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Commercially reared bumblebees are important pollinators of agricultural crops. Biobest Group NV was the first company to commercially rear bumblebees for pollination purposes and constantly aims to optimize the pollination services that are provided by their bumblebee colonies. One potential threat to the bumblebees’ pollination capacity is drifting, which is the behavior whereby an individual leaves its natal colony and enters a conspecific foreign one. This can lead to a broad range of negative consequences for the drifter’s natal colony, as well as for the host colony, and the grower or farmer who is using the bumblebee colonies for pollination purposes. For example, some colonies experience a reduction in the number of workers due to workers drifting out of the colony, which can negatively impact the colony’s survival chances and pollination capacity. Moreover, drifting can cause horizontal disease and parasite transfer between colonies. Drifting can be an accidental behavior, meaning that a worker unintentionally enters a foreign colony due to orientation errors. Therefore, the amount of drift is considerably higher in artificial settings, such as greenhouses, where unnaturally high densities of colonies are present and which lack visual cues to aid in orientation, than in wild colonies. However, more recent studies found that some drifters lay eggs in foreign colonies, and that drifters produce significantly more male offspring than native bees from the host colony, indicating that drifting might be not a purely accidental behavior. The aim of this master thesis study was to investigate whether proximal visual cues (hive figures) can reduce the amount of drifting of commercial Bombus terrestris colonies, and to get more insights into the drivers of bumblebee drifting: is it an accidental behavior resulting from orientation errors, or do workers drift intentionally? To investigate this, B. terrestris workers were individually marked using numbered tags in a colony-specific color and the presence of drifters inside the colonies was determined by performing observations inside the colonies. The amount of drift was then compared between a control treatment where no hive figures were present, and two figure treatments. Moreover, to gain more insights into the drivers of bumblebee drift, natives (i.e., workers who never drifted) and drifters were dissected. The ovarian development was then compared between natives and drifters to evaluate whether they differ in the probability of being egg layers. We found that the use of geometric figures can lower the amount of drifting, which indicates that drifting has an important accidental aspect. However, we also found that resident drifters had a higher probability of having stage IV ovaries and more mature eggs inside their ovaries compared to natives, indicating that drifting was most likely not a purely accidental behavior and that at least some residents may have drifted intentionally to reproduce in foreign colonies. The results thus showed that drifting of commercial bumblebees is likely not a purely accidental or purely intentional phenomenon, but rather a combination of both. Our study thus shows that hive figures can be used to reduce the amount of drift of commercial bumblebees. More research is needed regarding the effects of drifting on the colonies’ pollination capacities to assess whether the use of hive figures will be economically interesting.
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