1. From pollen to brood rearing

Nectar and honey mainly provide carbohydrates, and hence energy. Pollen supplies above all proteins and essential amino acids, but also lipids, fatty acids, sterols and numerous vitamins and minerals. These resources are complementary: an abundance of one does not necessarily compensate for the absence of the other.

Foragers gather the grains on their bodies, moisten them with nectar and secretions, and then transport them as pellets in the corbiculae of their hind legs. In the hive, they deposit these pellets in cells, where the pollen is packed down, mixed with sugary substances and acidified. The stored product is called bee bread.

This transformation contributes above all to preservation. The available data demonstrate neither a uniform and prolonged 'lactic fermentation', nor a general nutritional enrichment, nor a systematic improvement in the digestibility of stored pollen (Anderson et al., 2014).

Pollen is consumed mainly by young workers. It supports the development of their hypopharyngeal glands, their fat body and their physiological reserves. The nurse bees then convert part of these nutrients into brood food, which is distributed to the larvae, the queen and other adults. The brood is therefore highly dependent on pollen, while consuming only small quantities of it directly.

Prolonged deprivation affects the physiology of nurse bees, reduces the capacity to rear brood and can decrease the number of workers available a few weeks later. It also influences longevity as well as certain immune and detoxification mechanisms. Good nutrition supports the colony's defences, but replaces neither Varroa control nor the diagnosis and treatment of diseases.

2. Quantity, quality, diversity and continuity

Not all pollens have the same composition. Their value depends in particular on the content and profile of essential amino acids, on the lipids and sterols, on digestibility, on the quantity consumed and on the physiological requirements of the bees. A pollen rich in crude protein can therefore remain unbalanced, while two pollens showing the same protein level can produce different biological effects.

A diversified flora increases the possibilities for complementarity and reduces dependence on a single plant. It nevertheless guarantees neither a sufficient quantity nor the presence of all the necessary nutrients. Conversely, a temporarily dominant pollen is not automatically deficient if its composition covers requirements and if other resources subsequently take over (Di Pasquale et al., 2013).

The risk in landscapes dominated by a few crops therefore lies less in the word 'monoculture' than in three possible situations: an abundant but nutritionally inadequate source, increased exposure to certain residues, or an abrupt interruption when flowering ends. The continuity of resources over the course of the season counts as much as their diversity at any given moment.

3. Observing pollen income without over-interpreting it

The arrival of pellets at the hive entrance confirms that foragers are finding pollen and that the colony is collecting it at that particular moment. It does not, however, measure the intensity of the nectar flow, which concerns mainly nectar income, nor the nutritional quality of the pellets, nor directly the intensity of laying.

Open brood generally increases demand and stimulates collection. But the flow observed also depends on the weather, the time of day, the flowering plants accessible, the number of foragers, the stores present and competition between colonies. A momentary decline therefore demonstrates neither a deficiency nor a health problem.

The quantity of pollen visible in the cells must be interpreted with the same caution. A small store may correspond to fresh income rapidly consumed, to a genuine shortage or, conversely, to low demand because the colony is rearing little brood. An abundant store may be old, little consumed or of unknown quality.

4. What to do when a shortage is likely?

1. Check the context. Observe several favourable flying days, the open brood, the stores and the flowering plants actually accessible. Comparing colonies makes it easier to distinguish a general problem in the apiary from a disorder specific to one colony.
2. Address the cause first where possible. If the floral supply is lastingly insufficient, the choice of site, moving the colonies or adapting the number of hives to the resources of the surroundings are more coherent than repeated supplementary feeding.
3. Reserve protein supplementation for targeted situations. A protein patty can help to bridge a temporary interruption, but no substitute has been shown to reproduce, in a general way, all the functions of an appropriate pollen. The fact that a product is consumed or momentarily stimulates brood rearing does not prove its nutritional equivalence.
4. Avoid stimulating demand that cannot be sustained. Encouraging brood rearing when income is about to be interrupted again may simply shift the problem to the following weeks. Supplementation should therefore be limited in time, given under strict hygiene and assessed on the basis of the colony's overall development.
5. Do not separate nutrition from health. At the end of the season, an adequate pollen supply contributes to building up the physiological reserves of the future winter bees. Its benefit will nevertheless remain limited if the pressure from Varroa and viruses is not brought under control at the right time.

Transferring pollen frames from a diseased colony or of uncertain origin must be avoided. Stored pollen can carry residues or infectious agents, and freezing does not constitute disinfection.

5. Key points

• Pollen mainly feeds the young workers, who redistribute its nutrients to the brood in the form of brood food.
• Bee bread is primarily pollen that has been prepared and preserved, not pollen systematically enriched by fermentation.
• The quantity, composition, diversity and continuity of income must be distinguished from one another.
• Neither the pellets observed at the hive entrance nor the area of stored pollen is sufficient to diagnose nutritional status.
• Protein supplementation can address a temporary interruption, but it does not durably replace an adequate floral resource.
• Good nutrition supports the colony's resilience; it does not compensate for a failure to control Varroa or for disease.

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See also:

• All About Pollen
• Feeding bees protein: helpful or overrated?
• Nurse bees do not feed only the brood: brood food also circulates among adult bees
• Practical Guide: 3.2. Dearth periods
• Observations at the hive entrance
• Vitellogenin and the keys to the colony

References

• Anderson, K. E., Carroll, M. J., Sheehan, T., Mott, B. M., Maes, P., & Corby-Harris, V. (2014). Hive-stored pollen of honey bees: Many lines of evidence are consistent with pollen preservation, not nutrient conversion. Molecular Ecology, 23(23), 5904–5917. https://doi.org/10.1111/mec.12966
• Brodschneider, R., & Crailsheim, K. (2010). Nutrition and health in honey bees. Apidologie, 41, 278–294. https://doi.org/10.1051/apido/2010012
• Di Pasquale, G., Alaux, C., Le Conte, Y., Odoux, J.-F., Pioz, M., Vaissière, B. E., Belzunces, L. P., & Decourtye, A. (2016). Variations in the availability of pollen resources affect honey bee health. PLOS ONE, 11(9), e0162818. https://doi.org/10.1371/journal.pone.0162818
• Di Pasquale, G., Salignon, M., Le Conte, Y., Belzunces, L. P., Decourtye, A., Kretzschmar, A., Suchail, S., Brunet, J.-L., & Alaux, C. (2013). Influence of pollen nutrition on honey bee health: Do pollen quality and diversity matter? PLOS ONE, 8(8), e72016. https://doi.org/10.1371/journal.pone.0072016
• Requier, F., Odoux, J.-F., Henry, M., & Bretagnolle, V. (2017). The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands. Journal of Applied Ecology, 54(4), 1161–1170. https://doi.org/10.1111/1365-2664.12836
• Wright, G. A., Nicolson, S. W., & Shafir, S. (2018). Nutritional physiology and ecology of honey bees. Annual Review of Entomology, 63, 327–344. https://doi.org/10.1146/annurev-ento-020117-043423