Espresso, fidelity and constancy – The Apiarist


Synopsis : Why do bees gather pollen of just one kind when foraging? Why do they forage repeatedly in the identical space? What has espresso received to do with this?


Foraging is what my bees ought to be doing now. The summer time nectar movement ought to be sturdy – lime, blackberry, rosebay willow herb (RBWH, fireweed) then heather – it’s bonanza time.

However word the qualifier ’ought to’.

Thus far, it’s not trying promising.

The lime was hopeless, the blackberry flowered nicely however doesn’t seem to have yielded a lot, the fireweed is sort of over (early) and the heather … nicely, let’s not prejudge something, however I’m not hopeful.

Going, going, gone … rosebay willow herb, mid-July 2023

Not solely do these 4 crops/bushes yield nectar, however in addition they produce pollen and you’ll typically inform what the bees are foraging on by the color of the packed corbiculae on the hind legs of returning staff.

Regardless of their overlapping flowering intervals the pollen baskets are nearly at all times a single color. For instance, you don’t get deep purple pollen baskets from RBWH speckled with a lot paler borage pollen, regardless of the very fact you will discover each flowering – in a subject and its margins – concurrently.

It’s because honey bees are likely to forage on one plant species on any foraging journey. This function of the foraging behavior of honey bees is termed fidelity.

In the event you marked a foraging employee on a patch of RBWH, watched it fly off to the hive and waited a bit you may nicely see the marked bee return to the similar patch of RBWH and begin amassing pollen or nectar once more.

This isn’t fidelity however is as an alternative termed constancy.

Each constancy and fidelity have penalties for plant pollination. It’s subsequently unsurprising to find that some crops have advanced to affect these foraging habits of bees … which is the place the espresso is available in.

However I’ll get to that sooner or later.


We’ve recognized about fidelity for hundreds of years. Aristotle (Who else?) defined it one thing like this:

“On every expedition the bee doesn’t fly from a flower of 1 variety to a flower of one other, however flies from one violet, say, to a different violet, and by no means meddles with one other flower till it’s got again to the hive.”

Who else?

Properly … Charles Darwin, Hermann Müller, Gaston Bonnier and Karl von Frisch amongst others.

A bit of little bit of the historical past is reviewed by Verne Grant (1950) who additionally discusses among the research of fidelity from the early/mid 20th Century. A few of these observations – largely referring to producing uncontaminated vegetable seed pedigrees in the course of the Second World Conflict – are confusingly overlapping with constancy, so bear that in thoughts when you go rummaging by the archives.

You possibly can quantify fidelity in two fundamental methods:

  • analyse the pollen a great deal of returning foragers
  • mark staff and observe them returning to the identical flower kind over a number of foraging journeys

When the primary research have been performed with marked staff it quickly turned clear that fidelity is maintained each inside a foraging flight and between successive flights.

Moreover, these flights might be unfold over a number of days. In 1850 Hermann Müller demonstrated a single marked forager returning to flowering Salvia daily for 11 days.

The pollen evaluation actually must contain microscopic commentary. In spite of everything, a 50/50 mixture of pollens may be seen to the bare eye, however 95% of 1 kind combined with traces of others wouldn’t.

Bee species and fidelity

When pollen is analysed microscopically it turns into attainable to quantify fidelity after which make comparisons between completely different bee species.

Grant (1950) tabulated knowledge from a lot of research which I’ve summarised, simplified, averaged and graphed:

Fidelity in several genera of bees (see Grant, 1950)

Bees are grouped into genera – with honey bees listed beneath Apis (for Apis mellifera) – and aren’t essentially comparable because the samples have been analysed from Quebec, Colorado and the UK. Moreover, pattern sizes weren’t equal; over 3,000 honey bee pollen masses have been analysed, however solely 26 from Megachile (leaf cutter bees) and 5 from Anthophora (the biggest household of solitary bees).

Nevertheless, it’s clear from the graph that honey bees present one of many highest ranges of fidelity, and we might be fairly assured of this fidelity due to the pattern measurement.

The graph is a bit of deceptive; don’t assume {that a} ‘combined’ pollen load is a hotchpotch of various pollen sorts. Within the overwhelming majority of instances the majority of the combined pollen is definitely of a single kind. Each one of many ~20% of ‘combined’ pollen masses from Apis mellifera have been really 95-99% pure. Equally, no less than half of the ‘combined’ pollen masses carried by Bombus sp. have been over 95% pure.

Determinants of fidelity

Fidelity is clearly widespread in bees, even people who gather a proportion of combined pollen masses.

Monoculture ... beelicious ...

Fidelity assured

In a stunning yellow 100 acre subject of oil seed rape, the place there’s little or no different selection, fidelity is maybe unsurprising. Nevertheless, fidelity additionally happens in additional pure environments comparable to a combined wildflower meadow, or by bees foraging in subject margins.

