06
Jan
Mixture of Common Pesticides and Environmental Stressors Dramatically Elevates Unregulated Adverse Effects
![©[Tom Fisk via Pexels] via Canva.com](https://beyondpesticides.org/dailynewsblog/wp-content/uploads/cache/2026/01/DN-1_6_25/3647409916.jpg "©[Tom Fisk via Pexels] via Canva.com")
(Beyond Pesticides, January 6, 2026) Editor’s Note: We begin the new year with a clarion call for meaningful strategies to eliminate petrochemical pesticide and fertilizer use, based on the preponderance of science that documents both the hazards of their use and the abject failure of regulations in the U.S. and worldwide to accurately account for their harm to health and the environment. Over the holiday season, we have been cheered by letters to the editor, one from a pediatrician in Missoula, Montana and another from a student in Cedar Falls, Iowa, calling for the elimination of pesticide use in their communities. This call for action in communities targets the places where we live, work, learn, and play—where critical decisions on the use of poisons and contaminants are being made daily in our parks, playing fields, schools, open space, and other public properties. We have the tools to eliminate pesticide use with defined organic practices and compatible materials. We should accept nothing less. The scientific study we write about today (below) details an outrage of huge proportions, a synergistic effect of chemical interactions of widely used synthetic pyrethroid insecticides in combination with environmental stressors—resulting in adverse effects 70 times greater than when evaluated individually. 2026 is the year to take bold action and eliminate petrochemical pesticides and fertilizers in our communities, backed by a proven track record in land management systems that are beautiful, productive, resilient, and cost-effective. And, with this, we become advocates for changes that tackle the most serious toxic chemical threats to our health, biodiversity, and climate. In the new year, Beyond Pesticides is here to support this work in communities with technical hands-on resources. To become an advocate in your community, please reach out to us here, contact us at [email protected], or call us at 202-543-5450.
Researchers studied the effect of multiple climate stressors and pesticides in the environment and published their disturbing findings of elevated harm in “Double trouble: The synergistic threat of environmental stressors and pesticide mixtures,” Journal of Hazardous Materials (December, 2025). The researchers document synergism that is 70 times stronger than for the single chemical. The new work is a follow-up to a previous study covered in Beyond Pesticides’ October 25, 2024, news brief.
Among the manifold failures of pesticide regulators, the failure to address the effects of pesticide mixtures is paramount, since every living thing on the planet is exposed to mixtures rather than single chemicals in some kind of discrete order. Worse, pesticides interact with other stressors in the environment, such as climate change’s disruption of food webs and rapidly rising temperatures.
Climate stressors and pesticide interactions are vastly understudied, and only now are the scientific tools emerging to evaluate their dynamics. Testing for these effects is not required by regulators in the U.S. and globally, despite the expectation that in a warming world, environmental stressors will become far more intense.
Researchers at the Helmholtz Centre for Environmental Research in Leipzig and the department of Evolutionary Ecology and Environmental Toxicology, at Goethe University in Frankfurt, are revealing the interactions between two types of environmental stress—food deprivation and heat stress—and pesticides, both individually and in mixtures. In their laboratory study, the researchers exposed the water flea Daphnia magna, a tiny crustacean, to both types of stress in combination with exposure to the pyrethroid insecticide esfenvalerate and a mixture of 13 other pyrethroids. The degree to which the combination of pesticides and climate stresses multiplied damage to the Daphnia adds to the real-world hazards that are not captured in required regulatory reviews for pesticide registration by the U.S. Environmental Protection Agency.
In their earlier study, the researchers evaluated the effects of esfenvalerate on 24-hour-old Daphnia, coming to several ominous conclusions: first, food limitation and high temperature combined produce more stress than either alone; and that these environmental stressors dramatically increased Daphnia’s sensitivity to esfenvalerate.
Both studies employed a Stress Addition Model (SAM) developed in 2016 by some of the same researchers, which posits that each individual has an overall capacity to cope with stress and that it can be overwhelmed as specific independent stressors increase. Each independent stressor adds to the total sum of stress on a population, and the SAM provides a tool to measure the “highly synergistic direct effects of independent stressor combinations.” These direct stressors, in other words, result in effects greater than their individual influences on an individual and a population.
