A recent study led by researchers from Northwestern University and the Chicago Botanic Garden has found that wild bumble bees are selective in their pollen collection, seeking out specific macronutrient balances rather than gathering pollen at random. The research tracked eight species of bumble bees over eight years in the Colorado Rockies, recording flower visits and analyzing the protein, fat, and carbohydrate content of pollen from 35 plant species.
The findings show that bumble bee species can be grouped into two distinct dietary categories. Larger bees with longer tongues tend to favor pollen high in protein but lower in sugars and fats. In contrast, shorter-tongued bees collect pollen richer in carbohydrates and fats. The study also observed that individual bees change their diets as their colonies grow through the season.
“Despite the general importance of wild pollinators, especially bees, we know very little about their nutritional needs,” said Northwestern’s Paul CaraDonna, senior author of the study. “Given widespread pollinator declines that have been observed around the globe, this knowledge gap is surprising and concerning. Our research provides some of the best information yet on the availability of nutritional resources found in wildflowers and how pollinators use these resources. We can incorporate this work into our thinking about garden design, so we can select the right flowers that best support the nutritional needs of wild pollinators.”
CaraDonna is an adjunct associate professor in Northwestern’s Program in Plant Biology and Conservation, a partnership between Weinberg College of Arts and Sciences and the Chicago Botanic Garden. Justin Bain, who recently completed his Ph.D., is first author on the publication.
The research highlights significant variation among plant species regarding macronutrient content in pollen. Some plants offer pollen with protein making up only 17% of its content; others reach as high as 86%. Seasonal changes also affect nutrient availability: spring flowers tend to have more protein-rich pollen while late-summer blooms contain higher levels of fats and carbs.
“We know that bees forage exclusively from flowers for pollen and nectar,” CaraDonna said. “Beyond that, we are in the dark. That is like humans shopping at a grocery store and assuming that all food items in the entire store have similar nutritional value. Clearly, that is a bad assumption.”
Bain explained further: “All pollen contains protein, fats and carbs,” he said. “But each type of pollen has a different mixture of these macronutrients. Some are very high protein like a steak. Others are more like a salad. So, the nutritional profiles are very, very different.”
The team noticed transitions within colonies as well: queen bees emerging early after snowmelt gather more protein-rich pollen for themselves and initial broods; later-season worker bees shift toward collecting fattier pollens as colony needs change.
“Queen bees emerge in the spring to establish their colonies,” Bain said. “They forage when the snow first melts, collecting protein-rich pollen for themselves and their first brood. Later in the summer, worker bees take over foraging, and half of the species shifted toward pollen with less protein and more fats. Seeing these clear transitions between queens and workers was especially striking, and it highlighted how differently species meet their nutritional needs across the colony life cycle.”
In another observation related to bee morphology: long-tongued bee species collected higher-protein pollens while short-tongued ones gathered those with more sugar or fat—a difference linked to which flowers each could access based on tongue length.
“We now have a better idea of what bees are bringing home in their ‘grocery bags,’” CaraDonna said. “Although this work is from one ecosystem in the Rocky Mountains, it paints a very important picture for scientists to build upon. We found that not only is there a huge amount of variation in macronutrients available in natural ecosystems to wild pollinators but our wild bees use those nutrients in distinct ways. The nutrient needs of bees are not ‘one-size-fits-all.’ But we also see that two distinct ‘nutritional niches’ emerge suggesting that there may be some general hot spots in terms of what pollinators are seeking out nutritionally.”
Researchers say these insights could help conservationists plan gardens or restoration projects by choosing plants whose nutrition matches local bee populations’ requirements—an approach aimed at supporting healthy colonies amid ongoing threats such as habitat loss or climate change.
The full study appears August 26th online via Proceedings of The Royal Society B: Biological Sciences.