We know that bees, including honey bees, require protein to survive and thrive. But, does all that protein come from pollen, or is their diet more diverse? The answer to that question lies in their past, 128 million years ago when bees evolved. Science writer Dave Black takes us through that journey, exploring the scientific advances required, bees’ ancestry and their assumed vegetarian status.
By Dave Black
Genome sequencing has become a vital tool for understanding the deep roots of biological phenomena in an organism. In the late ‘70s Frederick Sanger improved on early methods to investigate the folded chain (or ‘sequence’) of amino acids (‘bases’) in a DNA strand, in which it was chemically ‘cut’ at a particular ‘base’ and the size of the fragments inferred how the strand was composed. Sanger used chemical ‘templates’ to connect the bases to a marker, and eventually their own fluorescent labels. While this was extremely accurate, it was laborious and slow, and genome sequencing, in the beginning, took decades.
Sequencing a human’s genome (actually it was group of anonymous volunteers) in the Human Genome Project took 13 years (1990-2003). That is now possible in a single day using ‘Next Generation Sequencing (NGS). NGS, or ‘massively-parallel-sequencing’ simply conducts the analyses in parallel; many strands are sampled at the same time, and less time means lower costs. Lower cost brought it within the reach of the bee scientists.
In 2017 a group of US scientists led by Michael Branstetter published the results of a study that described the phylogenomic relationships between 189 members of the Aculeata, that is, how the genetics of bees, ants, and wasps show they are related to each other[i]. Such a large and detailed study was made possible using NGS. It confirmed some things we thought we knew (that’s what good science does) and provided some insights about things we’d been thinking about. Especially, it helps to explain how bees, and social bees, evolved.
Eat what you like
The study unequivocally links Anthophila (bees, think pollen – ‘Anther-lover’), which includes all the bee ‘families’, Colletidae, Stenotritidae, Halictdiae, Andreneidae, Megachilidae, Apidae and Melittidae, to the mid-Cretaceous period, and to their ancestors, the Crabonidae wasps in the early Cretaceous. A subsequent study by others narrowed this down to a particular group of Crabonidae wasps, the Ammoplanidae, 128 million years ago[ii]. These small wasps hunted thrips, and these thrips fed on pollen; they use their mouthparts to pierce the grain and suck the fluid contents out[iii].
Eating pollen is difficult, ask Patricia Wiltshire. She is a famous Welsh palynologist and forensic ecologist who for 30 years or more has used the skills she has learnt to provide evidence for hundreds of difficult UK criminal cases and archaeological site reconstructions around the world. She can do it partly because pollen is nearly indestructible, capable of surviving millions of years virtually unaltered[iv]. How do you eat something so durable?
Not all pollen is the same, but essentially a grain of pollen usually has two protective layers, helpfully known as the exine and the intine. The external exine is made of sporopollenin, a complex carbohydrate that builds the intricate but characteristic shapes you see under a microscope, and then an intine, made mostly of cellulose and pectin. Inside that lie the useful, nutritious, cell contents. The ‘wall’ formed by the two external layers is made to resist three ‘D’s, dehydration, decay or digestion, but often contain a few pores through which the cell can germinate – like a microscopic seed. Eating pollen means you must be successful at doing one or more of the following: destroying the pollen wall mechanically, piercing the pollen wall with sharp mouthparts, dissolving the pollen wall with enzymes, inducing germination, bursting the pollen wall through osmotic shock, or penetrating the pollen wall with digestive enzymes[v].
Most animals, and bees, are thought to extract pollen nutrients by making it germinate (‘pseudo’ germination), osmotic shock bursting the grain, and/or penetration of the pollen wall with digestive enzymes. What may have provided bees with the ability to do that came as their ancestor identified an opportunity to diversify from eating animals that ate pollen, to eating the pollen itself. The newly evolving bees-née-wasps had already mastered the navigational, visual and olfactory floral features which wasps used to locate their prey (the flower-visiting thrips). The pollen-fed and pollen-covered thrips were transported to the nest by the female wasp to feed larvae, conditioning their gut microbiome to cope. This might have expedited the switch from involuntary and accidental pollen consumption to exclusive and obligatory pollen feeding by the early bee’s larvae. All of a sudden, we have our vegetarian wasp.
You don’t choose your family
The Integrated Taxonomic Information System lists 133 described species of Ammoplanidae wasps that exist today. That contemporary bees, with over 20,000 described species, are sisters to such a small group suggests the switch from thrip predation to consuming pollen was a significant driver of diversification in bees. It’s difficult to describe the variety of bees that exist now. One of the most shocking, and recent (1982), discoveries has been bees that obtain their protein exclusively from meat, not pollen[vi]. They have ‘re-invented’ and ancient ability.
Scientists have so far identified three stingless bee species, (Trigona hypogea, T. necrophaga, and T. Crassipes) that are known to be ‘obligatory necrophages’ (their protein comes from things like lizard carcasses) and some others known to supplement their floral diet with carrion. For researchers studying these bees there is a sort of chicken vs egg question, have genetic variations in certain gut microbes enabled shifts in diet, or have diet shifts changed the microbiome? In the case of stingless bees, they seem to readily obtain a variety of microbiome bacteria from the environment, but only ones that suit their diet prosper.
