How Bees Digest Pollen

Four Levels of Pollen Digestion in Adult Apis mellifera

Apis mellifera uses pollen as their primary source of proteins, fats, vitamins, and minerals (honourable mention to the starches). Pollen consumption contributes to development, brood rearing, and maintaining overall colony health. However, the grains are well-protected by tough outer walls (“exines”) that resist degradation​. Over evolutionary time, honey bees developed specialised techniques to access the nutrients inside: nectar rehydration to swell the germination pores, digestive enzymes like proteases to cleave the internal structures and symbiotic relationships with gut microbes (e.g. Gilliamella apicola & Snodgrassella alvi) for further nutrient release.

We’ve known since Anderson’s 2014 paper that bee bread does not make pollen “easier to digest” and that bees will prefer consuming fresh pollen over bee bread; this addition of nectar to pollen is for preservation purposes, rather than digestion. And now, from the creators of “four levels of honey making”, comes four levels of pollen digestion in (adult) A. mellifera (detailed larval digestion will part 2).

There’s a shirt that goes with this article – designed & pressed by Yours Truly.

Child: Pollen is Broken Down by the Bee’s Stomach Juices

Bees eat pollen to get strong and healthy. Pollen comes from flowers, and in many colours– yellow, pink, red, white… it all depends on which type of flower the bee gathers from. Each flower pollen provides bees with the healthy fats and protein it needs to grow, and to feed its baby larva.

But pollen is tough, it has a strong shell that is hard to break, and bees can’t chew it like we chew food, so instead, they add spit and honey to the pollen to make it soft, and then swallow it. The pollen goes into their second stomach (called a midgut), where the bee’s body makes special juices called enzymes that break open the pollen and let the healthy stuff come out. The bee’s body take these protein and healthy fats, and the empty pollen shells get pooped out.

The bee’s body is not alone in breaking down the pollen, there are also tiny bacteria inside their stomach that also help break down the pollen for easier eating.

Teen: Pollen Digestion is by Force, Enzymes and Gut Bacteria:

The digestion of pollen is a mix of physical softening (with saliva and nectar), enzymatic breakdown & microbial help from gut bacteria. Bees eat pollen for essential nutrients- proteins, fats, and vitamins. But these pollen grains have a hard outer wall that makes them tough to digest. In the European honey bee, Apis mellifera the breakdown process occurs like this:

1. After the pollen is collected from the flowers and returned to the hive, the bees mix it with nectar or honey and a saliva to make a soft paste. If not used right away, it is stored in the hive as bee bread.
2. When a bee swallows pollen, it travels into the midgut to begin its breakdown. The outer wall itself cannot be digested, but bees can extract nutrients through small openings in the pollen called germination pores.
   • The bees produce digestive enzymes, like proteases to break down proteins into its smaller parts, the amino acids, amylase to break down starches and lipase to break down fats, releasing the nutrients.
3. The bees’ gut has beneficial bacteria and other microorganisms that help break down parts of the pollen and help them absorb more nutrients. Once the nutrients are absorbed, the indigestible pollen shell is excreted.

Beekeeper- Nurse Bees Process Pollen for Larval & Adult Feeding:

The forager collects pollen and packs it into her hind legs’ corbiculae (“pollen baskets”). She unloads the pollen in the hive and begins its conversion to bee bread through the addition of saliva and nectar. This (slightly-fermented) pollenis easier to digest than the raw counterparts, and¹* can be consumed by the larvae and adult bees, or stored in the comb for later use… I’m sure you’ve seen those colourful cells on the periphery of your frames– that’s the bee bread.

Pollen Digestion in Adult Bees

The digestive process occurs in the midgut. The bee doesn’t break the entire shell- she uses enzymes and microbial activity to access the granule’s internal components via the germination pores.

Enzymatic breakdown involves multiple enzyme groups cleaving pollen proteins into amino acids (proteases), degrading starches (amylases) and the lipids found in pollen oils (lipases). These enzymes are most active in nurse bees, whose job is to feed the hive, however that doesn’t mean the other adults cannot feed themselves; foragers break down pollen for self-sustenance when in the field.

Animals have gut microbiota. In A. mellifera, there is a core group of 5 bacterial phylotypes each with their own role in disease resistance, immune function and larval development. Species like Gilliamella apicola and Snodgrassella alvi help degrade complex carbohydrates and detoxify plant compounds (“polyphenols”). The former G. apicola, breaks down pectin, a major component of the pollen inner layer, aiding nutrient release (Zheng et al., 2016).

