Do bumble bees make optimal nutritional choices?

Nutrition is crucial for bees, impacting their health, survival, and pollination performance in ecosystems and agriculture. Bees get essential nutrients such as carbohydrates, proteins, lipids, vitamins, and minerals, primarily from nectar and pollen. Many bee species are experiencing declines linked partially to nutritional stress, often exacerbated by climate change, pesticides, and pathogens, highlighting the need to understand and support optimal bee nutrition to mitigate these stressors. Bumble bees, such as Bombus impatient and Bombus terrestris, essential pollinators in agriculture, are known to regulate their nutrient intake. However, whether their dietary choices improve fitness is poorly understood. We tested diets with varying protein, lipid, and carbohydrate compositions, analyzing impacts on consumption, body mass, egg laying, and ovarian activation. Results showed that bees overconsumed pollen on protein-enriched diets and under consumed it on lipid-enriched and glucose-based diets. Nectar overconsumption was observed on low-concentration sucrose diets. These patterns, however, did not correspond to improved fitness, as egg laying and body mass were negatively correlated with consumption in diets enriched with protein and sugar. Ovarian activation was largely unaffected across most diets, indicating it may not be a reliable indicator of diet quality. These findings raise doubts about whether bees make optimal nutritional choices and suggest that diet consumption alone may not be a reliable indicator of their optimal diet. Alternatively, bees made the best possible decisions under circumstances that presented a lose-lose tradeoff across all the diets provided. These data can inform future studies on nutritional stress, enhance interpretations of bee diet preferences in bioassays, and guide bumble bee management practices.