Short answer:
It depends on the species of rhododendron – and also on the sub-species of honey bee visiting the rhododendron.
The common rhododendron, Rhododendron ponticum, certainly does produce toxic nectar. However, Irish research suggests the nectar may only have a negative effect on honey bees in countries where the rhododendron is an invasive species outside its native range – and even then, they’ll probably avoid visiting it anyway. Here’s why…
Long answer:
Rhododendron ponticum is known as a source of toxic nectar which causes bees to produce ‘mad honey’, used by European armies through the ages as a weapon of war.
By Rasbak – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=800114
The honey would be left in the path of invading legions; the soldiers would eat the sweet treat and end up vomiting and dizzy from grayanotoxin, a toxin contained in rhododendron honey. The effects rarely prove fatal to humans but probably would have halted or slowed down armies for a while. The grayanotoxin is the plant’s defence against herbivore attack.
Just two to three teaspoons of the spring honey made by Himalayan Giant honey bees (a subspecies of Apis dorsata) can result in temporary paralysis for a day or so, according to Mark Synnott’s article ‘The Last Honey Hunter‘ in National Geographic. A market for the honey exists because some people believe it improves sexual performance – this can go very wrong!
Which bees visit Rhododenron.ponticum?
Rhododendron ponticum By First Light at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6644210
R.ponticum was introduced to Ireland in the 18th century and has invaded large areas of the countryside, where it is regarded as a pest. Yet research led by Prof. Jane Stout, Professor in Botany and Dr Erin Jo Tiedeken, Research Fellow at Trinity College Dublin, has found that its flowers are visited almost exclusively by bumblebees, with occasional visits from solitary bees, flies, ants and wasps.
Professor Stout and Dr Tiedeken found that the nectar’s grayanotoxins cause palpitations, paralysis and death within hours – for honey bees.
They also tested the grayanotoxins on an Irish species of mining bee, Andrena scotica; this species did not suffer increased mortality but had its behaviour severely affected, for example becoming paralysed or spending increased time grooming.
In contrast the nectar has no apparent effect on worker buff-tailed bumblebees. Professor Stout suspects that the subspecies of honey bee that makes mad honey in the rhododendron’s native range has probably evolved to resist the toxins in a similar way to the bumblebees.
Who knows their rhododendrons…from their rhododendrons?
My beekeeping buddy Emma (Mrs Apis Mellifera) sent me a useful link from the Poison Garden website which suggests that most of us will be unable to tell what is R.ponticum and what is a hybrid – and therefore potentially less toxic – plant.
Pontifications on poison: rhododendrons
“Rhododendron is thought to appear in around 1,000 species and those species produce innumerable hybrids. This means there are very few people expert enough to identify exactly what Rhododendron a particular plant is.
In terms of appearance and flowering, that doesn’t matter too much but it has been found that the concentration of the main toxin is species/hybrid dependent so plants that appear to the layman to be identical may produce different degrees of poisoning.” – John Robertson, thepoisongarden.co.uk
It even depends where the rhododendron is growing…
To complicate matters further, Kew Gardens researchers have discovered varying levels of nectar toxin levels even within different R.ponticum plants: Hidden poisons in rhododendron nectar (see the section ‘The changing chemistry of invasive plants’). A landscape scale chemical analysis comparing R.ponticum plants in Ireland with R.ponticum plants in the species’ native range of Spain and Portugal found that toxin levels were lower overall within the Irish plants.
The researchers suggest that within their invasive range (as studied in Ireland) the plants “reduce or stop entirely the production of nectar toxins to ensure sufficient pollination success (Egan et al., 2016)“. That makes sense – it’s in the best interests of the plants to attract pollinators.
So, do beekeepers need to worry?
In conclusion, it sounds like rhododendrons are unlikely to cause British beekeepers many problems. If we do have R.ponticum near us, the research carried out by Professor Stout and Dr Tiedeken suggests that our honey bees will probably avoid it; additionally the plants are likely to be less toxic than in their native range. If we only have hybrid rhododendron species nearby, the hybrids are likely to be less toxic too.
Rhododenrons may only be a problem for beekeepers surrounded by large areas of R.ponticum, which could smother out other plants and reduce the amount of forage available for honey bees.
