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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Thursday 30th June 2011

At last, for those of us further to the north of the UK, there has been a bit more warmth and even some sunshine resulting in more flowers opening and more activity from the bees and other pollinating insects.

As a child, I used to accept it when I was told that bees pollinated flowers and made honey with the nectar they collected. As a city boy, I didn’t distinguish between honey bees and bumble bees. They were all just bees. But, it turns out, it’s a bit more complicated than that and so is the answer to the question ‘Why don’t bees get poisoned?’

Papaver somniferum, opium poppy

Now I know that not only are there lots of different species of bees, even though I haven’t made a huge effort to learn how to identify them, but that many other insects play a role in plant pollination.

The Viburnum* that we planted, ten years ago, from a cutting taken on our one-day gardening course, is now a substantial shrub and has been literally buzzing with wasps for a few weeks now. And my Papaver somniferum, opium poppies, are usually a strong attraction for marmalade hover flies, especially the peony flowering varieties. Plus, of course, there are quite a few plants, in addition to the dramatic Dracunculus vulgaris, dragon arum, that rely on flies to do the business for them.

I don’t know what relative contributions to total pollination these different insects make but I do know that there is more to it than the headlines we keep seeing that, if the honey bees’ problem isn’t dealt with, we’re facing a calamity. Though I can’t name them, I would say I’ve seen at least five different species of bee in the garden so far this year though many people, I suspect, would assume they were all honey bees and all, therefore, at risk.

Dracunculus vulgaris, dragon arum

At this point, I should find myself guilty of the crime I frequently find in others. If we were to use the zoological names for these insects there could be no confusion. Saying that problems are occurring for Apis mellifera, which is the specific species kept for honey production and subject to so-called colony collapse disorder, avoids any confusion with the up to 43 other species and sub-species of honey bees.

But, back to the original question, with the revision that it should be ‘Why don’t pollinating insects get poisoned?’ and adding the important second question, ‘Can poisonous honey be produced?’ because, though people are more likely to be concerned about the wider world these days, they are much more interested in knowing if they are taking a risk by putting honey on their toast.

There are a number of answers. First, there is the question of dose, as there always is with poisons. Pollinating insects, or rather I’ll stick to bees as they are the creatures who make honey consumed by humans, usually visit hundreds of different plants in the course of a day. Those plants can be different genera or different varieties of the same species but they ensure that the bees are collecting only a tiny amount of any toxins that might be found in the pollen and nectar they take back to the hive. There is very little chance that, if you directly collected all the material from one bee, it would contain a high enough proportion of toxins to be a potential poison.

There are exceptions of course and these arise when one plant is dominant in a particular area. In the north-west of Scotland, huge numbers of Rhododendron ponticum are the only flowering plant at a certain time in the spring. It is known for bees to die after visiting this one species during a day’s work. Commercial bee-keepers, and informed amateurs, know this and will keep their hives closed for this period. Since, in this case, the toxins in the plant are sufficient to kill the bees we can assume that they don’t manage to produce toxic honey.

Senecio jacobaea, ragwort

Of course, we can’t be sure that the toxins in a plant survive the process of being converted from nectar to honey. Some do, as we’ll see in a moment, but it may be that the manufacturing process changes the chemicals so as to reduce their toxicity.

The other plant that can dominate an area is Senecio jacobaea, ragwort, and there was sufficient concern about this possibility for the UK’s Ministry of Agriculture to conduct trials. In short, the work did find examples of honey containing potentially harmful concentrations of the pyrrolizidine alkaloids that make ragwort hepatotoxic but, and this is key, those samples were very dark in colour, foul-smelling and totally inedible.  

The only occurrence I know of where poisonous honey has been consumed resulting in harm to humans is the Coriaria arborea, tutu, found in New Zealand, but here the mechanism for the production of the honey is unusual. Bees collecting nectar directly from the plant do not produce poisonous honey. But, a vine hopper insect also feeds on the nectar of the plant and excretes a sweet ‘honeydew’ containing a high concentration of plant toxins. Especially in times of drought, bees may gather this honeydew rather than nectar from the plants. Because this is a well-known problem, however, there have been no instances of poisoning from commercially produced honey since 1974.

When four people were taken ill in 2008, the source was traced to honey produced by an amateur who was not aware of the problem. Another instance of the flaw in the belief that the more ‘natural’ something is the better it is for you.

*It turns out that the Viburnum is actually Cotoneaster lacteus. Another example of the true extent of my gardening knowledge. (Added 19th August 2013.)