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To Protect Pollinators We Must Address All Risk Factors

By Aimée Code on 10. October 2019
Aimée Code

Recent media coverage of a study on Tilia trees could lead to a dangerous misinterpretation of existing science—incorrectly exonerating neonicotinoid insecticides, which are known to harm pollinators.

A recent study, Linden (Tilia cordata) associated bumble bee mortality: Metabolomic analysis of nectar and bee muscle, adds valuable information to the effort to understand the natural phenomenon of bumble bees dying under linden (Tilia spp.) trees. Unfortunately, recent media coverage of the study could inadvertently mislead people to believe that it is okay to use neonicotinoid insecticides on Tilia trees—a dangerous misinterpretation of existing science.

Wilsonville, Oregon, is the city that experienced a high-profile mass bee kill in a Target parking lot in 2013. The incident was caused by an application of the neonicotinoid dinotefuran. However, a recent news article from the Wilsonville Spokesman, “Wilsonville event sparks bee research breakthrough: New study shows that environmental factors may be another cause of mass bee deaths,” states that the research “found that the convergence of factors such as temperatures lower than 86 degrees, it being late in the blooming season, a low amount of available nectar and bumblebees' attachment to resource-depleted linden trees can lead to mass deaths.”


Dead bees lay scattered on asphalt. A pencil is laid alongside them to help with estimates.
Dead bees lay scattered across the asphalt in Wilsonville in 2013. This bee kill—one of the largest reported in North America—was caused by neonicotinoid insecticides applied to Tilia trees. (Photo: Xerces Society / Rich Hatfield)


While this is really important information to elucidate an age-old question of why bumble bees die under Tilia—a problem that was noted as early as the 1600s—this is not representative of the entirety of the issue, or an accurate depiction of current research. Similarly, a recent New York Times article that also announced the new study on bumbles deaths under linden trees, “Seeking a Culprit When Bumblebee Carcasses Pile Up: Blame the linden trees? Maybe pesticides left by humans? Or is nature just cruel sometimes? could also mislead readers, by emphasizing natural causes, rather than the well-documented dangers of neonicotinoids. Indeed, a casual reader could easily walk away from either of these articles thinking that neonicotinoid insecticides aren’t to blame in recent high profile incidents, or even that they can be used safely on Tilia.

As someone who has witnessed first-hand bees dying under Tilia trees after neonicotinoids were applied, and reviewed official incident reports that document toxic contamination levels from legal applications, I fear that such misinterpretations could lead to further die-offs of bees. Numerous peer-reviewed and widely accepted studies have shown that neonicotinoids harm bees at levels that are being found in our environment—levels that could contaminate Tilia and other trees treated with neonicotinoids. Even the pesticide registrant Valent USA contributed to a study—Uptake and dissipation of neonicotinoid residues in nectar and foliage of systemically treated woody landscape plants—which found that treating woody plants systemically with neonicotinoids led to harmful contamination levels in nectar even months after an application.


A wider angle of the Wilsonville bee kill reveals thousands of dead bees scattered across a single parking space.


Therefore, the study’s authors recommend avoiding many neonicotinoid uses on bee-attractive trees, even when the application occurs months before bloom. That recommendation echoes conclusions from a 2017 study Do linden trees kill bees? Reviewing the causes of bee deaths on silver linden (Tilia tomentosa), which recommends that all insecticide use be prohibited on Tilia trees.

Bottom line: If we are to succeed in our efforts to bring back the pollinators, we cannot downplay any of the risks that bees face. While natural causes can lead to bumble bee deaths under Tilia trees, the role of highly toxic, long-lived systemic insecticides in current pollinator declines must also be considered and controlled.


Further Reading

Read more about neonicotinoids.

Learn more about the Xerces Society’s Pesticide Program.



Aimée Code joined the Xerces Society in 2013 to direct its new pesticide program. In that role, she has built a program focused on securing practices and policies that promote ecologically sound pest management. She and her staff evaluate the risks of pesticides, develop technical guidance, and advocate for actions that reduce reliance on and risks of pesticide use in both urban and agricultural settings. Aimée received her master's of science in environmental health with a minor in toxicology from Oregon State University.

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