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A Pollen Puzzle

Opening up a bee hive to see the variety of different pollens collected is something all beekeepers experience at certain times of the year. However, what drives these diverse pollen foraging activities remains a ‘puzzle’ to science, explains science writer Dave Black.

By Dave Black

There are many things we don’t fully understand about honey bee foraging, but here is one puzzle most people assume has already been worked out. We know that honey bees are particularly good at returning to, and recruiting nestmates to, abundant floral sources of pollen and nectar. We call this ‘patch fidelity’. Honey bees are also known for their ‘floral constancy’, that is, once they learn a rewarding flower the tendency is to keep visiting that type, even when other flowers might prove a better prize.

How do honey bees know what pollen to collect for optimal performance and why do they often collect such a wide variety? Science still doesn’t have a resounding answer.

Isn’t enough, enough?

Viewed from a nutritional perspective patch fidelity and floral constancy might actually be counter-productive. For bees harvesting nectar to an extent one flower is much like another, for honey bees it’s essentially just ‘sweet’ water. That’s not the case for the bees collecting pollen. There are large differences in the nutritional composition across the many varieties of pollen available on any occasion. From their harvest, the right recipe for half-a-dozen or so amino acids and several minerals and fatty acids from pollens is essential to ‘bake’ a balanced diet for their nestmates. If the bees, between them, can collect and store enough of all the different types of pollen around them, then surely they will have enough of what they need to prepare a healthy meal?

There is another observation. If the bees are randomly ‘sampling’ all the pollen sources that surround them then, while some might be fiddly or unwanted for some reason, what they collect should mirror the proportions of what we can find in the landscape surrounding the hive? Plenty of studies have shown that’s not the case. When I open a hive, look at the pollen cells, and congratulate my bees on the expansive variety of pollen colours on show it’s quite encouraging, but it isn’t happening by chance, and they aren’t just collecting what’s out there. They don’t just collect a little bit of everything.

So far, there has been no indication from scientific research that bees choose pollen on the basis of its nutritional value. It has been shown that they can train to detect particular amino acids (in the laboratory) but these foragers are not consuming the pollen, they don’t appear to know what its nutritional value is, and it’s not obvious how they could know what is needed by the colony even if they did. Honey bees are not like other bees; how does the ‘shopper’ know what the cook needs?

Nutrient balancing

In both natural and artificial situations, where monotonous diets occur, bees find a way to supplement their diet with other pollens, and it appears that they actively compensate by collecting the ‘right’ pollen, not just any old pollen. If the crop lacks the particular amino acids needed they go and collect pollen that has the missing compounds, it’s called a ‘compensatory behavioural shift’, which leads to ‘nutrient balancing’. For example, pollen analysis suggests bees restricted to foraging for kiwifruit pollen could face a shortage of lipids, and of the amino acid isoleucine. This constituent has also been shown to be missing for foragers on blueberries. These nutrients, if the hive’s store is exhausted, has to come from plants away from the crops

Choices, choices … there have been several theories put forward by scientists to determine how field bees choose what flowers to collect pollen from, but none are conclusive.

One test of this idea[i] prepared three pollen substitute diets for colonies to choose from. One, lacking a particular essential amino acid (EAA), was fed exclusively at the outset for a week. Another diet was similar, but still lacked the same EAA, but the other was ‘complimentary’ – it contained the missing EAA. The bees had the choice to pick them all, favour the ‘new’ ones to diversify their diet, or if they knew there was an EAA missing, they could prefer the food that contained the missing EAA. It turned out that while they collected some of all the diets, they collected much more of the complimentary diet that rebalanced their nutritional requirement.

That’s the puzzle. While honey bees manage their dietary requirements somewhat passively with a ‘little of everything’ strategy, they also actively compensate for missing ingredients by changing what they are foraging for. How? In 1998 the suggestion was that the ability of nurse bees to feed foraging bees could be acting to regulate pollen quantity, but the question of regulating quality did not arise[ii]. To work out how they might be doing that we have to look at research from other insects, because for most of the last 20 years honey bee research has been concerned with things other than nutrition.

New ideas

Two other insects are popular candidates as study animals, locusts, and drosophila flies. For both of these several studies[iii],[iv] suggest that the amino acids circulating in the haemolymph (their ‘blood’ supply) modify the sensitivity of the taste receptors in their mouthparts, increasing their preference for the amino acids they were missing, and reducing their appetite for the ones that were present. It doesn’t just happen with amino acids, but with other nutritional compounds too, and similar mechanisms had been suggested in some bumble bee research[v]. In 2019 a study published in Nature identified the amino acid receptors in the mouthparts of honey bees[vi]. These were already known in Drosophila so they compared the genomes, identified the most likely candidate, worked out where the genes were being expressed, and confirmed by measuring electrical signals from the neurons that the receptor responded to different amino acids.

In the case of honey bees, foraging bees are older workers so their need for protein (made from amino acids) is less than a growing bee, but it’s not negligible. Foragers have quite low levels of the enzymes needed to digest pollen, but still need some proteins to repair tissues, so they rely on hive bees to feed them. Twenty-five percent of the worker jelly the nurse bees produce, using amino acids in the pollen the foragers collect, is fed back to foragers,[vii] so potentially this allows some ‘feedback’ about the colony’s nutritional status. While stored pollen enables some ‘buffering’ of EAAs, eventually the foraging force will be affected by a deficiency and that (if the theory is right) will bias them to preferably forage for pollen containing more of the missing EAA.

All we need now is someone to try and disprove it. And figure out how amino acids interact with the fatty acids that coat pollen grains.

Dave Black is a commercial-beekeeper-turned-hobbyist, now working in the kiwifruit industry. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here.

References

[i] Harmen P. Hendriksma & Sharoni Shafir (2016) Honey bee foragers balance colony nutritional deficiencies. Behav Ecol Sociobiol (2016) 70:509–517, DOI: 10.1007/s00265-016-2067-5

[ii] Scott Camazine, Karl Crailsheim, Norbert Hrassnigg, Gene E. Robinson, Bernhard Leonhard and Helga Kropiunigg. (1998) Protein trophallaxis and the regulation of pollen foraging by honey bees (Apis mellifera L.), Apidologie, 29 1-2 (1998) 113-126, DOI: https://doi.org/10.1051/apido:19980107

[iii] Simpson SJ, Simpson CL (1992) Mechanisms controlling modulation by heamolymph amino acids of gustatory responsiveness in the locust. J Exp Biol 168:269–286

[iv] Ganguly, Dey et al. (2021) Dietary Macronutrient Imbalances Lead to Compensatory Changes in Peripheral Taste via Independent Signaling Pathways J. Neurosci., December 15,  41(50):10222–10246

[v] Stabler D, Paoli PP, Nicolson SW, Wright GA (2015) Nutrient balancing of the adult worker bumblebee (Bombus terrestris) depends on its dietary source of essential amino acids. J Exp Biol 218:793–802. doi:10.1242/jeb.114249

[vi] Lim, S., Jung, J., Yunusbaev, U. et al. Characterization and its implication of a novel taste receptor detecting nutrients in the honey bee, Apis mellifera. Sci Rep 9, 11620 (2019). https://doi.org/10.1038/s41598-019-46738-z

[vii] Karl Crailsheim. (1991) Interadult feeding of jelly in honeybee (Apis mellifera L.) colonies, J Comp Physiol B 161:554



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