The story of dead bumblebees at the Botanics that had apparently been killed by the toxic effect of nectar from silver lime (Tilia tomentosa) http://stories.rbge.org.uk/?p=5319 has taken a new twist thanks to the experts at Bumblebee Conservation Trust. Anthony McCluskey at Bumblebee Conservation Trust has explained that the observed mass deaths may just reflect aspects of bee biology and behaviour and be nothing to do with toxic nectar. According to Anthony…
“Older papers on this subject suggest that it might be toxic nectar killing the bees, but the latest research suggests that it’s because of the bees’ behaviour when feeding from certain plants. Lime trees produce lots of nectar, but at times when the nectar is low (e.g. warm days, and at the end of the flowering period) the bumblebees continue to try to feed. So they waste a lot of energy doing this, and eventually become lethargic and die. For some reason, honeybees don’t die like this (despite feeding on the same flowers), so it’s a bit of a mystery as to why bumblebees behave this way.”
“It’s hard to know whether lime trees do more harm than good. I certainly wouldn’t recommend cutting them down (as some have demanded), and there is a fair chance that the bees being found dead under them are those which are prone to dying at this time of year – the older workers and males.”
This agrees with my observation yesterday that a large Eucryphia ‘Nymansay’ near Inverleith House in the Garden was also surrounded by dead bumblebees. I find it hard to believe that two trees so attractive to bees that they can literally sound like buzzing high voltage power lines are both producing toxic nectar! It’s much more plausible that dead bees are most often seen around plants that are highly attractive to bees. See the webpage at Bumblebee Conservation Trust for more information.
So the worries about silver lime seem to be unfounded and anyone with a silver lime in their garden can be proud of the service their tree is doing for local bees.
A big thank you to everyone who passed on their concern and comments via Facebook and Twitter.
About two/three weeks ago I came across a mass bumble bee death under a flowering hedge (privot) on Rodney Street in Edinburgh. It was the first time I had ever seen such a thing. Another lady, who was passing, said they she came across a lot of dead bumble bees just a few days earlier. Again they were found under a flowering privot hedge, this time near Meadowbank in Edinburgh. Then a few days later I read the article about the “toxic” nectar of the lime trees! I wonder if it has been worse this year or are we just noticing it because of being out and about more in the nice weather?
I think this just adds further evidence to the idea that plants that attract bees are more likely to have dead bees nearby. Privet is nectar rich and sweet smelling and I would imagine highly attractive to bees.
I too have seen a heavy sprinkling of dead bees under what I think are silver lime trees. I could not believe that this was just a statistic of bumble bee frequency. I was so unconvinced I went looking for more information; and it seems that the chemical mechanism of bee death is already known to science and that someone has also done the statistics, counting the number of bees visiting and the number of bees dying, suggesting strongly that tilia tomentosa is not benign.
The article I found is in the Journal of Apicultural Science, Vol. 54, No. 2, 2010 by Tadeusz Pawlikowski.
Elaine, many thanks for your comment and interesting you have seen the same sort of mass bumblebee death that we see here in the Garden each year. The reason I am sceptical about the poison nectar theory is that a number of plant species that are highly attractive to bumblebees have been noted as having large numbers of dead bumblebees below them in this late summer period. The ones I know of are privet and Eucryphia. I can’t discount the possibility that all these unrelated plants have toxic nectar, but it would seem unlikely. It strikes me as very odd that any plant which displays insect pollination syndromes should actually poison insect pollinators. This maladapted situation would quickly be prevented by natural selection. I would not take publication in a scientific journal as evidence of proof of the toxicity theory. The biggest problem for the theory is that bumblebee life cycles are very different to honeybees. It just so happens that silver lime (Tilia tomentosa), the lime that seems to be involved in these mass deaths of bumblebees, has a flowering time that coincides with the time of year when bumblebee workers are at the end of their natural lives. Only a new generation of queen bumblebees survive the winter (in hibernation underground in most cases). All the workers of the relatively small bumblebee colonies will perish during late summer/early autumn. I think that any plant that is highly attractive to bumblebees at this time will have a disproportionate number of dead bumblebees below it, and this seems to be exactly what we observe. On that basis I would regard what has been said about silver lime nectar with great scepticism.
Please consider the effect of Nosema ceranae in urban trees, evidence is in Apis and Bombus bees found dead found under Tipuana tipu trees blooming.