ow do the sterol requirements of bee species affect their pollen foraging choices?

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BackgroundIn recent decades there has been an observed global decline in many insect groups with bees experiencing some of the greatest losses as a result of pesticide usage on crops, spread of disease, parasites and changes in land use(Potts et al., 2016).Loss of floral food resources, such as pollen and nectar, has also been a suggested explanation for declines, with bee population trends mirroring those of their floral hosts (Biesmeijer et al., 2006; Kleijn and Raemakers, 2008).Bees are entirely dependent on flowers for food with adults consuming nectar for energy and pollen providing the main food source for larvae. The quality and type of these resources consequently has a direct impact on growth and survival (Vanderplanck et al., 2014; Moermanet al., 2017)and changes in host availability directly affect bee abundance and distribution. Approximately 75%ofcrop species are dependent on anima pollination with bees providing the largest share (Klein et al., 2007; Rader et al., 2016). Successful maintenance of this service is strongly reliant on our understanding of bees' nutritional needs which remains poor. We do however know that larval development and survival can be greatly reduced when only poor quality pollen is available (Genissel et al., 2002; Moerman et al., 2017). Quantifying bee nutritiona lrequirements and the provisioning abilities of different plant species therefore has clear applications in the improvement of pollinator planting schemes in resource scarce environments and will also be key to successful bee conservation efforts. Aims and objectives Bees are able to selectively forage for nutritionally preferred pollen (Somme et al., 2015; Vaudo et al., 2016; Vanderplanck etal., 2019)and visit a more targeted range of plants for pollen compared to nectar (Goulson, D. and Dar, 2004; Goulson et al., 2005).Pollen containsa range of important dietary amino acids, lipids, vitamins and proteins for bees that vary across species (Roulston and Cane, 2000)but sterols in particular have high intraspecies variation (Villette et al., 2015)and can be highly specific to the bees that collect them (Moerman et al., 2017). Bees are unable to synthesise their own sterols and dietary sources must therefore fulfil all their physiological requirements(BehmerandNes,2003; Roulston and Cane, 2000). Pollen choices could be, as a consequence, constrained by these requirements, providing a potential explanation for narrow or in flexible foraging strategies in different species. Taxon specific nutrient requirements and changes in plant distribution and abundance could therefore be a key variable driving fluctuations in certain pollinator populations. There are major knowledge gaps regarding the role of pollen sterols in bee diversity and abundance. We do not know the sterol requirements of most bee species and our knowledge of the diversity of sterols found in pollen is also critically lacking. My work will address this deficit by generating sterol profiles for a novel selection of both bee and plant species found in the UK to answer the following questions:Can the distribution of pollen sterol resources in the UK help explain patterns of bee diversity and abundance?How do sterol nutritional requirements and foraging choices differ across bumble bee species?Do recommended pollinator-targeted seed mixes provide the pollen nutritional diversity required to support diverse bee communities in the UK?Do pollen specialist bee species show conserved sterol selection across different floral landscapes?