24
Jul
Synergistic Effects Documented for Multiple Pesticide Exposure on Honey Bee Health
![A study finds that the cumulative impact of a pyrethroid insecticide and a fungicide hexaconazole "induced [an] acute synergistic effect" in honey bees.](https://beyondpesticides.org/dailynewsblog/wp-content/uploads/cache/2026/07/DN-7_24_26/1425964145.png "A study finds that the cumulative impact of a pyrethroid insecticide and a fungicide hexaconazole")
(Beyond Pesticides, July 24, 2026) A study published in the Journal of Hazardous Materials finds that the cumulative impact of the pyrethroid insecticide flumethrin and the triazole fungicide hexaconazole “induced [an] acute synergistic effect” in honey bees (Apis mellifera L.). While extensive independent scientific research has shown the debilitating and deadly effects of neonicotinoid insecticides on managed and native bee populations, this study adds to the body of science that establishes the range of threats from pesticide use leading to unsustainable bee decline.
Synthetic pyrethroid insecticides are widely used products that are synthesized derivatives of pyrethrins found in pyrethrum, an extract of dried chrysanthemum flowers. Compared to their natural counterpart, synthetic pyrethroids take significantly longer to degrade in the environment and thus pose longer-term risks to humans and wildlife. The chemicals interfere with the proper function of the body’s sodium channels, resulting in harm to the central nervous system. Acute symptoms of poisoning in humans include headache, nausea, incoordination, tremors, and facial swelling, with severe incidents causing diarrhea, convulsions, paralysis, and death. Studies associate numerous synthetic pyrethroids with endocrine disruption, which can result in a range of neurological, reproductive, and organ system effects, including breast cancer and numerous other cancers. The Endocrine Society states, “According to studies using cellular and animal models and newer human-based studies, many carcinogens may also be EDCs [endocrine disrupting chemicals], and could influence the development and progression of cancer by acting like hormones. EDCs can also influence cancer development across generations, including children and grandchildren.â€
The growing evidence of these adverse impacts, from pollinator to human health, underscores calls from communities and experts across the nation to ban the use of these products and transition to organic land and building management that fosters a mutualistic, rather than parasitic, relationship with the biosphere.
Methodology and Main Findings
Researchers in this study maintain honey bee colonies in groups of 15, drawing individual honey bees from three independent colonies. These honey bees are evenly and randomly distributed across all treatment groups to ensure genetic diversity and health variance.
For assessing acute toxicity, there are different approaches for individual compounds versus mixtures. Acute assays follow Guideline 213 from the Organisation for Economic Co-operation and Development (OECD) parameters for assessing acute oral toxicity; each concentration (flumethrin and hexaconazole) has three replicate cages of 15 bees, with one pesticide-free group serving as the control group. In terms of assessing mixtures, there is a similar three-replicate cage approach to assess additive, synergistic, or antagonistic interactions. Moreover, mixture tanks are developed based on respective 4-day LC50 values from the individual compound assays. LC50 refers to the median lethal concentration that kills 50 percent of a test population over a specified exposure time—in this experiment, that is a four-day time horizon for mortality.
Separately, subchronic effects on biochemical, histological, and molecular impacts were assessed in three concentration gradients (Low, Medium, and High) for each compound. The cumulative group combined their respective Low, Medium, and High levels for those tanks as well. Each treatment used four cages of 15 bees, the same model for assessing acute toxicity. For further information on statistical analysis, please see pages 3-4 of the study PDF.
The main findings from this study include:
- Acute Lethality of Individual Ingredients. Flumethrin is more acutely toxic to honey bees than hexaconazole on its own based on the 4-day LC50 values assessed in this study.
- Synergistic Lethality of Cumulative Exposure. When both active ingredients are combined, the mixture is more lethal, with synergistic ratios of 27.9 at the 2-day interval and 22.3 at the 4-day interval. “These markedly elevated values suggested that co-exposure to [both active ingredients] generated a toxic response far exceeding the expected additive effect of the two compounds,†the researchers say.
Previous Coverage
There is overwhelming evidence from research efforts led by grassroots projects and premier institutions flashing alarms about the adverse effects of synthetic pesticides and the industrial agriculture system on native and honey bee populations across the globe.
A study of honey bee colonies in Florida and California, published in Environmental Toxicology and Pharmacology, finds elevated mortality from pesticide residues, including those that have been documented to threaten pollinators. As the authors describe, “While bees die from multiple, often interacting, stressors, here we show single contributors at levels capable of causing acute harm.†The presence of miticides, fungicides, herbicides, and insecticides within the bee colonies, including in the bodies of dying bees, further highlights pesticides as drivers of bee declines. By sampling both dying bees and in-house bees for chemical residues, the researchers are able to compare symptomatic colonies and control colonies. The authors note, “Our findings differ from previous screenings, which cast a broad net, screening agrochemicals in colonies nationwide, and not necessarily from impacted operations.†This study, however, shows the presence of specific pesticide residues in commercially managed colonies after die-off incidents. The neonicotinoid insecticide imidacloprid, in particular, is widely detected and found in high levels, with the researchers identifying the compound as the largest contributor to bee death. (See Daily News here.)
