Glyphosate is the most widely used herbicide on the planet, sprayed across millions of acres of farmland, gardens, and public green spaces every year. Understanding how glyphosate affects honey bees has become increasingly urgent as pollinator populations decline worldwide. The effects of glyphosate on honey bees have long been underestimated because the chemical targets a plant enzyme that these pollinators do not possess. For decades, it was assumed to be harmless to honeybees because they lack the specific enzyme the chemical targets in plants. But a new study from Virginia Tech has upended that assumption, revealing that even sublethal exposure can disrupt brain chemistry, impair foraging, and potentially threaten the long-term stability of entire colonies.
Virginia Tech Study Reveals Glyphosate’s Hidden Impact on Honeybee Brains
Researchers at Virginia Tech’s Department of Entomology set out to answer a question that had long lingered in the background of pollinator science: if glyphosate does not kill bees outright, could it still harm them in ways that are harder to see?
Led by Associate Professor Margaret Couvillon and Ph.D. student Laura McHenry, now a postdoctoral researcher at Penn State, the team designed an experiment using two artificial feeding stations. One contained a sucrose solution laced with glyphosate at a concentration of 5 mg acid equivalent per liter, while the other offered clean sugar water. Honeybees were trained to visit the feeders, and their behavior was monitored over several days.
Published in the Journal of Experimental Biology, the study produced a striking result. After only three days, bees exposed to glyphosate reduced their foraging activity by 13.4 percent compared to their unexposed counterparts. On its own, that number might seem modest. But across a colony of tens of thousands of bees, the implications are far more serious.
How Glyphosate Alters Brain Chemistry and Impairs Foraging
The researchers did not stop at observing behavior. They dissected the brains of a subset of the experimental bees and measured levels of three key biogenic amines, which are neurotransmitters that regulate behavior, learning, and motivation in insects, along with their amino acid precursor, tyrosine.
The brain chemicals analyzed were:
- Octopamine, often described as the insect equivalent of noradrenaline, influences arousal, foraging drive, and dance communication
- Tyramine, a precursor to octopamine, also functions as a neurotransmitter in its own right
- Dopamine is involved in reward learning, motivation, and motor control
- Tyrosine is the amino acid building block from which all three amines are synthesized
Bees that had consumed glyphosate showed significant shifts in the balance of these brain chemicals. The correlation between octopamine and tyrosine levels was altered in the exposed group, suggesting that glyphosate may interfere with the neurochemical pathways that drive normal foraging behavior.
These findings are especially notable because they provide a mechanistic explanation for behavioral changes that had previously only been observed at a surface level. Rather than simply documenting that bees forage less, the Virginia Tech team traced the effect down to measurable chemical disruptions inside the brain.
A 13% Decline: What Reduced Foraging Means for an Entire Colony
A 13.4 percent reduction in individual foraging may sound manageable, but Couvillon warns that the colony-level consequences could be substantial.
“For a colony, a 13 percent 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.”
To understand why, it helps to consider how a honeybee colony operates. A single healthy hive contains between 30,000 and 60,000 bees, of which roughly one-third are active foragers at any given time. If every forager brings back 13 percent less nectar and pollen, the hive’s total food intake drops by the same proportion. Over weeks and months, that deficit compounds. Less food means weaker larvae, reduced honey stores for winter survival, and diminished capacity to fight off diseases and parasites.
McHenry, the study’s lead author, emphasized the broader regulatory implications. “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,” she said.
Why Glyphosate Was Long Considered Safe for Bees
Glyphosate works by blocking an enzyme in what is known as the shikimate pathway, a metabolic process that plants, fungi, and some bacteria use to produce essential aromatic amino acids. Animals, including insects, do not possess this pathway, which is why glyphosate was long classified as having low toxicity to non-target organisms like honeybees.
The logic seemed sound: no target enzyme, no effect. But that logic overlooked two important realities.
First, while bees may lack the shikimate pathway, the beneficial bacteria living in their guts do not. A landmark 2018 study published in PNAS demonstrated that glyphosate exposure perturbs the honeybee gut microbiota, reducing populations of protective bacteria and making bees more susceptible to opportunistic pathogens. This gut disruption alone can weaken colonies over time.
Second, the Virginia Tech study now adds another layer: glyphosate appears to affect the neurochemical balance in the bee brain through mechanisms that are still being unraveled but are clearly independent of the shikimate pathway. The sublethal effects, Couvillon’s team suggests, may be akin to the way an antihistamine relieves allergy symptoms while also causing drowsiness. The primary target is hit, but unintended side effects ripple through the system.
Beyond Glyphosate: The Biggest Threats to Honeybees Today
So what is the number one killer of honeybees? The answer is not a single culprit but a combination of pressures that beekeepers and researchers often refer to as the “four Ps”: parasites, pathogens, poor nutrition, and pesticides.
The Varroa destructor mite remains the single most destructive threat to managed honeybee colonies worldwide. These parasitic mites feed on the fat bodies of adult bees and developing brood, transmitting viruses such as deformed wing virus in the process. According to the Bee Informed Partnership, Varroa is consistently identified as the leading cause of colony losses in the United States.
Researchers are actively developing solutions, including a safe new compound that can stop honey bee colony collapse in its tracks.
