A Virginia Tech study found that honeybees exposed to agricultural concentrations of glyphosate foraged 13.4% less and showed changes in brain chemicals associated with motivation, learning and foraging behavior.
By yourNEWS Media Newsroom
Honeybees exposed to low levels of glyphosate reduced their foraging activity and experienced measurable changes in brain chemistry, according to new research from Virginia Tech that raises questions about the herbicide’s sublethal effects on pollinators.
The study, published in the Journal of Experimental Biology, found that bees consuming glyphosate at concentrations associated with agricultural use made 13.4% fewer foraging trips than unexposed bees during a three-day experiment.
Researchers also detected changes involving neurotransmitters associated with learning, motivation and the drive to forage, suggesting that glyphosate may affect honeybees through neurological pathways even though insects do not possess the plant enzyme traditionally targeted by the herbicide.
The research was led by Virginia Tech Associate Professor Margaret Couvillon and Ph.D. student Laura McHenry, who is now a postdoctoral researcher at Penn State.
The researchers placed honeybees at artificial feeding stations containing glyphosate and tracked their foraging behavior for three days.
The decline observed among exposed bees could become significant if replicated throughout an entire colony, Couvillon said.
“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.”
A healthy honeybee colony can contain approximately 30,000 to 60,000 bees, with roughly one-third of the population engaged in foraging at a given time.
Because foragers supply the colony with nectar and pollen, a sustained reduction in their activity could affect honey reserves, food availability for developing larvae and the broader resilience of the hive.
Researchers sought to determine whether the change in behavior was accompanied by alterations inside the bees’ nervous systems.
After the exposure period, they dissected bee brains and examined concentrations and relationships among several biogenic amines and related chemicals involved in behavioral regulation.
The analysis included octopamine, tyramine, dopamine and the amino acid tyrosine.
Bees exposed to glyphosate showed altered relationships among those neurochemicals, according to the study.
Those changes suggest the herbicide may interfere with brain signaling pathways involved in learning, motivation and normal foraging behavior.
The findings challenge a long-standing assumption behind the view that glyphosate presents relatively little direct danger to insects.
Glyphosate, the active ingredient in Roundup and numerous generic herbicides, works primarily by inhibiting the shikimate pathway. Plants and some microorganisms depend on that biochemical pathway, but insects do not possess it.
Because honeybees lack the herbicide’s conventional biological target, glyphosate has historically been considered less likely to harm them through its primary mechanism of action.
The Virginia Tech findings indicate that absence of the target enzyme does not necessarily rule out other biological effects from exposure.
Rather than producing immediate mortality, glyphosate may influence behavior and neurological function at doses bees can survive, according to the research.
That distinction is important because the health of a social insect colony depends not only on whether individual bees survive but also on whether they can continue performing essential tasks.
A modest decline in foraging across thousands of workers, repeatedly occurring over time, could reduce the amount of food entering a hive and weaken its ability to withstand other environmental pressures.
McHenry said understanding those effects could help inform decisions about herbicide use.
“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.
The study contributes to broader research examining effects of glyphosate on honeybees that do not involve immediately killing the insects.
A 2018 study published in the Proceedings of the National Academy of Sciences found that glyphosate exposure disrupted honeybee gut microbiota and increased susceptibility to disease.
Taken together, the studies raise the possibility that glyphosate exposure can affect more than one aspect of bee biology, including gut microbial communities and neurological processes related to behavior.
Researchers continue to distinguish pesticide and herbicide exposure from other major pressures affecting honeybee colonies.
The Varroa destructor mite remains considered the leading cause of U.S. colony losses, according to the Bee Informed Partnership.
Chemical exposure, however, can add another source of stress to colonies already dealing with mites, disease, nutritional problems and other environmental challenges.
The new study does not establish how much glyphosate exposure bees typically receive across real agricultural landscapes or whether the 13.4% decline measured under experimental conditions would occur at the same magnitude in entire colonies in the field.
Couvillon and McHenry said additional research is needed to determine how the herbicide affects honeybees under real-world conditions.
The work was supported by the National Institute of Food and Agriculture.
The findings nevertheless underscore the difference between testing whether a chemical immediately kills a pollinator and examining whether repeated lower-level exposure changes its behavior or physiology.
For honeybees, functions such as navigation, learning, motivation and foraging are central to the survival of the larger colony.
A compound that leaves individual bees alive but reduces how effectively they gather food could therefore have consequences that are not captured by traditional assessments focused mainly on acute toxicity.
The Virginia Tech research suggests glyphosate may warrant closer examination under that broader standard.
While the herbicide’s primary plant-killing mechanism does not exist in insects, exposed bees in the study both foraged less and displayed changes in neurochemical relationships associated with normal behavior.
Researchers now say larger and more realistic field studies will be necessary to determine what those findings mean for managed and wild pollinators exposed to glyphosate in agricultural environments.
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