Key pollen host plants provide balanced diets for wild bee larvae: a lesson for planting flower strips and hedgerows.
Bee-friendly plants are defined by the quantity of food they produce and the visitation rates of adult insects foraging for nectar. However, it is pollen nutritional quality that enables proper larval development of bees, affecting their populations. Not all plants produce pollen that satisfies the nutritional requirements of bee larvae, and we lack an understanding of how different plant pollens impact bee nutritional demands. This study examined whether nutritionally desirable key plant species may promote wild bee larval development, which is essential if the population is to thrive. The generalist solitary mason bee Osmia bicornis L. was used as a model species to examine differences between bee larva nutritional demand and host plant nutrient supply; an ecological stoichiometry framework was applied. The stoichiometric ratios of 12 elements were investigated in bee bodies and cocoons (reflecting nutritional demand) and in the pollen supplied by the mother (nutritional supply; N = 15 × 2 sexes). Similarly, the stoichiometry of 62 pollen taxa, including native, alien, and garden plants and crops, was compared with the bee demand based on the literature. Compared to males, females had higher demands for P, Cu and Zn and were supplied with pollen richer in these elements. Therefore, when collecting pollen for their progeny, Osmia provides daughters and sons with different pollen mixtures, reflecting sex-specific nutritional demand. Bees may be limited by the availability of P, Na, Mn, Mg, K, Fe, Ca, Zn and Cu, with high taxonomic variability in their concentrations in pollen. Female fitness may be particularly related to a high proportion of P in the diet. Synthesis and applications. Access to key plant species that allow nutritionally balanced larval diets may be essential for bee development, whether food is gathered intentionally or randomly. Such plant species-and not only those rich in nectar and pollen-should be promoted in wild bee conservation efforts, including planting flower strips and hedgerows. Bee-friendly plants should not be defined and planted solely based on the quantities of food they produce and on the visitation rates of adult insects foraging for energy.