How to really help bees: key pollen host plants may alter growth, development and fitness, thereby influencing populations, of a generalist solitary bee

Abstract

Floral resource limitation connected with land degradation and habitat loss was identified as potential threat that cause pollinator decline and food resource quality may be the main limiting factor for bee populations. To better understand the nutritional constraints of growing and developing organisms, their colonies and populations, ecological stoichiometry was developed with reference to the elements that, if environmentally scarce, prevent the building of biologically important organic molecules. The least understood aspect of bee nutritional needs concerns stoichiometric balancing and the need for adequate amounts and ratios of nutritional elements in consumed food. I used the framework of ecological stoichiometry to study differences in the demand and supply of nutritional elements and stoichiometric balancing of an important pollinator’s diet: the mason bee Osmia bicornis L. O. bicornis larva is supplied with pollen by its mother. I used a field study to investigate concentrations and stoichiometric ratios of C, N, S, P, K, Na, Ca, Mg, Fe, Zn, Mn, and Cu in the bee production (body and cocoon) of both sexes and their pollen supply, i.e., the only food eaten during larval development. Females had a higher demand for and were supplied with pollen richer in P, Cu and Zn than males. Female fitness may be in particular related to a high P proportion and a low C:P ratio in their diet. Additionally, males had a higher demand for Na and a lower demand for K than females, but these elements were similarly concentrated in the pollen supply for both sexes. Comparison of nutritional demand and supply of the bee suggests that adult females while collecting pollen supply for their progeny, may favor key species that allow for a dietary stoichiometric balance. Moreover, they may provide their daughters and sons with a different mix of pollen that better fulfills sex-specific nutritional demands. Bee production, growth and development may be limited by the availability of P, Na, Mn, Mg, K, Fe Ca, Zn and Cu, i.e., elements that show high taxonomical concentration variabilities in pollen. Therefore, it is likely that the presence of key plant species in the flora, which produce nutritionally balanced pollen for bees, influence bee development and shape bee populations.

Changes in bee habitat floral composition shape the available nutritional supply in the environment. In this context, the key plant species must be present in the flora to produce pollen that is nutritionally balanced for bees. Using literature data on the elemental composition of taxonomically different pollen, I suggested pollen species that either promotes or limits bee production, thereby influencing the bee populations. In conclusion, the quality of food sources for bees, not solely the quantity, should be considered in intervention strategies aimed at improving the nutritional base for bees and planting random plant species that offer pollen in large quantities is not a good practice.