Are sterols landscape limiting nutrients for wild bees in the UK?

Sterols are essential nutrients for insects (e.g., for membrane fluidity, hormones and physiological functions). Sterols in plant vegetative tissues predominantly comprise two sterols; beta-sitosterol and stigmasterol. However, in pollen - from which bees obtain their nutrient provision - sterols are structurally diverse and highly variable. Our pilot data show striking variations among 25 pollen sterols across more than 100 plants representing over 50 families sampled from a global collection at Kew with specific sterol groups occurring in closely related plants. Research on the nutritional requirements of honey bees - the only well-studied bee species in this respect - indicates that they cannot synthesise sterols and that their tissue sterol composition is determined by the specific sterols in their diet. Thus, in terms of sterols bees are what they eat. We also know from existing research on artificial diets that honey bee larval growth and survival is highly sensitive to their dietary sterols. For example, honey bees require 24-methylenecholesterol (24MC) for brood rearing and their development is improved significantly when diets are supplemented with this sterol. Our pilot data shows that 24MC is abundant in royal jelly provisioned to larvae and in a range of plant pollens popular with honey bees e.g., in the Rosaceae but is absent from many plant families including Asteraceae which is avoided for pollen by honey bees suggesting sterol composition could influence pollen choice in bees.
However, almost nothing is known about the nutritional needs of wild bee species, especially sterols, but our pilot work shows that the sterol profiles of bees vary dramatically across different species but closely match the sterol profiles of the pollen collected by these bees for food. For example, the sterols from ivy flower pollen (Hedera helix) are highly similar to those of the ivy bee (Colletes hederae), which forages exclusively on ivy for pollen. Furthermore, while the sterols of ivy bees and ivy pollen match they differ dramatically from sterols of bumble bees which have sterols matching the pollen they collect. These in turn differ from the sterols of red mason bees (Osmia bicornis). This close association between pollen and bee sterol chemistry strongly suggests that these other bees use the specific sterols in the pollen they collect. We do not currently know whether these differences reflect differences in sterol tolerances or differences in sterol requirements. This project will test this for the first time. Either way, these differences would provide a mechanistic basis for bee population and community dynamics, and thus also a valuable tool in conserving diverse bee communities and the ecosystem services they provide. We will undertake the first comprehensive assessment of sterol nutrients in pollen of UK plants and in UK wild bees. Using these data, we will identify key plant species that can support the greatest diversity of wild bee species. These data will be integrated into landscape scale models of pollen resources to inform pollen sterol provision in the UK. Landscape-scale modelling based on measurements of plant distributions will be used to estimate the capacity of the landscape to support pollinator populations, taking a major step forward from existing assessments on nectar and carbohydrates.
We will match these data with assessments of wild bee species to determine how important specific pollen sterols are for specific bees. We will conduct the first ever assessment of how sterols influence development in wild bees to understand fully the potential cost of sterol limitations in the landscape, how dependent bee species are on specific sterols and how this influences specialisation in wild bees. Our data will inform delivery of the UK government's 25 Year Environment Plan and National Pollinator Strategy and wider initiatives to renew floral landscapes in support of bees.