In the current study, the researchers compared SAM with two conventional models that derive additive effects of both chemical exposure concentration and chemical effects on organisms. The additive models consider combined effects for each component but do not account for their synergistic interactions.
The researchers chose esfenvalerate and the 13 pesticides in the mixture based on nationwide German monitoring data showing that they are frequently detected in agricultural streams and are known to be toxic to aquatic invertebrates. They exposed Daphnia to a suite of experimental conditions combining the environmental stressors of heat and food limitation with exposure to either esfenvalerate alone or the mixture at varying doses and temperature.
Food limitation strongly amplified the pesticides’ effects, and these became more intense over time. When the temperature was set near the upper limit of survivability for Daphnia, it did not produce any synergistic reactions with either the single pesticide or the mixture for two weeks, but after three weeks there was strong synergy with the mixture. This indicates a latency effect, in which a specific response to a toxicant does not appear immediately after exposure. The combined effects of food limitation, high temperature, and the pesticide mixture were much stronger than for esfenvalerate alone.
Further, the comparison of SAM with the other models showed that “SAM better predicted the combined effects of chemical and nonchemical environmental stressors by orders of magnitude. This was especially true, again, for the mixtures. The researchers predicted LC10 values (the concentration that kills 10% of the population) to measure the “long-term effects of ultra-low pesticide concentrations in the field” and found that the conventional models underestimated combined effects by up to 2,000 times compared to SAM.
Esfenvalerate is a synthetic pyrethroid insecticide registered for use against a range of insects in the U.S. and the European Union. It primarily disrupts calcium signaling, leading to seizures and death. In the U.S., it is applied to vegetables, fruits, nuts, cotton, and sorghum. Additionally, esfenvalerate is registered for both indoor and outdoor uses for pest control in residential, institutional, and commercial areas, including lawns and turf.
Yet in its Interim Registration Review Decision from 2020, EPA states, “There are no human health risks of concern for esfenvalerate.” Compare this to the information in the Pesticide Properties DataBase (PPDB), a European archive of some 2,300 pesticide active substances including a comprehensive suite of parameters, including human health and biodiversity risk assessments. The PPDB classifies esfenvalerate as a “highly hazardous pesticide”, acutely toxic to mammals and temperate freshwater fish. According to the PPDB, for humans, esfenvalerate is carcinogenic, genotoxic, a neurotoxicant, and a reproductive/developmental toxicant. Among nontarget insects, it is acutely toxic to honey bees and beneficial predatory mites, and moderately toxic to ladybugs and lacewings.
EPA’s position on esfenvalerate in its Interim Registration Review Decision is dismissive of the chemical’s known effects on organisms at all levels of the biosphere. While it acknowledges worrisome toxicity to both aquatic and terrestrial invertebrates, including bees, its requirements for protecting them consist of hopes that applicators will follow detailed instructions on usage and disposal that, under real-world conditions are unlikely to be effective.
See 2019 Daily News coverage of the Pyrethroid Working Group’s influence on application rules and child protection policies. For example, EPA reduced buffer distances between buildings and field margins and water bodies at the request of the chemical industry. Under the 1996 Food Quality Protection Act, EPA had set a margin of safety of 3X (out of a maximum of 10X) for pharmacokinetic differences (how the body absorbs, distributes, metabolizes, and excretes chemicals) between adults and children from synthetic pyrethroids. But based on information from the pesticide industry’s Council for the Advancement of Pyrethroid Human Risk Assessment (CAPHRA), EPA reduced the protective margin to the lowest level of 1X for children and adults, including women of childbearing age. This decision was a masterpiece of hand-waving corporate toxicology in which EPA accepted CAPHRA’s proposed physiologically-based pharmacokinetic modeling (PBPK) method. See this 2015 expose by Elizabeth Grossman and Valerie Brown detailing the chemical industry’s successful campaign inducing EPA to accept PBPK modeling as a valid regulatory tool.
The current German study demonstrates clearly that evaluating chemical by chemical, presuming the effects of each are independent, and disregarding the increasing perils of climate change, is a bankrupt approach to chemical regulation. The authors suggest SAM can replace this failed approach. Although conventional models can predict mixture toxicity to some extent, only SAM was able to predict the synergies among multiple pesticides and environmental stressors.
