Bumblebees (Bombus spp.) are the chunky black and yellow insects that dwell in our gardens. Their buzzy nature attracts children, home gardeners, naturalists, and scientists to take a closer look. Bumblebees do not produce large amounts of honey like the other types of bees do, but they are excellent pollinators of crops including cucumbers, tomatoes, squash, and a variety of berries. Unfortunately, bumblebees are in decline, and may eventually disappear all together.

Many European and North American cities have planted linden trees (Tilia spp., which can also go by the common name of basswood) in parks and along roads. These trees have lovely flowers and an "intoxicating" fragrance that attracts insects. However, a new study published last month in PLOS ONE suggests that, despite their appealing flowers, linden trees are killing bumblebees. 

I've observed this myself walking around my campus at the Institute for Insect Biotechnology at Justus Liebig University Giessen in Germany. In the last few days I have seen either crawling and dead bumblebees on the ground around the bases of linden trees. Bee mortality has previously been linked with mannose (a type of sugar) toxicity from the nectar. Bees that depend entirely on these trees are also at risk of starvation, because linden flowers bloom relatively late in the season and so nectar is only available at specific times of year.

The PLOS ONE study ties the story together. The researchers found that the mass death of bumblebees is due partly to ineffective metabolism of linden nectar and partly to a toxin in the nectar that impairs the bees' nervous systems. Specifically, in cool morning temperatures, the energy that the bees get from nectar isn't enough for constant flight, so the bees are forced to crawl on the ground and may not be able to regulate their body temperatures. Also, the linden nectar is loaded with toxic chemicals, so bees that continue feeding on it eventually die. 

The linden trees are just a small piece of the larger puzzle of what is causing bumblebee declines all over the world, but may point researchers working on other causes in new directions. Hopefully we can find a way to save the bumblebees, because they are responsible for much of the nutritious food we humans rely on each day.