Rhododendron nectar - poisonous to both bees and man.

May and early June are often the best weeks to visit the West Highlands of Scotland. The days are long, sunshine hours at their highest for the year and gardens and hillsides are ablaze with colour from rhododendron and azaleas. Although far from their native Himalayas, rhododendrons seem equally at home in the Scottish mountains. Rare species of rhododendron as well as the common R. ponticum flourish in the thin acid soils of the west. At the turn of the century West Highland landlords vied with each other as much over their collections of exotic plants, as the stag shoot and the size of the salmon bag. Today many of these same estates are being over-run by R. ponticum as well as native bracken.

That rhododendron nectar is poisonous to man has somehow seeped into our folklore. Perhaps we all remember from our history lessons the fate that befell Pompeys army after they feasted on the local honey crossing the mountains of the Caucasus. Yet most beekeepers are not sure how it is that with so many acres of the plant on the west, no-one seems to come to harm eating our honey!

The nectar from R. ponticum, the most common species found in the UK contains the poison grayonotoxin. Acute cases of honey poisoning reported from Nepal and Turkey indicate severe cardiovascular problems with very low blood pressure and slow pulse rate.(1) A Scottish case has been reported where a man licked R. ponticum nectar from his hands and rapidly experienced loss of co-ordination and an inability to stand.(2)

Rhododendron honey, however, remains toxic for only a very short period.(4) Honey that is stored in the comb, along with say the sycamore and bluebell, will have lost it's toxicity before the first extraction. Also, the bees themselves will consume most if not all of this nectar and honey for brood rearing during the spring and early summer build up.

Whilst the bees may build up well on the nectar and pollen of R. ponticum and most other rhododendron species, and appear to come to no harm themselves from the toxins, there are one or two species on which they do not fare so well. A study of the species that produce nectar toxic to bees was carried out on The Isle of Colonsay in the late 1950's. (3)

Niall McNeil who kept the bees at that time had experienced quite serious losses in hives kept near to the estate garden policies with it's famous rhododendron collection. The experts from the West of Scotland Agricultural College and Glasgow University Medical Faculty were called upon to investigate further. After a long, grey Glasgow winter, early summer in the Hebrides during the 1950's would have seemed as exotic a research location as say Madagascar for today's scientist !!

Nectar from different rhododendron species was collected, their toxins analysed and also fed to bees and injected into mice and cats. The species found to be especially poisonous to all victims were R. thomsonii, R arboreum and R. pratti.

The study describes well, from my own experience, the effect of nectar poisoning on bees. Every spring I find a good scattering of bees on the ground in front of hives or on hive floors, usually lying on their sides or backs, legs and wings trembling, as if having an epileptic fit. The tongue is nearly always extended. The severity of losses varies from year to year and between even neighbouring hives. Large numbers of bees might be recruited to the toxic nectar from one particular hive and not another A change in the weather will alter the foraging pattern of a hive when they next start flying. I suspect, also, that the amount of toxin produced in the nectar of the poisonous rhododendron species varies greatly each year. Thankfully, I have not yet experienced the extreme losses that prompted the original study on Colonsay.

For those interested in such things, the toxin isolated from the nectar of R. thomsonii etc, was found to be andromedotoxin ( acetylandromedol ) (3) The authors of the Colonsay study also indicate that the toxin grayanotoxin found in the nectar of the common R ponticum is identical to andromedotoxin!. This rather confuses the picture. Why do we not hear more often of bee poisoning with so much more ponticum on our hillsides? R. ponticum flowers later than most of the exotics and I have not noticed any poisoning casualties once the exotics have finished flowering. An interesting but not obviously valuable research project for someone.

References.
1. Newsletter of the WHO Surveillance Programme for Control of Foodborne Infections & Intoxications in Europe 1996 49/50 6
2. Cooper,Johnson Poisonous Plants & Fungi: an illustrated guide. HMSO 1991
3. McLeod-Carey,Lewis,MacGregor and Marin-Smith, 1959 Pharmacological and Chemical Observations on some Toxic Nectars. J. of Pharmacy and Pharmacology 11, Suppl. 269T-274T
4. SCIEH Weekly Report 1996 30 No. 96/51 p.275

Andrew Abrahams - Isle of Colonsay