See more:
- Grayanotoxin Poisoning: ‘Mad Honey Disease’ and Beyond
A scientific paper on mad honey. Contains a fascinating description from the Greek warrior-writer Xenophon in 401 BC on the effects of the honey on an army – “those who had eaten a great deal seemed like crazy, or even, in some cases, dying men”. - A rare case of “honey intoxication” in Seattle
Rusty at Honey Bee Suite reports on the rare case of a man who may have been poisoned by honey purchased at a local farmer’s market. Rusty’s observations have led her to believe “that rhododendron is not a preferred forage for honey bees and they probably collect it only in rare circumstances when other more favorable blooms are not available.” - Mad honey
A 2018 post by Scottish beekeeper & scientist The Apiarist, who has a particular interest in rhododendrons because he’s surrounded by them. - “Mad Honey” sex is a bad idea
That got your attention! - Hallucinogen Honey Hunters documentary
A tribe in Nepal hunt wild rhododendron honey with natural psychoactive properties. One falls unconscious after overdosing on the honey. - The strange history of ‘Mad Honey’
Emma Bryce writes about Turkey’s hallucinogenic rhododendron honey (deli bal), produced on remote mountainsides smothered with vast fields of cream and magenta rhododendron flowers. - ‘The Last Honey Hunter‘ by Mark Synnott (National Geographic, July 2017, p.80-97)
Stunning photos of 57 year old Mauli Dhan harvesting ‘mad honey’ from steep Nepalese cliffs. Mauli climbs for the honey since a spiritual dream many years ago set him on his path. In his people’s tradition it is bad luck for anyone who has not had this dream to take the honey. - ‘Poisons in rhododendron nectar‘
Philip Stevenson (Plant Chemist) and Alison Scott-Brown (Plant/Insect Ecologist) from Kew’s Natural Capital and Plant Health department report on the toxicity of natural chemicals in the nectar and leaves of Rhododendron. - ‘Bitter Sweet Nectar: Why Some Flowers Poison Bees’ by Stephanie Pain, BBKA News, February 2016
Adventuresinbeeland's Blog 
Very interesting! Jane Stout and I wrote a grant together, but it didn’t get funded haha…otherwise I would have been in your neck of the woods. Actually, many plants have different levels of toxins in the nectar and sometimes bees will seek it out to “self-medicate” if they have a parasite or pathogen. A very new area of research, I think!
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What a shame that it didn’t get funded, would have loved to see your photos of British bees. It’s interesting that some toxins can actually be beneficial for bees.
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Yeah that would have been fun!
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very interesting; by the way I like the idea of “randy rhododendron fans”!!
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Yes, and ‘heightening their Turkish delight’!
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Hallo Emma, Last Saturday was the national Bio-dynamic beekeeper day in The Netherlands, a real red letter day so to speak. In the foyer of the school where the event takes place there are people who have products and plants to sell specifically of interest to beekeepers. I bought 3 Aesculus Californica young trees. €10,00 per tree.
When I got home late that night I looked up in Wkipedia information about my trees. I was dumbfounded to read that the nectar and the pollen of these Calfornian Chesnut trees are fatally poisonous to European and Asian honey bees. I contacted the grower on the monday morning and he in turn was quite shocked. He reimbursed me half the money and told me to destroy the trees. His nursery is too far away for me to take them back and he didn’t want to drive half way either…
I feel bad about throwing these, otherwise healthy, trees in the bin but all my friends are bee keepers too so they won’t appreciate such a ‘present’. As well as this aspect of the problem, most legislative councils are pro active in having useful bee forage plants in public areas these days so I don’t know what to do and would appreciate some input from you. My terrain is 1ha or approx 2 acres if I include the piece we hope to buy in the future. Are there reader of your site who have practical experience of this tree, would honey bees avoid it if there was plenty of other forage available or is the risk too great? Thanks for your thoughts anyone else too.
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Sorry to hear that Lindy, what a shame. I have not heard anything about these trees to be honest, but will ask the question on the British Beekeepers Facebook page to see if anyone else knows and can help you.
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Hi Lindy, so I asked on the British Beekeepers Facebook group and have had a helpful reply already from a beekeeper called Andy Willis. He said:
“In the book “Plants and Beekeeping” by F.N.Howes. Aesculus califonica is mentioned……It flowers July – August and the blossoms are much visited by bumble and hive bees. This species yields honey in California but is suspected of poisoning or causing paralysis in bees.
I also happen to know that it is planted at the Sir Harold Hillier gardens as an ornamental, not far from the hives there, and I have not heard of any issues. It is probable that it needs a high light intensity ( like you would get in California) for it to yield copious nectar that may then contain concentrations of the toxin at a level that might cause an issue.
In a separate part of the book it mentions….. ‘buckeye poisoning ‘ troublesome to Beekeepers in the southern United States. In buckeye poisoning it is the brood or young bees rather than the adult bees that are affected. But doesn’t say there are issues elsewhere in the world where it is planted.
In short it would probably be just fine, but for future reference the Indian Horse-chestnut ( Aesculus indica ) would be far better for the local bees as it flowers in the June gap and the nectar is not reported to be toxic to bees. I hope this helps.”
So it sounds hopeful that the trees would not be a problem for you. Will let you know if I get any more replies 🙂
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Thanks, Emily. You are such a talented and interesting writer. And prolific! (How do you find the time!?). I’ve long been intrigued by the idea that a plant could poison its pollinators. It had not occurred to me that non-honey bees (bumblebee ssp) may have evolved to handle the poison. This makes sense – rhododendrons bloom in the spring when honey bees may be distracted by other flowers. So, the flower’s strategy is to avoid over-dependence on a bee that might not be faithful. Instead, the rhododendron nurtures bumblebees that depend on its flowers, creating a stronger symbiosis.
