Findings of dead bumblebees under lime trees - a review article
What you will learn here
- Why do we find dead bumblebees under flowering lime trees during the summer?
- Are lime trees toxic, or are there dead bumblebees for another reason?
Date of last update: 20/07/2021
Contents
Introduction
What about linden trees and the dead bumblebees you find beneath them?
Appearances can be deceiving.
First hypothesis
Second hypothesis
So, let's summarise.
All right, so bumblebees are lying under the lime trees, why is that?
Third hypothesis
Conclusion
Regarding the lime trees from our article
Introduction
To start with, straight away Thank you.
Thank you to our editorial colleague, Dr K. Kučera for his text and analysis. We published the article on our Facebook page Čmeláci PLUS in 2019. Since then, it has been frequently cited, and we believe, rightly so.
The „man-eating tree“ type of HOAX, so often cited in literature, certainly deserves this explanation.
What about linden trees and the dead bumblebees you find beneath them?
You've surely also noticed the repeated disaster reports of hundreds of dead or dying bumblebees under blooming non-native lime trees (primarily *Tilia tomentosa* (also sometimes with the Czech name applied as silver linden)) Silver Lime, lime trees*T. euchlora*.)
Are lime trees really murderous?
Photo O. Hercog 7/2021
Using common sense, simple logic and the „seeing is believing“ principle, the matter seems clear – a non-native species of linden contains something that kills bumblebees. Be it nectar or pollen, or indeed some kind of toxin contained within the flower or plant.
And so let's start the chainsaws and get rid of those invasive species...
So why do we write about it?
This is a prime example of, how little we know about nature, ...how little we understand about relationships in nature, and how things that seem clear at first glance, according to common sense, turn out to be complicated.
And also a bit of justification on why we believe that even for bumblebee husbandry cannot be accessed in the style of simple solutions There is an opinion that bumblebee farming is easy compared to bee keeping and pretty much for everyone, all you need is a hive, a lid and bedding... But on the contrary, it is still an unexplored area, where one must constantly search, gather information, evaluate it, even in the context of a rapidly changing climate, and adapt farming accordingly, which is why we have joined together and founded this website.
Well, if you are interested in how it is with this „mass bumblebee killer lime tree“, then read on ….
PS. You can put the pill back in its case with peace of mind.
Silver lime
Photo: Wikipedia
Silver lime
Photo: Wikipedia
Lime tree (Tilia euchlora)
Photo: Wikipedia
Lime tree (Tilia euchlora)
Photo: Wikipedia
Appearances can be deceiving.
The matter at first seemed clear – afterAmong the dead insects found in the mature lime trees, 87% bumblebees were identified, of which 99% were ground-nesting and tree-dwelling bumblebees (Muehlen et al. 1992).
It was therefore assumed that the problem was partly due to the small amount of nectar and partly to the nectar of these lime trees containing a toxin that causes mass die-offs. Lime trees tolerate urban environments well, they are decorative, and moreover, our „common“ tree, so it is not surprising that they are used in urban architecture. Lime trees are a rich source of pollen and partly of nectar for bees, which are their main pollinators.
There are approximately 30 known species of lime trees, but the death of bumblebees has been recorded not only beneath some exotic species but also beneath the small-leaved lime (also known as the small-leaved lime tree). (T. cordata) and to a lesser extent, large-leaved linden (T. platyphyllos).
All kinds of lime trees They bloom profusely in June and July, Their flowering periods overlap considerably.
The large-leaved lime first begins to flower and vice versa Lime tree is a species that Blooms latestThe flowers have numerous stamens and are easily accessible to short-tongued pollinators.
Small-leaved lime
Photo: iPrima
Small-leaved lime
Photo: iPrima
Large-leaved lime
Photo: Botany.CZ
Large-leaved lime
Photo: Botany.CZ
.
First hypothesis
First hypothesis, that the deaths of bumblebees are caused by a substance contained in pollen or nectar, has not been confirmed.
Scientists initially suspected that mannose, a nectar sugar that is difficult for bumblebees to digest, was to blame.
In 1977, Mandel and, in 1994, Hagen even classified mannose among bee poisons. However, mannose was not found in the nectar or pollen of lime trees (Baal et al. 1991, Denker et al. 1992, Jacquemart et al. 2018). Nectar can also contain a number of other potentially toxic substances from the alkaloid, phenol, and other compound groups. Suspicion therefore fell on the alkaloid nicotine, for which the lethal dose for bees is 12.3 mmol/l (Detzel and Wink 1993), whereas a toxic level was observed in bumblebees (Bombus terrestris) at only 0.1 mmol/l (Tiedeken et al. 2014).
Detailed cchemical analysis also did not find any presence with concentrations that could cause toxicity. (Jacquemart et al. 2018).
Silver linden and buff-tailed bumblebee
Photo: H. Kříženecká
Second hypothesis
The second hypothesis then it was, že Lime blossoms do not contain enough nectar, which is something like a bumblebee fuel.
Without an energy source from nectar, bumblebees are unable to fly or maintain thermoregulation and therefore perish under trees from exhaustion.
This possibility was reinforced by the fact that deaths are not regular, but only occur in certain years under a combination of other circumstances, primarily average temperatures and rainfall.
In 1993, authors Baal and Surholt demonstrated by weighing bumblebees entering and exiting a hive under lime trees that the bumblebees on the lime trees carried minimal nectar, and simultaneously, their mortality rate increased.
