timing of brood onset in hibernating honey bee colonies.

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.

Combined effects of waggle dance communication and landscape heterogeneity on nectar and pollen uptake in honey bee colonies.

The instructive component of waggle dance communication has been shown to increase resource uptake of Apis mellifera colonies in highly heterogeneous resource environments, but an assessment of its relevance in temperate landscapes with different levels of resource heterogeneity is currently lacking. We hypothesized that the advertisement of resource locations via dance communication would be most relevant in highly heterogeneous landscapes with large spatial variation of floral resources. To test our hypothesis, we placed 24 Apis mellifera colonies with either disrupted or unimpaired instructive component of dance communication in eight Central European agricultural landscapes that differed in heterogeneity and resource availability. We monitored colony weight change and pollen harvest as measure of foraging success. Dance disruption did not significantly alter colony weight change, but decreased pollen harvest compared to the communicating colonies by 40%. There was no general effect of resource availability on nectar or pollen foraging success, but the effect of landscape heterogeneity on nectar uptake was stronger when resource availability was high. In contrast to our hypothesis, the effects of disrupted bee communication on nectar and pollen foraging success were not stronger in landscapes with heterogeneous compared to homogenous resource environments. Our results indicate that in temperate regions intra-colonial communication of resource locations benefits pollen foraging more than nectar foraging, irrespective of landscape heterogeneity. We conclude that the so far largely unexplored role of dance communication in pollen foraging requires further consideration as pollen is a crucial resource for colony development and health.