Wildflower meadow

While it would appear to be widespread sense for a bee to stay to 1 flower kind – maybe for foraging effectivity – that’s a reasonably troublesome idea to review from an evolutionary perspective. And, earlier than you ask, nearly any organic phenomena might be defined in evolutionary phrases.

It’s price remembering why the bees are visiting Salvia, or oil seed rape or lime or no matter. They’re there to gather pollen and nectar, and the nectar is being produced by the plant to encourage the visits by the pollinator.

And fidelity may nicely be extra necessary to the plant than it’s for the bees.

Fidelity reduces the losses of pollen to crops of various species.

If the bee flitted from fireweed to borage to bramble after which zoomed off to a lime tree then the quantity of pollination that occurred can be lowered.

Due to fidelity a plant is extra more likely to develop into pollinated because the bee in all probability visited an identical plant earlier on the identical or latest foraging journey.

Color, form and odour …

Research relationship again over 100 years have demonstrated that the color, form and odour of flowers assist decide fidelity. Crops have advanced distinctive flower shapes and hues to facilitate their recognition by pollinators.

I don’t have time – or know the literature sufficiently nicely – to evaluate the seminal research by von Frisch and others on the associated areas of color imaginative and prescient, form and odour recognition by honey bees.

The predominant colors of insect pollinated flowers – yellow and blue (or mixtures of those and/or white) – have advanced as a result of these are the colors readily detected by bees , and that in all probability might be greatest discriminated towards the inexperienced of foliage.

… and reward

However there’s extra to a flower than form, color and odour that may be detected and in contrast by a visiting bee. The sugar content material and quantity of nectar may also be measured by bees and more moderen research have proven that these additionally affect fidelity.

The sugar content material and quantity of nectar varies between plant species and may range quickly over time.

Ratnieks and colleagues (Grüter et al., 2011) created an array of synthetic blue and yellow flowers after which ‘rewarded’ bees visiting the blue or yellow flowers with ‘nectar’ containing field-realistic ranges of sucrose resolution. They did this manually and so have been additionally in a position to range the variety of rewards and the quantity of nectar offered by the ‘plant’. Once more, volumes have been field-realistic, just like these returned by a person flower.

Having skilled the bees (24 coaching mixture, with 9 bees on every), they then recorded subsequent visits to the colored flowers.

Nectar high quality and amount will increase fidelity (click on for full legend)

These research confirmed that rising rewards, quantity or the sucrose focus all elevated fidelity, with the least rewarding mixture resulting in 75% fidelity, and probably the most rewarding scenario leading to over 98% fidelity.

In conclusion, fidelity – the repeated visits to a single flower kind by a forager – is widespread in bees and decided by the popularity of the flower by the visiting pollinator. This recognition contains, however in all probability isn’t restricted to, the color, form, odour of the flower and the reward supplied.

Most of those research have been carried out on honey bees, however I’d guess the conclusions additionally broadly apply to different pollinators. Bats are necessary pollinators for some tropical crops. I’d not count on color to be related (visits are more likely to be at evening) however odour and nectar power in all probability are, and form may be distinctive.


How distant are you able to see a bee from? A number of metres maybe (to tell apart a employee from a drone, or a bee from a syrphid)?

Over what distance may you discriminate a bee marked with a spot on her thorax from an unmarked one? 

Hermann Müller’s demonstration of fidelity with marked foragers in 1850 was attainable as a result of the bees returned, to not any flowering Salvia, however to the exact patch of Salvia through which the bees had been marked and Müller had chosen to look at.

So, the truth that the employee returned to Salvia confirmed fidelity, however the employee returning to the similar patch of flowering crops demonstrated constancy.

Constancy has not been studied in something like the identical quantity of element as fidelity. We all know it happens and we now know that honey bees differ of their constancy from among the different bee species investigated. Nevertheless, we don’t correctly perceive what determines constancy.

Understanding constancy might be necessary the place makes an attempt are made to enhance habitat for pollinators; how massive do patches of flowers must be to draw after which retain bees in order that they spend extra time amassing nectar and fewer time looking for nectar?

Since one of many finish merchandise of a pollinator go to is pollination the constancy of bees will affect each seed set (how nicely crops are pollinated) and the gene movement of crops in an setting.

If constancy is low then pollination is more likely to even be low, however genes from one plant will in all probability be extensively unfold within the setting. Conversely, if constancy is excessive, pollination charges and consequently seed set ought to be good, however genes from any particular person crops shall be extra localised, reasonably than being extensively dispersed.

Sufficient concept … are honey bees excessive constancy?

