Competition between Bombus terrestris and Osmia cornuta and its effects on their foraging behavior in indoor facilities

Summary

In recent years, the large earth bumble bee (Bombus terrestris) and the horned mason bee (Osmia cornuta) have been increasingly used as managed pollinators to support and enhance pollination efforts in agriculture. In particular, B. terrestris and O. cornuta are frequently used in early flowering orchards, as they already fly at low temperatures. This master thesis investigates if it is possible to rear these two bee species in indoor facilities and whether competition affects their foraging behavior. The experiments were conducted in 12 flight cages, supplied ad libitum with pollen and nectar, and O. cornuta cages were equipped with nesting boxes for egg-laying as well as soil mixtures for nest building. Resource utilization of pollen and nectar, weight changes, survival, and reproduction rates were recorded to evaluate the feasibility of indoor rearing. The bees were also offered black mustard (Brassica nigra) flowers for a behavioral observation experiment. To investigate intra- and interspecific competition the bees were either observed in scenarios where a single species was present in a cage or two species were present. The behavioral observations were recorded with the software Observer XT on pollen dishes, nectar dispensers, and B. nigra flowers. Data analysis of the indoor rearing showed that both B. terrestris and O. cornuta bees consumed the provided pollen and nectar. However, O. cornuta did not produce any offspring during this study despite the observed mating activity between individuals. Furthermore, O. cornuta did not collect any soil mixtures for nest construction. From this finding, it is clear that B. terrestris and O. cornuta can be kept in indoor facilities, but ultimately the challenge is to identify the factors that influence reproductive success for O. cornuta, such as environmental conditions and resource quality. The findings of the behavioral assessments indicated that visitation rates and durations of visits were similar for both bee species when exposed to pollen dishes and nectar dispensers in both competitive and non-competitive scenarios. However, whereas B. terrestris reacted similarly when B. nigra flowers were present in both scenarios, O. cornuta showed different behavior. Interestingly, in the competitive setting O. cornuta visited flowers more frequently but spent less time on the flowers compared with when these bees were among their conspecifics. Overall, this study provides insights into the competitive behavior of B. terrestris and O. cornuta bees and their implications for foraging within indoor facilities with climate conditions. Understanding competitive interactions will facilitate the development of management strategies for conserving and managing bee populations, thereby safeguarding their vital contributions to agricultural and natural ecosystems.