The authors note that mixtures of toxicants that have different mechanisms of action put enormous stress on organisms because they have to cope with detoxifying multiple assaults at once. Metabolic demands rise markedly, and in conditions of low food availability and near-lethal temperatures, an organism may not be able to cope. “[C]umulative sub-lethal effects of the mixture…progressively exhaust physiological reserves,” the authors write. “Ignoring such combined effects may severely underestimate ecological risks.”
Despite all this, until EPA updates its framework for evaluating chemicals’ threats to the biosphere, this underestimation continues to serve the interests of the pesticide industry. In its Interim Registration Review Decision for esfenvalerate, EPA stated, “[T]he benefits of pyrethroids in agricultural crop production outweigh the risks, and the necessary mitigation are expected to allow continued use of pyrethroids in agricultural settings while putting reasonable measures in place to reduce risk to non-target organisms from runoff and spray drift.”
Thus, the fate of the modest water flea demonstrates the real cost of such reasoning. Daphnia provide enormous ecosystem services by filtering suspended particles out of water. They are food prey for fish and insect larvae and form the basis of most freshwater trophic webs and therefore, are considered a keystone species. What harms them harms their beneficiaries, and the mechanisms of toxicity, not to mention the environmental stressors from climate change, affect micro- and macro-organisms, including humans, worldwide.
Pesticide pollution and climate change, individually, are near-existential threats; combined, they threaten civilization itself and the stability of the ecosphere. The stress on the biology of Earth will be considerably reduced if, as Beyond Pesticides advocates, pesticides can be eliminated altogether by 2032. It is possible.
See Beyond Pesticides’ resources—Organic Agriculture, including sections on Why Organic? and Keeping Organic Strong, to view an array of information, guides, and research on the ecological, public health, and environmental justice implications of a wholesale organic food system. Plus, see Tools for Change to learn about organizing strategies to transition your community toward organic lawncare management programs!
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Sources:
Double trouble: The synergistic threat of environmental stressors and pesticide mixtures
Shahid et al
Journal of Hazardous Materials 2025
https://www.sciencedirect.com/science/article/pii/S0304389425032133
Study Shows Climate Change Exacerbates Synergistic Effects of Synthetic Pyrethroid on Biodiversity
Beyond Pesticides, October 25, 2024
https://beyondpesticides.org/dailynewsblog/2024/10/study-shows-climate-change-exacerbates-synergistic-effects-of-synthetic-pyrethroid-pesticide-threatens-biodiversity/
Scientific Studies Identify EPA Deficiency in Evaluating Safety of Toxic Chemical Interactions
Beyond Pesticides, September 8, 2025
https://beyondpesticides.org/dailynewsblog/2025/09/scientific-studies-identify-epa-deficiency-in-evaluating-safety-of-toxic-chemical-interactions-calls-for-action/
Common Household Pesticides Again Linked to Behavioral Problems in Children
Beyond Pesticides, March 7, 2017
https://beyondpesticides.org/dailynewsblog/2017/03/common-household-pesticides-linked-behavioral-problems-children/
Research Shows Commonly Used Pesticides Produce Greater Toxic Effect When Mixed
Beyond Pesticides, August 11, 2011
https://beyondpesticides.org/dailynewsblog/2011/08/research-shows-commonly-used-pesticides-produce-greater-toxic-effect-when-mixed/
Pyrethroid Pesticide Affects Puberty at Low Levels
Beyond Pesticides, September 22, 2008
https://beyondpesticides.org/dailynewsblog/2008/09/pyrethroid-pesticide-affects-puberty-at-low-levels/
Predicting the synergy of multiple stress effects
Liess et al
Scientific Reports 2016
https://www.researchgate.net/publication/307937573_Predicting_the_synergy_of_multiple_stress_effects
Pesticide Properties DataBase (PPDB)
An international database for pesticide risk assessments and management.
Lewis, K.A., Tzilivakis, J., Warner, D. and Green, A.
Human and Ecological Risk Assessment: An International Journal, 22(4), 1050-1064. (2016)
https://www.researchgate.net/publication/299518887_An_international_database_for_pesticide_risk_assessments_and_management