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That is very kind of you to say Miksha. At the moment I have time because I’m on maternity leave, with my baby due on 10th April. I know things will soon get very busy so am trying to write all the posts I can now before life goes crazy!
I think the main evolutionary advantage of the graynotoxins is that they are poisonous to grazing animals like horses, which helps prevent the plants from being eaten.
Also the research project’s press release at https://www.tcd.ie/news_events/articles/toxic-tastes-ireland-s-bees-and-non-native-rhododendron-nectar/6109#.VuyCm7QeNFJ has an interesting quote from the project’s chemist Professor Phil Stevenson: “What we don’t know is how bumblebees cope with this natural toxin and why some other bee species can’t or even why the plant apparently seeks to poison some potentially good pollinators. It may be a mechanism to select specialists that are more effective at pollinating the plant.”
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At this time of year (late summer/early autumn) every year we have bee deaths in elevated numbers. The first time it happened we suspected pesticide poisoning. That year was very dry and we suddenly had piles (hundreds) of dead bees in front of the hives every morning – not all hives, but maybe 20% of them. The past 2 years it’s been a different pattern (and we’ve had more rain). The deaths start slowly (tens, not hundreds), build in number of deaths and colonies affected, go away, come back and the whole drama takes about a month to finish doing its damage. I’m now convinced the bees eat something that is poisonous to them. I have no idea what it is. I’ve asked lots of beekeepers and have been told vaguely that there are plants that kill bees but no one has any specifics (nor can I find them anywhere). I guess it doesn’t really matter since I couldn’t stop the bees eating unless it’s a plant on my property that I could prune or chop down!
On thing’s for sure, no rhododendrons here. Any other suggestions?
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That is so perplexing and must be very upsetting to see 😦
Lindy mentions above that Aesculus Californica (Calfornian Chestnut) trees are known to be poisonous to bees. I found a list of other plants at http://www.buzzaboutbees.net/Plants-Toxic-for-Bees.html – scroll down to the blue ‘List Of Plants Toxic For Bees’ chart – perhaps you could check whether any of these grow near you.
In the UK lime trees have been linked to bumble bee poisoning, as large numbers of bumble bees can sometimes be found dead under lime trees. However the jury is out about whether lime nectar actually poisons bumble bees, or whether bumbles continue feeding on lime nectar even when levels are low and so they run out of energy and collapse.
Do the dead bees in front of your hives look different from normal bees in any way?
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Thanks for this link. I’m bookmarking it and will have to study it.
As for the dead bees… they look normalish. There are a high number that have their proboscis extended which I have read is an indicator of pesticide poisoning (could it be the same for nectar poisoning?) I’ve also, more than once, found bees landing in the grass about 50 cm in front of hives affected this way as if bees are having troubles finding home.
I can’t help but wonder how common this kind of thing is but perhaps not noticed by beekeepers. If you go out at dawn, the affected hive looks normal – no dead bees. If you look at noon, the hive looks normal. The problem is only observable in the time gap from about 1 hour after dawn to mid day where the bees have pushed the dead bees out of the hive but not removed them from the immediate area in front of the entrance. Throughout the morning, the more energetic and house-proud bees actually drag the corpses away from the entrance so, without really studying the colonies at the right time, you’d never see the dead bees.
Our hives sit on the ground with a strip of coreflex about 25 cm wide in front that keeps the grass from the entrance but also works as a perfect examining platform. Without the coreflex I don’t think we’d notice the problem and just assume normal end-of-season population drop.
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This is news to me and I am glad to learn it. I never really noticed bee activity on Rhododendron, but will now. My Akita ate one once and he never got sick, but it was a hybrid. I was very mad at him too.
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Glad your Akita was ok. Hopefully your reaction will have put him off eating any more!
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He is gone now but lived much longer (13.5 years) than most of his breed. He never ate another one I can tell you that.
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I saw the documentary about Nepalese honey hunters. The locals know the effect of the honey so they consumer it in minimal amounts. I heard that some people have started to raise their bees on Cannabis flowers.
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Do the cannabis flowers also produce ‘mad honey’?
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A fascinating post Emily, interesting to learn that bees may self-medicate and that rhododendron is poisonous to honey bees. Common rhododendron is regarded as an unwelcome alien invader and being eradicated from Nature Reserves here in North Wales, so not a good hunting ground for ‘mad honey’ here!
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I have a bush of this type which I knew nothing about until about a week ago when I started finding dead bumblebees near it ! I am now convinced that it is killing them and much as it pains me to, I am going to eraditcate it as I feel that, sadlt, bees need all the help they can get these days !
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Hi Mel, sorry to hear that. I’ve never heard any reports of rhododendrons killing bumblebees though, only honey bees. Bumblebees seem to be immune to the toxins. It may be that the bush is not a rhododendron, or that something else is killing the bees.
Dead bumbles have been found under lime trees before and one theory is that they have died of exhaustion after being able to find alternative flowers when the lime nectar ran out.
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