Of course, even in the case of bees like ours that do rely on pollen consumption, their vegetarian status can’t be assumed. That we do assume it is because we can’t easily see the omnipresent collection of native bacteria, and fungi that dominate a honey bee colony. But they’re there, and when we do acknowledge all these microbes it’s usually only to attribute a role in ‘pre-digesting’ that difficult-to-eat pollen!
Our daily bread?
Pollen being converted into ‘bee bread’ might be useful shorthand, but it’s not an accurate description of what is going on. Honey bees (or other bees) do not need to store pollen in order to eat it, they prefer eating fresh pollen to stored pollen[vii]. When it is stored it isn’t for long, most being eaten within a week, and that storage period isn’t long enough for the biology to ‘do its stuff’ and predigest pollen into ‘bread’, nor are there the number and range of microbes present that would be necessary to do that[viii]. During that period the microbe content actually wanes (because bread isn’t a good environment for them). The symbiont community therefore appears opportunist, preservative, and a reserve of prebiotics and probiotics that populate the bee’s gut microbiome when the store is eventually consumed.
But just as their ancestors couldn’t avoid eating pollen grains, modern bees can’t avoid eating microbes. We know they do because we can look at the proteins in bees, and proteins in the microbiological ‘food’, specifically the ratio of isotopes of nitrogen in those proteins, and work out where in a food web a bee sits from what it eats[ix]. Most bees sit somewhere between herbivores (level 2.0) and carnivores (level 3.0, level four is for predators of carnivores). Bees are really omnivores. For example, Andrenidae scored 3.09; Apidae, 2.62; Colletidae, 2.11, and so on. We also know that if we eliminate or alter the microbe portion by manipulating their diet in a lab (say, giving them sterile food) they lack essential nutrients and fail to develop or thrive. The microbe community associated with the pollen is a vital part of its nutritive value[x].
If you want a ‘take home message’ it is this. Bees are all different, but most are not strictly vegetarian. The community of bacteria and fungi they depend on is much more complex than we think, so using antimicrobials (antibiotics & antifungals) on them or their food has consequences that are very hard to predict.
Dave Black is a commercial-beekeeper-turned-hobbyist, now retired. 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]Branstetter, M.G., Danforth, B.N., Pitts, J.P., Faircloth, B.C., Ward, P.S., Buffington, M.L., Gates, M.W., Kula, R.R., Brady, S.G., 2017. Phylogenomic Insights into the Evolution of Stinging Wasps and the Origins of Ants and Bees. Current Biology 27, 1019–1025. https://doi.org/10.1016/j.cub.2017.03.027
[ii]Sann, M., Niehuis, O., Peters, R.S., Mayer, C., Kozlov, A., Podsiadlowski, L., Bank, S., Meusemann, K., Misof, B., Bleidorn, C., Ohl, M., 2018. Phylogenomic analysis of Apoidea sheds new light on the sister group of bees. BMC Evol Biol 18, 71. https://doi.org/10.1186/s12862-018-1155-8, and Sann, M., Meusemann, K., Niehuis, O., Escalona, H.E., Mokrousov, M., Ohl, M., Pauli, T., Schmid‐Egger, C., 2021. Reanalysis of the apoid wasp phylogeny with additional taxa and sequence data confirms the placement of Ammoplanidae as sister to bees. Systematic Entomology 46, 558–569. https://doi.org/10.1111/syen.12475
[iii]Grinfeld, 1959. reviewed in Kirk, W.D.J., 1984. Pollen‐feeding in thrips (Insecta: Thysanoptera). Journal of Zoology 204, 107–117. https://doi.org/10.1111/j.1469-7998.1984.tb02364.x
[iv]Patricia Wiltshire, video 15.3Mb, 33min, https://www.bbc.co.uk/sounds/play/m000cz0p
[v]Roulston, T.H., Cane, J.H., 2000. Pollen nutritional content and digestibility for animals, in: Dafni, A., Hesse, M., Pacini, E. (Eds.), Pollen and Pollination. Springer Vienna, Vienna, pp. 187–209. https://doi.org/10.1007/978-3-7091-6306-1_10
[vi]Maccaro, J.J., Figueroa, L.L., McFrederick, Q.S., 2024. From pollen to putrid: Comparative metagenomics reveals how microbiomes support dietary specialization in vulture bees. Molecular Ecology 33, e17421. https://doi.org/10.1111/mec.17421
[vii]Carroll, M.J., Brown, N., Goodall, C., Downs, A.M., Sheenan, T.H., Anderson, K.E., 2017. Honey bees preferentially consume freshly-stored pollen. PLoS ONE 12, e0175933. https://doi.org/10.1371/journal.pone.0175933
[viii]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. Mol Ecol 23, 5904–5917. https://doi.org/10.1111/mec.12966
[ix]Steffan, S.A., Dharampal, P.S., Danforth, B.N., Gaines-Day, H.R., Takizawa, Y., Chikaraishi, Y., 2019. Omnivory in Bees: Elevated Trophic Positions among All Major Bee Families. The American Naturalist 194, 414–421. https://doi.org/10.1086/704281
[x]Dharampal, P.S., Hetherington, M.C., Steffan, S.A., 2020. Microbes make the meal: oligolectic bees require microbes within their host pollen to thrive. Ecological Entomology 45, 1418–1427. https://doi.org/10.1111/een.12926
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