More than mesolectic: Characterizing the nutritional niche of Osmia cornifrons

Characterizing the nutritional needs of wild bee species is an essential
step to better understanding bee biology and providing suitable
supplemental forage for at-risk species. Here, we aim to characterize
the nutritional needs of a model solitary bee species, Osmia
cornifrons (Radoszkowski), by using dietary protein-to-lipid ratio (P:L
ratio) as a proxy for nutritional niche and niche breadth. We first
identified the mean target P:L ratio (~3.02:1) and P:L collection range
(0.75 to 6.26:1) from pollen provisions collected across a variety of
sites and time points. We then investigated the P:L tolerance range of
larvae by rearing bees in vitro on a variety of diets. Multifloral and
single-source pollen diets with P:L ratios within the range of surveyed
provisions did not always support larval development, indicating that
other dietary components such as plant secondary compounds and
micronutrients must also be considered in bee nutritional experiments.
Finally, we used pollen metabarcoding to identify pollen from whole larval
provisions to understand how much pollen bees used from plants outside of
their host plant families to meet their nutritional needs, as well as
pollen from individual forager bouts, to observe if bees maintained strict
floral constancy or visited multiple plant genera per foraging bout. Whole
larval provision surveys revealed a surprising range of host plant pollen
use, ranging from ~5–70% of host plant pollen per provision. Samples from
individual foraging trips contained pollen from multiple genera,
suggesting that bees are using some form of foraging decision-making.
Overall, these results suggest that O. cornifrons have a wide nutritional
niche breadth, but while pollen P:L ratio tolerance is broad, a tolerable
P:L ratio alone is not enough to create a quality diet for O. cornifrons,
and the plant species that make up these diets must also be carefully
considered.