What remains of the pollen grains are its undigested pollen outer wall “exine”– they remain intact and are passed in faeces, giving bee poop its yellowish colour… I wonder if a colony was fed exclusively on non-yellow pollens, will it produce a different-coloured poop?²

Pollen Digestion in Larvae

Larvae don’t eat raw pollen directly and are fed a pre-digested mixture by the nurse bees. When nurses consume the pollen, they secrete proteins from their hypopharyngeal gland (“royal jelly”) along with the pollen to the larvae.  For the first three days of hatching, all larvae receive royal jelly, afterwards, the worker brood are fed a pollen-based diet (“worker jelly”) whereas queen larvae continue to receive royal jelly until cell capping.

Once larvae are fed pollen, they being producing worker-specific enzymes (like proteases, e.g. PS4 in the gut; Sagona et al., 2022) that aid digestion. Sagona and colleagues suggest that these proteases are a response to the “rougher” diet of pollen and honey, compared to the easily-digestible royal jelly that queens consume exclusively.

Microbiota wise, Larval guts are barren. They don’t have the same gut microbes as adults until after pupation. Their microbial communities develop from continued nurse-bee feeding and their hive environment, eventually establishing the adult communities typical of A. mellifera.

Bee Brofessor: Grain Rehydration, Gut Enzymes & Microbiome Facilitate Pollen Digestion

Adult A. mellifera workers ingest pollen grains while foraging or inside the hive in its stored form (bee bread). To expose the germination pores, pollen is moistened with nectar or honey during collection, making it easier to access the nutrients within the shell (Harano et al., 2013).

After ingestion, pollen is transported to the midgut, the primary site of digestion. In the midgut, digestive enzymes are secreted to commence digestion of the internal pollen structures via the aforementioned pores, whilst the tough outer shell passes through the digestive system intact. Studies show that when pollen is pooped *ahem* defecated, the pollen grains are hollow, supporting the fact that bees consume pollen for its internal (“cytoplasmic”) contents for nutrition (e.g. Human & Nicolson 2003 & Human & Nicolson 2006).

Digestion via Enzymatic Activity is Highest in the Nurse Caste

Gut enzymes release internal pollen nutrients for gut absorption. Proteases cleave proteins into their component peptides and amino acids, and their numbers are upregulated when bees consume pollen (Sagona et al., 2022). The presence of enzymes is caste-specific, with young nurse bees having higher protease activity than the older foragers, correlating with their role as pollen digestors and brood feeders.

Amylases breakdown starch and other polysaccharides present in pollen grains​, enabling bees to access pollen-borne carbohydrates for energy. This activity is exclusive to worker bees who use the starches to power their flight muscles. Drones cannot do this as they lack amylases (Hrassnigg et al., 2005), reinforcing the idea that foraging and defending are more demanding than sex and reproduction… who would have known?

Pollen fats are broken down with lipases for the absorption of fatty acids. Unsurprisingly, nurse bees have the highest lipase activity, and this increases as they mature, reflecting their role in processing and distributing nutrients around the hive (Loidl & Crailsheim, 2001).

Contribution of the Gut Microbiome: Digestion of Tough Components

Symbiotic gut bacteria break down pollen parts that the bee’s own enzymes cannot fully process. The adult worker’s hindgut harbors a consistent community of specialised bacteria, of which there are five core phylotypes.

Each group produce certain enzymes to degrade structural components (mainly polysaccharides) in the pollen’s interior. For example, Gilliamella apicola produces pectate lyases and other pectin-degrading enzymes that digest the pectin and cell wall materials ​(reviewed in Engel et al., 2012). Pectin is a major component of pollen cell walls that is normally indigestible, and only Gilliamella strains can degrade it. Once the pectin is removed, the typical pollen grain is left with minimal defences and comes apart, creating an opening for the bee’s own enzymes to continue digestion.

As in our own the bodies, bacteria weaken or dissolve parts of the ingested material and release the (otherwise-inaccessible) nutrients for absorption. Other phylotypes like Bifidobacterium and Lactobacillus ferment carbohydrates and produce organic acids which preserve gut pH and suppress pathogens​ (Brown et al., 2022). The work of Brown and colleagues showed that honey bee workers with an intact, “natural” gut microbiome gain more weight and live longer when fed pollen, whereas bees lacking their normal gut bacteria (in this case, due to antibiotics) cannot fully utilise the pollen nutrients.

