Does mother know best? How plant preference influences pollen nutrition in Osmia cornifrons

Prior work suggests social bee species may mix pollen across plant taxa, which vary in protein and lipid content, to reach an ideal protein-to-lipid ratio (P:L) target intake, but it is still unclear if solitary bees also selectively forage for pollen in this manner. We investigated the dietary preferences and tolerances of the solitary bee O. cornifrons, a species commonly used in orchard pollination. Larval pollen provisions collected from O. cornifrons nests revealed that while the mean ratio (2.9:1) was consistent across sites and time periods, there was a broad range of P:L ratios (.75-7:1) even within sites and locations, indicating that larval tolerance to different diets is higher than previously expected. Pollen metabarcoding of individual forager pollen loads and whole pollen provisions revealed that individual forager collections were less diverse than whole pollen provisions, which may indicate that, across foraging trips, bees are balancing collections by species, and thus P:L ratios. O. cornifrons were raised from egg to adult on pollen diets with different P:Ls to understand dietary tolerance bounds, and if what bees were collecting locally was the most beneficial diet for growth and development. Surprisingly, bees had faster developmental rates and larger body sizes when fed diets high in protein, which significantly differed from average survey collections.  Overall, these results suggest that O. cornifrons larvae may be able to physiologically tolerate a broad range of P:L ratios, allowing female bees to forage broadly across plants with pollen of diverse nutritional content.