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.