In short, the microbiota “pre-digests” tough components and produces beneficial metabolites, acting as an extended digestive system for the adult bee.

Caste-specific Pollen Digestion: Nurses are Most Efficient

Not all pollen content is digestible by bees. Nurse bees can extract up to ~95% of the nutrients from ingested pollen grains, leaving only empty husks (Roulston & Cane, 2000). In contrast, older bees digest a smaller fraction- ~50-60% depending on the pollen source. This could be attributed to age-related declines in enzyme levels or more likely because foragers eat pollen to feed themselves, rather than for brood rearing duties.

As mentioned above, the difference in efficiency aligns with enzyme activity: protease expression is lower in foragers and highest in nurses, allowing the latter to utilise pollen more completely​. The average bee still retrieves a substantial portion of the nutrients in pollen before excreting the indigestible shell, so this ‘partial inefficiency” should not be deemed wasteful from an ecological perspective. Consider that all bees defecate outside the hive (but if they do it inside you may have a case of Nosema). This returns nutrients to the soil, and even releases viable, intact pollen back to the environment where it could be picked up by another insect, or the target flower (no matter the odds!).

Figure 1: Percentage of pollen grains in the bee faeces by adult age. Nine-day old adult bees (nurse bee) had the highest percentage of empty pollen grains, suggesting they have the most efficient digestion. Table adapted from Roulston & Cane, 2000.

Bees Add Nectar to Pollen (“Bee Bread”) for Preservation, not Digestibility

Bee bread is formed by mixing collected pollen with nectar and saliva. It is stored in comb for later use by the nurse bees for brood feeding. Although Nicolson & colleagues suggest that the evidence is mixed on bee bread being more nutritious than fresh pollen (and why wouldn’t it? There are plenty of pollen varieties out there), bee bread is a stored form of pollen for when there is lack a of forage during brood rearing (see footnote above; Nicolson et al., 2018). The added nectar/honey increases moisture, causing pollen grains to rehydrate, swell and expose their germination pores. However this is insignificant from a digestive standpoint, and is backed up by the aforementioned work of Anderson & colleagues, as well as the observation that bees greatly favour consuming fresh pollen over the stored bee bread form.

Influence of Pollen Type on Digestion

Plant pollens vary in protein content (ranging from <5% to >50% protein by weight- sunflowers and aloe are examples of low and high protein content respectively) as well as in starch, fat, and fibre content. A. mellifera’s broad enzymatic profile handles a variety of pollen types– high-starch pollens can be digested by amylase, and high-protein pollens are addressed with abundant proteases. There is evidence that bees prefer or perform better with certain pollen types, possibly because those pollens are more digestible or nutritionally balanced- Nicolson et al., found that adults consumed more of a lower-protein pollen (sunflower) to compensate for its poorer nutritional value compared to one of higher quality (aloe; Nicolson et al., 2018).

Deeper Down: The Pollen Grain Structure Influences Digestion

Pollens with thin exines or large germination pores are more easily broken down by the bee . Compare the digestion of corn pollen with that of dandelion or citrus: The faeces of corn-pollen fed bees showed more intact grains than of other flowers, suggesting that the usual approach of osmotic shock and enzymatic penetration is ineffective ​(Nicolson et al., 2018).

I know I’m referencing Nicolson’s 2018 work a lot, and here I am doing it again– the bee groups fed aloe pollen (higher in protein and easier to digest than sunflower) found that they cleared significantly more pollen grains (>90%) than the tougher pollens (around 50%). This clearance rate is improved when the pollen is hydrated or softened via the addition of water or nectar.

The swelling of the pollen grain can “trick” it into germinating in the bee’s stomach³, weakening the wall and opening the nutritious interior. However this approach is unreliable, as the bee’s internal gut conditions need to mimic the germination cues pollen would encounter in the wild; different plants have different requirements to initiate the sprouting process. Nonetheless, pseudo-germination is thought to be another possible mechanism (along with changes in osmotic pressure and microbial action) that bees use to breach the grains.