Adding to the wide body of science highlighting the adverse effects of pesticides on pollinators, a study published in Insects finds threats to Italian honey bees (Apis mellifera ligustica) following exposure to insecticides with contrasting toxicity levels. Both the high toxicity and low toxicity compounds impact honey bee gut bacteria and gut microbial composition, showing how even “reduced risk†insecticides can have sublethal effects and jeopardize pollinator health. As the authors point out, “Honeybees depend on a small but highly specialized community of gut bacteria that help them digest food, resist infections, and cope with environmental stress.†Because of this, chemicals that disrupt the honey bee gut microbiome can threaten their survival. In the current study, the researchers analyze two compounds to determine adverse impacts on honey bees’ gut microbiota: emamectin benzoate-lufenuron (EB-LFR), an avermectin insecticide with high toxicity, and RH-5849 (1,2-dibenzoyl-1-tert-butylhydrazine), a non-steroidal ecdysone agonist (mimicking the action of the insect molting hormone) and insect growth regulator with reported lower toxicity. (See Daily News here.) A study of ecotoxicity risk from neonicotinoid insecticides, published in Environmental Chemistry and Ecotoxicology, finds that chemicals in this class of pesticides, particularly dinotefuron, increase the body temperature of European honey bees (Apis mellifera) and subsequently accelerate the translocation (movement) of contaminants into hives by the honeybees. The research indicates that neonicotinoids affect acetylcholine receptors in the nervous system, leading to an “elevation in octopamine titer [neurotransmitter/hormone] and subsequent increase in the body temperature of honeybees,†the authors report. They continue: “Furthermore, we observed a considerable upregulation [of] the expression of a flight gene, flightin, in honeybees. This gene accelerates the homing behavior of honeybees and facilitates the rapid and frequent transport of neonicotinoid pesticide-contaminated nectar to the hive.†In describing their results, the researchers state: “For the first time, we propose that neonicotinoid pesticides accelerate the homing ability of honeybees by affecting their body temperature, which leads to more neonicotinoid pesticides entering the hive and explains the prevalence of neonicotinoids and at higher concentrations in terms of their effects on the honeybee body temperature that enhances homing.†This accelerated movement of neonicotinoid pesticides into honey bee hives heightens the toxicity risks of honey bee populations. (See Daily News here.)
Meanwhile, there is demonstrable evidence that organic farms can offer oases for pollinators from an otherwise more severely polluted environment. Public health and environmental advocates, as well as experts in pollinator and biodiversity, continue to warn of the impacts this will have on the reliability of peer-reviewed science, given that “control” fields/areas without pesticide exposure are the exception now rather than the norm. For example, a study of two pollinator species, honey bees (Apis mellifera) and small carpenter bees (Ceratina calcarata), finds oxidative stress (OX)— an imbalance between antioxidant defenses and excess reactive oxygen molecules (species), or ROS—resulting from exposure to non-living (abiotic) stressors, such as synthetic chemicals, leading to cell damage. Regulatory bodies, including the U.S. Environmental Protection Agency (EPA), do not routinely evaluate oxidative stress as a standalone or required endpoint in standard pesticide registration protocols. In comparing pollinator responses to different pesticides and pest control management practices, the lowest levels of OX are exhibited in organically managed systems, as described in the research published in Physiological Entomology. (See Daily News here.) A study of organic tomato agroecosystems with managed and wild bees, published in Apidologie, affirms the importance of protecting natural systems to support organisms that contribute to crop productivity. The study finds that the strategy of introducing social bees, even those native to other nearby areas, to enhance pollination in open-field conditions provides no direct benefits to the crops that are better served by wild bees. In evaluating the addition of Melipona quadrifasciata stingless bees, not native to the study site, for assisted pollination of tomato plants cultivated in open organic fields, the researchers note that “the presence of M. quadrifasciata hives did not influence fruit quality, indicating that wild bees primarily drove pollination benefits.†(See Daily News here.)
Additionally, researchers in Germany and Brazil investigated the biodiversity of agricultural landscapes in organic and nonorganic areas in “bee hotels,†finding that there is a positive correlation between organically managed fields and numerous indicators of improved pollinator health, including an “increase in bee abundance, species richness, and diversity.†This study, published in Global Ecology and Conservation, “was conducted at 17 sites in the southern part of Germany, Baden-Württemberg, including eight conventional and nine organic farming systems.†(See Daily News here.)
Call to Action
For more peer-reviewed science on biodiversity impacts of synthetic and organic pest management, please visit What the Science Shows on Biodiversity.
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One way to take action in the protection of pollinator health includes telling Congress and USDA to preserve the Beltsville agricultural research facilities that support farming and beekeeping nationwide. Â
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: Journal of Hazardous Materials