Fragoso and Burnet (2023) in contrast the constancy of honey and bumble bees utilizing a mark-reobservation examine on patches of alfalfa (Medicago sativa). They planted a sequence of enormous and small patches of alfalfa in an in any other case grass-filled and invariant setting. The density of alfalfa crops in massive (225 crops) and small (100 crops) patches was an identical.

Honey bees or wild Bombus impatiens (widespread japanese bumble bee) visiting the patches throughout peak bloom in two successive years have been captured, uniquely marked and launched.

Revisits of marked bees to the unique patch, or others within the examine space, have been recorded by handbook commentary each hour over 6 hours a day for ~1 month .

Constancy subject format (left) for research on bumble (centre) and honey bees (proper)

Outcomes are expressed by way of patch-faithful (pale color) and non-faithful (darkish color) for every of the 5 examine patches.

Three issues are instantly apparent from the outcomes;

  • honey bees present a markedly larger degree of patch constancy than bumble bees (for any patch, the proportion of the bar that’s within the pale color is bigger for honey bees). Over the 2 12 months examine, patch-faithful observations have been 76% for honey bee and 47% for bumble bees.
  • each honey bees and the bumble bee species studied favoured the bigger patches of alfalfa (the LF and LN bars are larger).
  • patch constancy assorted for bumble bees, who have been extra more likely to return to a bigger reasonably than smaller patches, than for honey bees (the proportion of the bar that was a lighter color is bigger for bigger patches)

So, honey bees do exhibit larger constancy than bumble bees … or no less than this species of bumble bee on alfalfa.

Not all pollinators are equal

Most of these research are time consuming to conduct and could also be influenced by the setting. Comparable research, of various species of bumble bees foraging on Delphinium or Gentiana have proven constancy might be larger at 67% or 78% respectively (in contrast with the 47% recorded on alfalfa), although I’m not conscious of a simultaneous comparative examine of bumble bees and honey bees.

Inevitably, there shall be delicate variations between pollinators of various sorts. These variations shall be influenced by different forage obtainable, the wants of the colony, the actual rewards offered by the crops.

The variations between honey bees and bumble bees spotlight the – frankly crass – greenwashing tried by firms who parachute a number of honey bee hives into an setting to “help pollination”.

Not solely will the honey bees probably compete with native bees, however the penalties of the pollination shall be completely different. The larger constancy of honey bees will scale back gene movement within the plant inhabitants, leading to a patches-within-patches sample.

The variations in constancy are more likely to be influenced by the foraging technique of various bee species; honey bees use cooperative foraging and dance communication to ‘promote’ appropriate areas whereas bumble bees follow repeated sequential visits to a sequence of places (trapline foraging).

Regardless of the technique, for a bee to efficiently return to a location it has to keep in mind the place that location is throughout the panorama.

Which, lastly, permits me to debate espresso.

Caffeine enhances a pollinator’s reminiscence of reward

This website runs on espresso. Beneficiant supporters pay for the server charges and backup house , and for the espresso that fuels my late evening writing marathons that produce posts like this one.

It’s the comparatively low doses of caffeine in espresso that enhances cognition, will increase alertness and attentional efficiency and boosts reminiscence retention

In excessive doses it’s deadly.

For a human the LD50 is ~175 mg/kg, equal to ~80 espressos for a 70 kg grownup.

Caffeine is produced by a variety of crops native to Africa, East Asia and South America. It has a bitter style and doubtless advanced to forestall grazing by herbivores. Along with being current within the leaves, some crops – together with the genera Citrus and Coffea – produce scented flowers, the nectar of which accommodates low doses of caffeine.

Notably, when pollinated by bees, Coffea and Citrus crops produce extra fruits and seeds.

This prompted Jeri Wright and colleagues to research whether or not caffeine impacts the educational and reminiscence of pollinators (Wright et al., 2013). The speculation was easy; caffeine stimulates reminiscence, bees are rewarded with nectar after they go to a plant, repeated visits (necessitating reminiscence) are wanted for pollination … maybe the low concentrations of caffeine in some nectars enhance the ‘reminiscence of reward’ and so would encourage repeat visits (over hours or days)?

Caffeine in nectars

Wright and colleagues quantified the quantity of caffeine in a variety of Coffea and Citrus species. Ranges have been low, maybe from ~80% to as little as 1% of the extent of caffeine in immediate espresso . Nevertheless, all 7 species examined produced detectable quantities of caffeine of their nectar, and the extent of caffeine was unrelated to the sucrose focus current.

Notably the extent of caffeine in nectar was nicely beneath the quantity decided to repel honey bees, and a tiny fraction of the degrees of caffeine within the vegetative and seed tissues of the plant that are current to discourage grazers.