Habitat loss and monoculture farming have reduced the diversity and availability of pollen and nectar sources, leaving bees nutritionally stressed. Poor nutrition weakens immune systems, making colonies more vulnerable to diseases and pesticide exposure alike. Researchers have found that improving bee nutrition could be key to saving them from these compounding threats.
Pesticides, including neonicotinoids and glyphosate-based herbicides, add another layer of stress. Where neonicotinoids are known for their acute neurotoxicity to insects, glyphosate’s danger is more insidious: a slow, sublethal erosion of foraging ability, gut health, and brain function that may tip already-stressed colonies past the point of recovery.
Bee-Safe Alternatives to Glyphosate
For gardeners, farmers, and land managers looking to reduce their impact on pollinators, several herbicide alternatives offer effective weed control with lower risk to bees. The table below compares the most common options.
| Alternative | How It Works | Risk to Bees | Best For |
|---|---|---|---|
| Horticultural vinegar (acetic acid) | Desiccates leaf tissue on contact | Low, as it is non-systemic with no residual activity | Driveways, paths, spot treatment |
| Manual removal (hand-weeding, hoeing) | Physical removal of weeds by root | None | Small gardens, raised beds |
| Mulching | Smothers weeds by blocking sunlight | None | Garden beds, around trees and shrubs |
| Flame weeding | Brief heat burst ruptures plant cells | Low if applied when bees are not foraging | Gravel areas, between pavers |
| Corn gluten meal | Natural pre-emergent that inhibits seed germination | None | Lawns, before weeds emerge |
| Timed application of herbicides | Spray when bees are not foraging (early morning or late evening) | Reduced, as it avoids direct contact | Situations where chemical control is necessary |
For those who must use herbicides, the EPA’s Bee Advisory Box on product labels recommends applying pesticides only when bees are not actively foraging, typically early morning or late evening, and avoiding spray drift onto flowering plants that attract pollinators. Home gardeners can also support pollinator health by providing a bee waterer to help hydrate pollinators during hot weather.
Frequently Asked Questions
Will glyphosate hurt honey bees?
Glyphosate is not acutely lethal to honeybees at typical environmental concentrations, but the Virginia Tech study and earlier research confirm that it causes sublethal harm. Exposed bees show reduced foraging activity, altered brain chemistry, and disrupted gut microbiomes, all of which can weaken colonies over time.
What are the effects of glyphosate exposure on honeybees?
Documented sublethal effects include a 13.4 percent decline in foraging activity within three days of exposure, disruption of biogenic amines in the brain (octopamine, tyramine, and dopamine), perturbation of the beneficial gut microbiota, impaired learning and memory, and reduced navigational ability.
Does Roundup kill honey bees?
Roundup, the most recognized glyphosate-based herbicide, is not directly lethal to honeybees at field-realistic doses. However, research suggests that the inert surfactants and co-formulants in some glyphosate products may be more harmful than the active ingredient itself. A study published in the Journal of Applied Ecology found that a glyphosate-free version of a commercial herbicide caused 96 percent mortality in bumblebees, while the glyphosate-containing version caused only 30 percent mortality, indicating that surfactants and other additives, not glyphosate alone, were responsible for the deaths.
How do I know if my honey has glyphosate?
Glyphosate residues in honey can only be detected through laboratory testing. Some commercial testing services specialize in honey purity analysis. To minimize the likelihood of residue, source honey from beekeepers whose apiaries are located away from large-scale conventional agriculture, or look for organic certification, which prohibits the use of synthetic herbicides like glyphosate.
What herbicide is safe for bees?
No herbicide is entirely risk-free for bees, especially during active foraging hours. The lowest-risk approaches are non-chemical: mulching, hand-weeding, flame weeding, and corn gluten meal as a pre-emergent. Horticultural vinegar carries relatively low risk due to its non-systemic, contact-only action. Scientists have also developed an antidote that gives bees immunity to certain pesticides, offering another promising avenue for pollinator protection. If chemical herbicides are necessary, applying them when bees are not foraging and avoiding flowering plants can substantially reduce exposure.
What is the number one killer of honey bees?
The Varroa destructor mite is widely considered the single most destructive threat to managed honeybee colonies, transmitting viruses and weakening bees at every life stage. However, colony collapse is rarely caused by one factor alone. Nutritional stress from habitat loss, pathogen pressure, and pesticide exposure, including both neonicotinoids and glyphosate-based herbicides, often act together to push colonies past the point of recovery.
Protecting Pollinators Means Rethinking Herbicide Use
The Virginia Tech study adds a critical piece to a growing body of evidence: even when a chemical does not kill bees outright, it can still chip away at the foundations of colony health in ways that are subtle, cumulative, and scientifically measurable. Glyphosate, once considered benign to pollinators, now joins the list of environmental stressors that beekeepers, farmers, and regulators must account for. After all, bees have been declared the most important species on Earth by scientists, and their decline has consequences that reach far beyond the hive.
As McHenry and Couvillon’s work makes clear, protecting pollinators is not just about banning the most toxic chemicals. It means understanding the full picture, including the sublethal, brain-level effects that unfold quietly inside individual bees and, multiplied across millions, determine whether a colony thrives or collapses.