In their 2018 paper, Jacquemart et al. found that the common lime (T. europea) (Hybrid of large-leaved lime and small-leaved lime) and woolly lime they have a denser bloom than small-leaved lime and large-leaved lime. Downy-leaved lime also has Many more flowers overall in comparison with the other 3 monitored species. The amount of nectar in silver lime and small-leaved lime was twice as high as in the flowers of common lime and large-leaved lime.
Even more interestingly, the analysis of sugars in nectar by the mentioned authors turned out as follows. In the woolly lime tree, there were sugar concentration 2x higher than in large-leaved and common lime trees, and even 4x larger than in small-leaved lime tree. This variability was primarily due to the concentrations of the key sugar – sucrose. The ratios of sucrose (disaccharide) / hexoses (monosaccharides) differed significantly between lime species. Sucrose was the dominant sugar in the nectar of woolly-leaved lime, whereas in the nectar of common lime (T. europea) was the dominant sugar monosaccharide.
The above therefore does not correlate well with the initial hypothesis.
The attempt by the aforementioned authors was also interesting, where bumblebees had no other pollination option for 6 days than from the specified lime flowers. Bumblebees pollinating exclusively small-leaved lime and bumblebees pollinating exclusively felted lime survived much better than their colleagues from the control group (pollinating freely growing plants). After 6 days, the mortality in the lime group was 5-12 individuals, while in the control group there were around 30. Thus, even this hypothesis, so clear at first, is, so to speak, afloat...
Tree bumblebee (Bombus hypnorum) on a small-leaved lime (Tilia cordata)
Photo: Keila
So, let's summarise.
- Four studied lime species (large-leaved lime, small-leaved lime, downy-leaved lime, and their hybrid common lime) differ both in the quantity of their flowers and their composition.
- The felt-leaved lime offered more flowers per cubic metre, more pollen grains per flower and more nectar than the other studied lime species.
- The total sugar content in lime nectar ranged from 0.24 mg per flower (in _Tilia tomentosa_) and 0.06 – 0.10 mg (in the remaining three other species). Similar sugar volumes and concentrations were also found by other scientists (Käpylä 1978; Pigott 1991; Krasenbrink et al. 1994; Baal et al. 1994; Gašićet al. 2014; Somme et al. 2016; Argoti2016).
- The linden nectar contained the optimal composition of nitrogenous substances and amino acids necessary for the development of larvae. The authors found that the “most deadly downy-leafed linden“ had the best nectar composition for pollinators.
- No potentially toxic components were found in the nectar, whether it was mannose or nicotine alkaloids. However, the toxic influence of other minor substances cannot be ruled out, although this seems very unlikely.
- Jacquemart et al. did not record any increased mortality, even though the experiment took place in 2016, when climatic conditions during the monitored period (May to August) were normal, and a rainy spring was followed by a summer with average rainfall.
Conclusion = neither of the possible hypotheses convincingly explains the cause of death of bumblebees under the lime trees.
Are lime trees really murderous?
Photo O. Hercog 7/2019
Alright, so the bumblebees are lying under the lime trees, why is that?
Key to explain the mystery is not in the lime trees, but rather in the lifecycle of bumblebees.
Bumblebees are, however prone to starvation And adverse periods shorten the colony's lifespan.
Authored by Illies and Mühlenem, it was monitored in 2007 bumblebee mortality, when their maximum corresponded to the peak of the linden blossom which seems to be paradoxical as a single flower of this linden tree is capable of producing 3-4mg of nectar per day. This is much more than all other commonly planted trees in cities are capable of. Consequently, even after more than a decade, research into this relationship has not been fully elucidated.
Are lime trees really murderous?
Photo O. Hercog 7/2020
Hypothesis three
However, a third hypothesis also presents itself, which is that Everything is actually perfectly fine..
Don't believe your eyes? Don't, because you might be witnessing an optical illusion, a logistical redistribution in nature. Lime trees (and that woolly one last) because tovetoed at a time when most bumblebee colonies are reaching their peak and nests are entering a period of decline.
It's connected with the natural death of workers, which mostly occurs outside the nest (startle), but sometimes also in males.
Given that individual bumblebees have a short lifespan of around a month, The mortality rate at this time is high.
MÜHLEN et al. (1992, 1994) found that the majority of dead bumblebees are exclusively workers, which can support this hypothesis. Given that bumblebees tend to choose the best source they remember and forage there until their bodies are torn apart, and considering the fact that urban green space management does not offer them another nectar and pollen source as rich as linden trees, it seems quite logical that they are all on the linden trees. And since linden trees are planted in areas with paving, bare earth, or at most some short-mown, sun-scorched grass, the dead bodies of the workers are more easily visible to us than naturally deceased bumblebees anywhere else. However, this is also a hypothesis, and we currently lack scientific data. If you look closely, you will not find bumblebee queens under the linden trees.
Garden bumblebee (Bombus hortorum) male
Photo: O. Hercog
Conclusion
- Don't believe in toxic linden trees. Toxins in them have not been proven.
- Under the lime trees, you will primarily find bumblebees whose lives are ending – worker castes and sometimes males, primarily of early species.
Well, would you have expected it? Something so clear, given by simple common sense? We certainly wouldn't have. So let's not fall into the delusion that nature is simple and that we understand it.
We'll say goodbye with a quote from the greatest Czech thinker (and undoubtedly a bombusophile) Jára Cimrman and his theory of knowledge: „We know everything = we know nothing".„.
A modest addition from us: „Let us approach everything with this awareness, so that we do not cause more harm than good.“.
Let's be glad for our national trees – the beautiful lime trees. They are beautiful and deserve our attention and love.
Solopysky – lime tree and chapel
Foto cestyapamatky.cz