Researchers found that chronic exposure to a field-realistic concentration of atrazine damaged intestinal structures in two bee species and reduced Western honey bee body weight, while a third species showed greater apparent ability to detoxify the widely used herbicide.
By yourNEWS Media Newsroom
A widely used agricultural herbicide can produce measurable intestinal and physiological changes in important pollinators at concentrations designed to reflect environmental exposure, according to new research comparing the effects of atrazine across three different bee species.
The study, published in the September 2026 edition of Pesticide Biochemistry and Physiology, exposed Western honey bees, buff-tailed bumblebees and mason bees to atrazine and found striking differences in how the species responded.
Researchers reported structural damage to the midgut of Western honey bees, or Apis mellifera, and buff-tailed bumblebees, Bombus terrestris. Honey bees also experienced a significant reduction in body weight.
Mason bees, Osmia excavata, did not show the same significant intestinal damage, a difference the researchers linked to stronger activation of biological pathways involved in eliminating foreign chemicals.
The authors described atrazine exposure as “found to be associated with structural damage to the midgut.”
Their results suggest that herbicide risk assessments focused primarily on whether a chemical kills bees could miss less obvious effects involving digestion, metabolism and detoxification.
The experiment evaluated atrazine at 1.1 milligrams per kilogram, a concentration the researchers characterized as representative of field-level exposure. Atrazine residues have previously been detected in food sources available to bees.
Importantly, the pesticide did not significantly increase mortality or reduce consumption of sugar solution in any of the three species during the study.
Instead, some of the most pronounced changes appeared beneath the surface.
Researchers found deterioration of microvilli in the intestines of honey bees and bumblebees. Microvilli are microscopic projections along intestinal cells that expand the surface area available for the absorption and processing of sugars, proteins, nutrients, water and other compounds.
Damage to those structures can therefore potentially interfere with digestive and metabolic functions even when an exposed animal survives.
The findings distinguish the research from traditional acute pesticide toxicity tests that rely heavily on mortality as the principal endpoint.
Three bee species showed markedly different responses
The research involved scientists associated with Central State University in Ohio and institutions in China, including the Hunan Academy of Agricultural Sciences and other agricultural research organizations.
A total of 464 individual bees were included, with bees housed by species in cages containing 10 individuals. Survival was monitored daily during a 10-day exposure period.
The researchers measured more than whether the bees lived or died. They tracked body weight, examined midgut tissues and conducted transcriptomic and metabolomic analyses to identify changes in gene activity and metabolism.
Beyond Pesticides described the experimental preparations as including a control without atrazine and atrazine treatments prepared with the solvent and adjuvant dimethyl sulfoxide, or DMSO. The journal publication identifies 1.1 mg/kg as the field-level atrazine concentration evaluated across the three species.
Western honey bees experienced a significant decline in body weight following atrazine exposure. That effect was not detected in either bumblebees or mason bees.
The intestinal findings followed another species-specific pattern.
Both honey bees and bumblebees displayed compromised intestinal integrity, including significant degradation of microvilli. Mason bees did not experience a statistically significant adverse intestinal effect under the same field-level exposure conditions.
Those differences prompted researchers to examine what was happening at the molecular level.
Detoxification genes may explain why mason bees fared differently
One of the clearest differences involved ATP-binding cassette, or ABC, transporters.
These cellular transport systems help organisms move foreign compounds and metabolic products through and out of cells and play an important role in pesticide detoxification.
Researchers found that genes associated with ABC transporters were significantly down-regulated in honey bees but up-regulated in mason bees after atrazine exposure.
The authors suggested that the decrease in the honey bees’ transporter activity may have impeded the efficient removal of atrazine.
“In mellifera, the downregulation of related genes may have impaired the timely efflux of atrazine, thereby preventing a reduction in its toxicity, which could ultimately lead to the observed weight loss,” the researchers wrote.
Mason bees showed essentially the opposite response.
“In contrast, O. excavata likely achieved timely detoxification through upregulation, reducing the toxicity of atrazine, which may explain the absence of significant intestinal damage in O. excavata,” the authors wrote.
Researchers said similar species-dependent patterns appeared in pathways involved in energy metabolism.
The study also found changes involving hormone-synthesis genes across all three species.
Metabolomic analysis identified several biological pathways affected across the pollinators, including pyrimidine metabolism, tryptophan metabolism and the citric acid cycle, also known as the TCA cycle, which plays a central role in cellular energy production.
Taken together, the findings suggest that the same pesticide concentration can produce substantially different biological responses depending on the pollinator species exposed.
That has implications for pesticide testing because a chemical evaluated primarily in Western honey bees may not necessarily produce identical effects in wild or solitary pollinators — and the reverse may also be true.
Herbicides can affect bees without targeting their nervous systems
Concerns about pesticide effects on pollinators have historically focused heavily on insecticides, particularly neonicotinoids, because those chemicals are specifically designed to kill insects.
Herbicides operate differently.
They are designed to kill plants and generally do not attack insect nervous systems in the manner associated with many insecticides. One of the best-known consequences for pollinators is therefore indirect: eliminating flowering plants can reduce the nectar and pollen available to bees.
The new atrazine findings point to another concern — direct sublethal effects on bee physiology following chemical exposure.
