Lime trees: delicious or deadly for bees?
5th Jul 2021
Have you walked under a large tree this July and noticed a heady fragrance? Chances are that tree was a lime (or 'linden') tree; the source of the scent its abundant sprays of dainty pale yellow-green flowers. It's a sweet aroma that you can drink in on a sunny day, or also as a cup of linden tea made from the dried flowers.
Usually planted as park or street trees due to their size, the commonly found lime trees in the UK are small-leaved lime (Tilia cordata) and large leaved lime (Tilia platyphyllos) – both natives – along with the less common hybrid Tilia x europaeus, cultivated forms of silver lime (Tilia tomentosa) and Caucasian lime (Tilia x euchlora). The latter has the advantage that it doesn't attract aphids in the same way as the others, which produce so much honeydew it leaves sticky patches on anything underneath the tree canopy. Anyone who has parked a vehicle in the shade of a lime will know the problem!
Lime trees don't only attract aphids. They have a reputation as being a sort-of bee narcotic and in some languages their common name is 'bee tree'. In June-July, bees flock to limes for their abundant pollen, but the observation of numerous dead bees, especially bumble bees, under lime trees has led to the idea that lime nectar is toxic for them. Specifically, the presence of nicotine and a sugar called mannose in the nectar have been incriminated. Another hypothesis is that bees feeding on lime trees cannot obtain enough nectar from them to survive, so they starve to death.
A 2018 Belgium-based study put these hypotheses to the test, measuring levels of nicotine and mannose in four species of lime tree and observing whether bees exclusively fed on lime died in significantly high numbers.
Intriguingly, the results found no trace of mannose or nicotine in any of the lime flowers sampled. In their analysis, the authors suggest that the mannose hypothesis originated before more accurate, modern methods were available for studying chemical composition of nectar. They also cite evidence that even if low levels of nicotine have been found in other studies of lime nectar, these levels are not dangerous to bees.
However the authors do acknowledge that toxicity tests of the nectar's volatile secondary metabolites are lacking. A 2019 study in Italy, for instance, reported 13 of these chemical compounds in Tilia tomentosa for the first time. Secondary metabolites in plants often have medicinal properties for humans (you may have heard of phenolic compounds or flavonoids, for example), but who knows what they might do to bees?
Neither did the study find that there was any particularly high rate of mortality among bees fed exclusively on lime. Indeed, they concluded that lime produces plenty of nectar, even more than many other trees planted in the urban landscape, so the starvation hypothesis is dubious.
They argue that the correlation between lime trees and bee death could in fact be spurious. Bee populations peak in mid-summer, but their individual life expectancy is short, so this time also coincides with a high rate of bee mortality. Meanwhile, lime trees are in flower and are often planted in urban areas. So perhaps it is just that dead bees are more obvious underneath these trees, where they are congregating, than in other habitats like meadows, for example? In other words, other floral hotspots may well turn out to look like bee graveyards at this time of year as well! To confirm this 'natural death hypothesis', researchers need to compare the number of dead bees found near lime trees with numbers found near other types of flowering plant. No scientific study has done this, yet. Till then, the jury on lime-bee-toxicity is still out!