Author: Tim Larson
On April 13, 2023, entomologist and Duluth-area farmer Claire Lande discussed die-offs of native bumblebees, with a specific focus on bumblebee mortality related to linden trees.
Claire began her talk by passing around insect exhibit boxes. One displayed several species of bumblebees of many shapes and sizes that are native to our region. Many of our bumblebees are declining in numbers. Claire described her love of farming and native plants and how these interests relate to her studies in entomology. She showed a second small insect display case featuring a wide variety of bees that she has found dead or dying beneath linden trees while she described her research experiences with bees in Oregon, where she studied pollinator plants and bees, and investigated bee mortality beneath linden trees.
Why we should save the bees
Bees matter. Two-thirds of global crops require pollination. Minnesota has over 500 species of bees. The majority of them are solitary bees rather than social species. Most of our local bee species are cavity-nesters and ground nesters. Do they need to be saved? Evidence of declines exists in many families of bees—particularly in seven groups: bumble bees, cuckoo, leafcutter, long-horned, mason, miner, and yellow-faced. Among bumblebees, 28% are threatened. Some species, however, are thriving. The declining populations are part of declining insect populations overall. Several species of bumblebees that were formerly very common now show reductions in abundance and distribution—several of these are threatened with extinction.
Why are bees in decline?
Contributing factors include pesticides: neonicotinoids affect ability to reproduce; glyphosate leads to poor health without outright killing of pollinating insects. Changing land use destroys nesting sites and reduces forage. Other factors are invasive species, predators, pathogens, and climate change causing mismatch of insect emergence with food sources as well as drought affecting nectar availability. Disease is another factor.
What are some meaningful things we can do?
1. Recognize that how we manage our landscapes affects bees.
2. Learn to appreciate and identify the many species of bees.
3. Promote bees and bee habitat, so that “bee-ing” becomes the new birding.
Why are bees dying under linden trees?
Linden-associated bumblebee mortality has been observed for a long time. People have wondered why since this was first observed in England in the early 1900s. Sujaya Rao, then at Oregon State University, became interested in it. Linden or Basswoods, Tilia Americana, bloom for seven to fourteen days, typically in early July in Duluth Some bees drop to the ground, crawl—unable to fly–and die after foraging on linden. Notably, die-offs occur on trees that have not been sprayed.
Two unsatisfactory hypotheses have previously been proposed: 1. Stressed trees might produce mannose—a sugar that is toxic to bees–in nectar. But this was never found to be true—nectar analysis detected no mannose; hypothesis #2. Bees were starving—because linden tree nectar production declines three days after blooming.
Observations showed that not every bumblebee was affected, and few honeybees were affected. Not all linden trees caused mortality, and not all the time.
Newer hypotheses looked at the metabolism of nectar in bees, themselves. Claire was involved in research that studied 29 bees found crawling on the ground and 28 healthy bees. An analysis of the thoracic muscles of bumblebees collected below Linden trees showed that crawling and healthy bees were in very different metabolic states. An alkaloid, trigonelline, was found in the nectar of sampled Linden trees at low levels. Trigonelline inhibits memory formation in bumblebees.
Conclusions so far?
Bee mortality requires several intersecting factors. Strange things can occur. But we should still plant linden and other flowering plants!
Claire’s deeper dive into the mystery of bumblebees dying beneath linden trees reveals significant new findings; yet we still have much to learn about how and why some individual bees that forage on linden trees weaken and die while others are not so affected.
Arrowhead Native Plant Explorers 