Are Linden Trees Killing Bumblebees?

Important pollinators, bumblebees are larger and fuzzier than honeybees.  Animals that transport pollen from male flower parts to female flower parts are crucial to the success of many crop plants. Scientists have been alarmed at the increase in diseases that affect honeybees and bumblebees, because they have severely decreased numbers of these important pollinators.  The bees collect and eat nectar and pollen from flowers, and they spend lots of time foraging from their favored sources.  As they move from flower to flower, they also deposit pollen from one source to other flowers.

 

Figure 1.  Female bumblebee gathering food.  Note the ‘pollen baskets’ on her hind legs, full of collected pollen.

Copyright © by Tony Wills (CC BY-SA 3.0) at https://commons.wikimedia.org/wiki/File:Bumblebee_05.JPG.

 

So, imagine the alarm when heaps of dead and weakly crawling bumblebees were found under lovely linden (basswood) trees, which are found in many parks, gardens, and urban landscapes.  Studies have attempted  to determine the factors that might lead to such massive losses.  At first, scientists suspected pesticides, which did have a role in some cases.  Where pesticides had not been applied, some researchers hypothesized that the culprit was a toxin contained in the flower’s nectar and in the sap of the linden trees.

Further observations and analysis by scientists at Oregon State University, as enumerated below, allowed them “to connect some dots” that modified their hypotheses and furthered understanding. 

1. Bumblebee deaths tended to occur late in the blooming season, when nectar production was lower. Naturally, less nectar per flower visited results in a greater effort to supply enough energy for flight and other activities. 
2. Simultaneously, morning temperatures also declined. Lower temperatures affect the bumblebees’ abilities to fly, because their flight muscles must be maintained at 30° (86°F) for the animals to fly.  They can shiver to produce sufficient heat, but it takes more energy to do that, and with lower ambient temperatures, more shivering is required. 
3. Comparative analysis of the flight muscles from crawling versus healthy flying bees revealed that crawling individuals had much lower metabolic intermediates of the Krebs Cycle (Biology for the Global Citizen, Module 7).  These data indicate that central metabolism is affected, reducing energy levels in crawling bees for flight.  Essentially, without access to the nectar, they starve to death. 
4. Analysis revealed that the nectar contains an alkaloid, called trigonelline (caffeine is another alkaloid).  It is possible that this chemical acts to reinforce the linden as a source of rich nectar, promoting bee’s loyalty to the tree’s flowers. 

Further experiments continue to assess the role of trigonelline in reinforcing bumblebee behavior.  At this point, it seems that pesticides, lower temperatures, lower nectar volumes, and perhaps nectar chemicals play roles in these massive die offs.  As is the case in many biological phenomena, explanations are rarely simple, but they are fascinating.

We now have evidence that tends to exonerate the beautiful linden trees.  We should not stop planting linden trees, but it is a good idea to place them in areas where other sources of nectar are available to the bees.

 

References

Koch, Hauke, and Philip C. Stevenson. "Do linden trees kill bees? Reviewing the causes of bee deaths on silver linden (Tilia tomentosa)." Biology letters 13, no. 9 (2017): 20170484.

Lande, Claire, Sujaya Rao, Jeffrey T. Morré, Gracie Galindo, Julie Kirby, Patrick N. Reardon, Gerd Bobe, and Jan Frederik Stevens. "Linden (Tilia cordata) associated bumble bee mortality: Metabolomic analysis of nectar and bee muscle." PloS one 14, no. 7 (2019): e0218406.