Cultivating flowering plants for pollinator gardens, commercial farms, or home landscapes often relies on the use of herbicides to manage unwanted weeds. Honeybees are attracted to these locations and play a critical role in their success. So what happens when foraging bees pick up a dose of weedkiller?
The study, published in the Journal of Experimental Biology, was led by Associate Professor Margaret Couvillon and Ph.D. student Laura McHenry in the College of Agriculture and Life Sciences' Department of Entomology.
The unseen risk of glyphosate
Glyphosate, the active ingredient in many agricultural weedkillers, is a pesticide bees are likely to encounter while foraging. The chemical works by blocking an enzyme in weeds to inhibit photosynthesis. Because honeybees don't have that enzyme, glyphosate was long thought to be harmless to them.
"We were interested in investigating the impact of glyphosate, the most widely used pesticide in the world, on the behavior and brains of honeybees, important pollinators that might encounter the weedkiller as they forage in the landscape," said Couvillon.
While glyphosate exposure isn't normally lethal to honeybees, researchers worried that contact with the chemical might cause sublethal impairments. These impairments would best be described as an unintended side effect, similar to over-the-counter antihistamines causing drowsiness while fighting allergy symptoms.
To test this idea, the research team set up artificial feeders—one with glyphosate and one without it. Honeybees were trained to forage at these feeders, and their behaviors were monitored over time. After just three days, researchers tracked a reduction in foraging of 13% and changes in bee brain chemistry—but only in the bees exposed to glyphosate.
"For a colony, a 13% reduction in foraging can be consequential," Couvillon said. "If the entire colony was exposed, this could lead to decreased pollination effectiveness and reduced honey production, risking colony survival and long-term stability."
In addition to measuring foraging, Couvillon's lab measured changes in brain chemicals such as amino acids and neurotransmitters. Changes were observed that correlated with glyphosate exposure, which could link the neurochemical balance of the bees and foraging effectiveness.
Implications for pollinator health
This discovery raises the question: What can be done to protect our pollinators from these ubiquitous chemicals? According to Couvillon's team, the goals should be regulation, strategy, and further understanding.
"Understanding how weedkillers affect beneficial insects like pollinators will help us make more strategic regulatory choices about when and where to use them for maximum benefit and minimum harm," said McHenry, now a postdoctoral researcher at Penn State, who conducted the study while a Ph.D. student at Virginia Tech.
By understanding the impact glyphosate-based weedkillers have on honeybees, the research suggests that further examination of how these chemicals interact with bees' biology is needed. As bees are one of the world's most important pollinator species, and glyphosate is one of the world's most-applied herbicides, the impact of the chemical on the bees could be greater than previously realised.
Publication details
Laura C. McHenry et al, Sublethal glyphosate exposure reduces honey bee foraging and alters the balance of biogenic amines in the brain, Journal of Experimental Biology (2025). DOI: 10.1242/jeb.250124
Journal information: Journal of Experimental Biology