Lime trees in flower on city streets are magnets for pollinators. But reaping their copious rewards is not without risk. It is well known that large numbers of dead bumblebees accumulate under the trees, and a new study has provided fresh clues to what’s going on.
By feeding lime tree nectar to captive-bred bumblebees, biologists at the Norwegian Institute for Nature Research have ruled out the possibility that the insects are being poisoned.
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A bumblebee flying to comfrey flowers. © Andi Edwards/Getty
What they have discovered is that the dead bumblebees are lighter than normal and often bear significant injuries. This suggests that individuals weakened by starvation are being targeted by hungry birds, with the resulting wounds hastening their demise.
Malene Kyrkjebø Vinnes, lead author of the research in Ecological Entomology, also suspects that the numbers of dead bumblebees might ultimately be because lime trees attract so many of them.
“A relatively small proportion of the bumblebees in the tree crown die,” she says.
Main image: Further work is needed to work out why only bumblebees seem to be affected by lime trees. © SViktoria/Getty