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.
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.
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.