Pollen’s Enzymes Aid in its Own Digestion

Many pollen grains contain proteases (e.g. cysteine or serine proteases) and other hydrolases as part of their own biology⁴. Unsurprisingly, individual plant species have different levels of enzyme activity, ranging from trypsin-this, carboxypeptidase-that and chymo-shlymo-trypsinases– complex molecules beyond the scope of beekeepers (further reading: Verissimo et al., 2011). These pollen-borne enzymes sometimes remain partially-active in the bee’s gut and could assist in breaking down the pollen’s own proteins (or even the body’s own proteins, as in the case of gut irritation from due to allergic reactions).

Note that pollen enzymes are minor contributors to the digestion processes– remember, each plant species has their own range of enzymes and activation requirements, so having “self-digestion” is more a happy coincidence than a biological necessity. Need further proof? A few minutes’ observation of your hive can show you that A. mellifera collect a wide array of pollen types without needing pollen-specific enzymes.

Not All Pollen is Good: Toxic Pollens to Bees

I end here. Toxic pollens harm bees because of their secondary metabolites, which cannot be detoxified by their digestive system. Examples include the Chinese Tea-oil tree Camellia oleifera, whose pollen contains “theasaponin” (Mamut et al., 2024), and “rhodojaponin III” in the Rhododendron Rhododendron molle (Feng et al., 2024). Such plants are pollinated by other animals, from birds to reptiles (or butterflies and bumble bees, but not honey bees in the case of R. molle).

Where enzymes cannot detoxify harmful compounds, the gut bacteria does so, and when both systems fail, you get signs of poisoning, whether due to pesticides or poor foraging. The latter is easier to manage, as bees will learn to avoid bad foraging sources. Pesticide-laced pollens (or nectars) are harder to circumvent, unless you have the means to move your hives away from the spraying; in some cases bees prefer plants that contain low concentrations of pesticides (e.g. in neonicotinoids in nectar- Kessler et al., 2015). No matter, the generalist appetite of A. mellifera gives it enough enzymatic variety to digest and detoxify many plant compounds.

Read the whole post and found puns, typos mistakes? Voice your displeasure with a strongly-worded message to admin@tgbee.com.au, or by filling this form.

Footnotes

1. I got it wrong, sorry about that. Bee bread is not easier to digest due to microbial action- Anderson and colleagues’ 2014 paper tested several hypothesis to challenge the notion that the bacteria in bee bread make it easier to digest:
   1: If the core gut bacteria digests pollen, then their numbers would increase in bee bread since they’re receiving nutrition from it and thus reproducing. This was not case, the bacteria numbers only increased the first 48 hours, then decreased
   1a: The authors suggest that this points to “preservation” of bee bread, rather than pre-digestion
   2: If there were enough enzyme-producing bacteria on the pollen’s shell, then they could have partially weakened the shell to make it easier on the bees’ digestive system.
   2a: Not the case, the calculations equated to about 1 microbe per 2,500 pollen grains. For comparison, a single pollen grain that is being “digested” (e.g. in the bee gut) may be surrounded by hundreds of bacterial species
   3: If the bacteria added during bee bread formation digested pollen, there would be visual differences between fresh vs. breaded pollen
   3a: Microscopic imaging showed no morphological differences
   As such, microbial action in bee bread is for preservation rather than digestion. Perhaps the addition of nectar to the pollen rehydrates the grains and makes them easier to crack open? (see Nicolson et al 2018). Thank you to reader Steve for pointing it out. ↩︎
2. Trivia Tidbit: In the 1980’s, U.S. intelligence corps declared that Soviet-supported Laotian and Cambodian regimes were using chemical warfare “yellow rain” in Thailand… it took two scientists (Meselson & Seeley) to declare that this “yellow rain” was in fact mass releases of bee poop from migratory swarms of Apis colonies (Nature, 1984) ↩︎
3. What occurs in the bee gut is more correctly “pseudo-germination”, as the pollen has not reached the female gamete for complete fertilization. Pollens are the male reproductive gametes from a plant, and since they are released into the environment, they require protection from animals and the elements to safely reach the female pistils. ↩︎
4. Pollen grains use enzymes for digesting the stigmatic tissues during fertilization, or as a defence against pests and pathogens. In the case of fertilization, they degrade the surface barriers (cell wall polysaccharides, proteins, and lipids) to adhere to the plant pistils and start growing. For defence, the enzymes can cause allergic reactions in insects, or produce reactive oxygen species (ROS) to protect the pollen tube against pathogen attacks. ↩︎