November 18, 2016

How Bees Choose Which Pollen to Collect

Researchers suggest that bees use multiple sensory cues, previous experience, and possibly feedback from other bees in their search for pollen.
By Laurie Anne Walden, DVM, ELS
How do bees decide where to look for pollen? What makes them choose one type of flower over another? In a review recently published in Functional Ecology, researchers suggest that bees use multiple sensory cues, previous experience, and possibly feedback from other bees in their search for pollen.
 
Flowers have evolved more than one type of signal to attract pollinators, say the authors. Plants use fragrances, bright colors, nectar, and pollen (a food source) to attract bees and encourage them to return. Individual bees seem to prefer certain types of pollen, but it is not clear which properties of pollen bees respond to. “We need more research that considers the behavior and neurobiology of bees to understand when and why they prefer some plants and some pollen over others,” said coauthor Dr Natalie Hempel de Ibarra, of the University of Exeter, United Kingdom, in a news release. “A breakthrough in this area could advance our efforts in both biodiversity conservation and crop production.”
 
A major question, say the authors, is whether bees assess the nutritional value of pollen when deciding where to forage. Bees evaluate the sugar content and flow rate of nectar while they are collecting it. However, they typically carry pollen back to the nest to feed offspring or the whole colony rather than eating it themselves at the flower, which makes it a challenge to understand how they gauge its nutritional content.
 
Pollen is bees’ main protein source, but according to the authors, published studies have not shown conclusively that bees select pollen on the basis of its protein content. Bees have gustatory receptors that can detect sugar, but whether they can also detect protein—or truly taste pollen—is an open question, complicated by the fact that pollen presents bees with cues aimed at multiple senses, not just taste.
 
Bees respond to fragrances, say the authors, and the odor of pollen is different from that of the whole flower. Bees also use visual cues to select pollen, as demonstrated by experiments in which bees learned to associate a pollen reward with a particular color. The authors note that during pollen collection, bees receive mechanosensory feedback (for example, signals indicating how much pollen a flower is releasing) and physical cues like the size of pollen grains. New research also indicates that bees can detect electric fields around flowers.
 
“From a bees' perspective pollen represents a multimodal stimulus, at once providing foragers with gustatory, olfactory, visual and mechanosensory cues, all of which could be used to guide their foraging choices,” write the authors. Because pollen is a complex substance and controlling for single sensory cues is difficult, determining which cues are most important to bees is not easy, they say. The problem is compounded by the difficulty of replicating fresh pollen in a controlled experiment. Commercially available pollen does not necessarily have the same properties as the pollen that bees from a particular location would encounter in nature.
 
Bees’ foraging choices are influenced by factors beyond the characteristics of pollen itself, say the authors. Studies have indicated that bees’ pollen preferences are modified by experience or learning. Other studies suggest that social bees change their foraging habits in response to changes in colony pollen stores, indicating that feedback within the colony influences pollen foraging.
 
“Our review is unique in considering pollen foraging from an individual bee’s perspective, asking which senses bees use to decide which flowers are worth visiting,” said study author Dr Elizabeth Nicholls, of the University of Sussex, Brighton, United Kingdom. The authors conclude that more studies of bees’ sensory and learning mechanisms are needed and note that new research methods, including genome sequencing, might provide insight into how pollen functions as a reward for bees.
 
Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.
 

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