This might recommend – if pollinators are ready to answer the caffeine current in nectar – that they’ve pushed the number of differential concentrations of the drug in several tissues of the plant, such that the caffeine in nectar will not be repellent however stays pharmacologically energetic.

So how do you take a look at if the degrees of caffeine current in nectars is pharmacologically energetic?

How do you take a look at the reminiscence of honey bees?

The proboscis extension reflex

A bee will prolong its proboscis in response to a scent they recognise (keep in mind) being related to a reward.

Bees have been skilled to answer a floral scented sucrose resolution by exposing the bee at 30 second repeats – the approximate fee of flower visitation by foragers – to the odour and reward.

The sucrose resolution was spiked with completely different concentrations of caffeine, and each the speed of studying and the long-term reminiscence was then examined.

Caffeine within the nectar brought on a weak however detectable enhancement in studying however profoundly enhanced long-term reminiscence.

Caffeine enhances reminiscence of honey bees (click on for legend)

When examined 24 hours after coaching, thrice as many bees remembered the florally scented resolution and responded. When examined 72 hours after coaching about twice as many bees responded to the floral odour.

Caffeine enhances reminiscence of reward in honey bees … however how?

Making reminiscences

The enhance caffeine supplies to mammalian reminiscence is partly as a consequence of its pharmacological exercise as an adenosine receptor antagonist. That is method exterior of the scope of a basic curiosity beekeeping weblog and I don’t need to lose the 0.0034% of readers remaining who no less than began this put up …

Suffice to say that caffeine induces long-term potentiation of neurones within the hippocampus and this can be a essential element of reminiscence formation.

And, experiments recommend that it really works in basically the identical method in honey bees.

Bees have Kenyon cells (KC) within the mushroom our bodies of the mind that are broadly equal to hippocampal neurones. Utilizing delicate electrophysiological strategies, Wright and colleagues immediately measured the exercise of the KCs and confirmed that they responded in an identical approach to that anticipated when uncovered to caffeine. Identified adenosine receptor antagonists (energetic in mammalian cells) additionally labored towards KCs.

Reminiscence, fidelity and constancy

This means that there are broad similarities between the methods reminiscences are fashioned within the human/mammalian mind and that of the honey bee and – to get us again on monitor considerably – that elements of nectar may improve reminiscence and so may probably improve fidelity and constancy.

Not all crops produce caffeine, however reminiscence formation of plant patches and nectar sources by honey bees doesn’t want caffeine, however is enhanced by the presence of caffeine at low ranges.

Formal experiments on a job for caffeine (or probably different compounds that work in an identical method) in enhancing reminiscence formation and so rising fidelity and/or constancy stay to be performed. Do bees present elevated fidelity for crops that specific caffeine?

Nevertheless, the variations noticed in fidelity or constancy between honey bees and bumble bees is not as a result of solely honey bees reply to caffeine … latest research have, unsurprisingly, proven that bumble bees additionally reply to caffeinated nectar in an identical method (Arnold et al., 2021).

Decaffeinated readers might not keep in mind the suggestion that the carpet of lifeless bees typically discovered beneath lime bushes could also be as a consequence of chemical deception i.e. caffeine, or a caffeine-like compound, inducing persistent foraging even after the bushes have stopped yielding nectar, resulting in eventual hunger.

Tilia tomentosa (the silver lime which is predominantly related to bee deaths) nectar does include caffeine, however so do different species of lime bushes not related to bee deaths.

All of which is a little bit of a moot level this 12 months as my honey supers echoingly affirm that none of my bees visited the close by lime bushes after they briefly flowered 🙁 .

I concern that the one fidelity and constancy my colonies will present this summer time is to fondant …


Arnold, S.E.J., Dudenhöffer, J.-H., Fountain, M.T., James, Okay.L., Corridor, D.R., Farman, D.I., et al. (2021) Bumble bees present an induced choice for flowers when primed with caffeinated nectar and a goal floral odor. Present Biology 31: 4127-4131.e4

Fragoso, F.P., and Brunet, J. (2023) Honey bees exhibit larger patch constancy than bumble bees when foraging in a standard setting. Ecosphere 14: e4606

Grant, V. (1950) The Flower Fidelity of Bees. Botanical Evaluate 16: 379–398

Grüter, C., Moore, H., Firmin, N., Helanterä, H., and Ratnieks, F.L.W. (2011) Flower fidelity in honey bee staff (Apis mellifera) relies on ecologically real looking rewards. Journal of Experimental Biology 214: 1397–1402

Wright, G.A., Baker, D.D., Palmer, M.J., Stabler, D., Mustard, J.A., Energy, E.F., et al. (2013) Caffeine in Floral Nectar Enhances a Pollinator’s Reminiscence of Reward. Science 339: 1202–1204


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