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…
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.
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?
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.
“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.
- 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