Atrazine works in susceptible plants by interfering with photosynthesis, a biological process bees do not possess. Despite that different mechanism, researchers found changes in intestinal tissue, body weight, gene expression and metabolism among exposed pollinators.
Earlier research has also examined how pesticides accumulate within hive products.
A study in Food and Chemical Toxicology reported residues of pesticides including tau-fluvalinate, coumaphos, chlorfenvinphos, chlorpyrifos and amitraz in materials including bee bread, propolis, beeswax and royal jelly, raising questions about chronic exposure within hives.
Other research examining the microbiomes of animals has suggested that gut microorganisms may influence the way organisms process triazine herbicides such as atrazine, adding another potential variable to pesticide toxicity.
The overall pesticide burden has become an increasing focus of environmental research as agricultural chemicals move beyond the fields where they are initially applied and enter soil, waterways and food sources used by wildlife.
Atrazine faces heightened scrutiny following cancer classification
The bee study arrives less than a year after the International Agency for Research on Cancer, or IARC, changed the international cancer hazard classification for atrazine.
On Nov. 21, 2025, IARC, the specialized cancer agency of the World Health Organization, classified atrazine and the herbicide alachlor as “probably carcinogenic to humans,” placing both chemicals in Group 2A.
IARC cited limited evidence of cancer in humans along with sufficient evidence in experimental animals and mechanistic evidence.
For atrazine, the agency reported positive associations involving a particular form of non-Hodgkin lymphoma.
The designation is a hazard classification, meaning it evaluates whether an agent is capable of causing cancer under some circumstances. It does not by itself establish the likelihood that a person will develop cancer from a particular real-world level of exposure.
Atrazine remains approved for use in the United States and is used extensively for agricultural weed control, particularly in crops such as corn, sorghum and sugarcane, as well as certain turf applications.
Syngenta is a major manufacturer of atrazine.
Beyond Pesticides and other environmental organizations have repeatedly urged the Environmental Protection Agency to ban the herbicide, including through advocacy efforts in 2025.
Atrazine has also generated decades of controversy over possible endocrine effects.
University of California, Berkeley, biologist Tyrone Hayes, Ph.D., has argued that atrazine can activate aromatase, an enzyme involved in converting androgens to estrogens, and has cited epidemiological correlations involving prostate and breast cancer.
Hayes has also published research reporting reproductive and sexual-development effects in male frogs exposed to environmentally relevant atrazine concentrations.
Those findings have generated substantial scientific and regulatory debate over the chemical’s endocrine effects.
U.S. water monitoring finds widespread atrazine detections
Separate research published in May by U.S. Geological Survey scientists placed atrazine among the pesticides presenting the greatest potential threats to aquatic environments in a decade-long national analysis.
The study examined pesticide monitoring data collected at 81 river sites across the contiguous United States from 2013 through 2022.
Atrazine was the leading pesticide for acute aquatic-life benchmark exceedances measured by several indicators, including the number of affected sites and total exceedances.
Researchers found that 72% of all acute plant benchmark exceedances recorded during the study period were attributable to atrazine.
The percentage of sites and samples exceeding acute aquatic-life benchmarks for atrazine was generally an order of magnitude greater than for other pesticides included in the analysis.
The USGS researchers identified atrazine, metolachlor and imidacloprid as three pesticides presenting particularly significant concerns for surface-water availability based on trends and benchmark exceedances.
The findings do not mean that every detection represented a threat to human health. The study separately found very few exceedances of human-health benchmarks in the river samples examined.
Instead, the results documented atrazine’s persistence and ecological significance in freshwater systems.
Restrictions on atrazine vary internationally and within North America. Quebec has imposed tighter requirements on the herbicide along with certain neonicotinoid insecticides, citing environmental and pollinator concerns.
Researchers say testing should account for multiple pollinators
The differences among honey bees, bumblebees and mason bees are one of the new study’s most significant findings.
Mason bees appeared better able to activate detoxification mechanisms under the experimental conditions, while honey bees showed both weight loss and intestinal damage and bumblebees experienced intestinal changes.
Those species-specific responses suggest that relying on one pollinator as a proxy for all bees can leave gaps in assessments of pesticide effects.
The researchers concluded that field-level atrazine exposure may produce sublethal effects on pollinator physiology and gut function and called for risk evaluations that examine multiple bee species.
Beyond Pesticides cited the findings in support of broader pesticide restrictions and expansion of organic land-management practices. The organization argues that pesticide regulation should more fully account for chronic and sublethal effects at environmentally realistic concentrations rather than relying primarily on measurements of acute mortality.
Study co-author Hongmei Li-Byarlay, Ph.D., is scheduled to discuss pesticide science and pollinator health during the Beyond Pesticides National Forum on Oct. 21. The event is titled “Advancing Science on Pesticides, Taking Action in Harmony with Nature, and Sustaining Life.”
The new research does not show that field-level atrazine exposure killed the bees during the 10-day experiment. Survival and sugar consumption were not significantly affected across the three species.
Instead, the findings identify a different category of potential harm: bees can remain alive while undergoing measurable changes in intestinal structure, body weight, detoxification pathways and metabolism.
For researchers evaluating the health of pollinators, those effects raise a broader question about pesticide safety — not simply whether an exposure kills a bee immediately, but what happens inside the animal after it survives.
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