How much nectar can wild bees carry? Allometric equations of nectar crop capacities for investigating bee nutrition and foraging ecology

The decline in bee populations is influenced by the scarcity of floral resources, mainly nectar and pollen, resulting from the loss of natural habitats and climate change. In this context, it is essential to understand the foraging success and nutritional status of wild bees, particularly as research on honeybees has overshadowed that of wild species. This study aims to establish allometric functions for the nectar crop capacities of wild bees, which may be used as predictive tools in research related to bee nutrition and foraging ecology. Allometric scaling models, which typically relate body size to morphological or ecological traits, are proposed as a method of estimating crop capacity in wild bees. The crop load was measured in 2358 free-ranging foragers of different sizes (body length 5-29 mm) belonging to 25 different genera, in southern France. Overall, we established consistent allometric functions for predicting crop load capacities from the body length of wild bees, with a satisfactory degree of confidence. Sex is an important determinant of crop capacity, with females carrying maximal nectar loads around three times greater than males of similar size. We also found evidence that taxonomic affiliation influences allometric properties of crop capacities. Further studies are however needed to refine the predictions by taxonomic group, and in particular for the eusocial bumblebees. Field validation involving complementary data on floral nectar availability indicated that an allometric framework can be used to address research questions related to foraging success and competition for nectar in large wild bees (body length similar or greater than that of honeybees). Future studies should aim at refining the taxonomic resolution of allometric functions to family or genus level, which would provide a better understanding of the ecological particularities of certain groups of bees, such as eusocial species whose foragers store nectar for the entire colony or, conversely, cleptoparasitic (cuckoo) female bees that do not supply their broods.