12
Jun
Literature Review Unpacks Synergistic and Cumulative Pesticide Impacts on Aquatic Life
(Beyond Pesticides, June 12, 2026) In a literature review published in Caspian Journal of Environmental Sciences, researchers assessed 27 peer-reviewed studies conducted between 2011 and 2025 on the adverse impacts of insecticides, including neonicotinoids, pyrethroids, organophosphates, chlorpyrifos, and fipronil. Across agricultural and suburban environments, pesticides were detected in the majority (88 percent) of samples. This review builds on the continuous flow of science that highlights the adverse impacts of synthetic pesticide dependency on ecosystems and wildlife that are essential to global biodiversity.
Main Findings
The researchers, based at a variety of research institutions in Jordan, Uzbekistan, Iraq, and India, refined their search to 27 studies after screening for geographic diversity and empirical robustness; for example, they excluded studies that did not include empirical data, relied solely on nonagricultural contexts, or only assessed exposure through urban wastewater exclusively.
Toxicological data assessed include pesticide occurrence, toxicity, and biological responses. This literature review is not a meta-analysis, but rather a narrative synthesis of various findings.
The main findings include:
- Documented Widespread Pesticide Occurrence. In the United States, based on data collected between 2013 and 2017, 88 percent of water samples contained pesticides, with a median of 18 compounds across all sites and 24 at agricultural sites, peaking between the months of May and July. (Covert et al., 2020). Great Lake tributaries show 96 percent of samples testing positive for pesticide residue, with a median of 16 compounds detected year-round. (Oliver et al., 2023)
- Sediment and Algal Mixtures as Exposure Pathways for Contamination. Glyphosate and one of its breakdown products (AMPA) were detected in periphyton (algal mixtures) at 50 percent and 30 percent of Ontario agricultural drain sites, respectively. Researchers have determined that mayfly survival, a pivotal source of energy for various aquatic organisms, can be compromised due to exposure. (Ijzerman et al., 2023) In Japan, the neonicotinoid insecticide imidacloprid was found in the sediment of 66 of 90 sites with evidence of fipronil metabolite persistence. (Furihata et al., 2019)
- Exceeding Regulatory Thresholds in Multiple National Contexts. In the U.S., 60.9 percent of agricultural samples exceeded both chronic and acute benchmarks set by the Environmental Protection Agency (EPA). (Covert et al., 2020) In small water bodies in Germany, 22 percent of samples exceeded legally acceptable concentrations. (Lorenz et al., 2025) The story for German protected streams was more dire—70 percent of stream samples exceeded legally acceptable concentrations, largely driven by fipronil (9 times the limit) and imidacloprid (6 times the limit). (Schweiger et al., 2025)
- Measurable Impacts Below Regulatory Benchmarks. In U.S. wetlands, sediment equivalents (NI-EQs) of neonicotinoids drove chronic insect effects (including adverse impacts on species abundance and richness), even when water concentrations remained below acute EPA benchmarks. Researchers at the U.S. Geological Survey (USGS) tested water and sediment levels for potential impacts of thiamethoxam (neonicotinoid)-coated and fungicide-coated seeds on benthic organisms in Missouri floodplains between 2016 and 2017. (Kuechle et al., 2022)
- Developmental and Immunotoxicity in Fish. As mentioned above, there is evidence of sublethal effects to fish embryos due to pesticide drift. In Costa Rica, fish exposed to agricultural drift exhibit elevated biomarker levels, with macroinvertebrate abundance lowest at sites with the highest concentration of pesticides. Researchers sought to understand the ecotoxicity of agrichemical pesticide runoff from pineapple, rice, and banana plantations on the aquatic biodiversity of the Madre de Dios River watershed. (EcheverrÃa-Sáenz et al., 2018)
- Undermining Ecosystem Services. Pesticides have demonstrated disruption of ecosystem services and basic functioning. For example, Japanese rice-paddy mesocosms reduce macrophytes by 25–51%, with phytophilous predators being most sensitive. Researchers in this study highlight the importance of assessing the impacts of indirect agrichemical exposure on species traits of aquatic predatory insects found in rice paddies to more adequately “predict the realistic risks of [these threats] in nature.†(Hashimoto et al., 2019) Glyphosate drift into semi-natural ponds causes persistent species richness reduction among zooplankton (a critical food source for aquatic insects), among other deleterious impacts on these aquatic environments in Canada. For example, “taxon richness decreased in more contaminated ponds [and] failed to recover.†(Hébert et al., 2021)
- Toxic Mixtures Contribute to Multi-Stressor Interactions. The neonicotinoid thiacloprid combined with synthetic fertilizers altered microbial, phytoplankton, and chironomid communities in an additive manner that is not considered in single-stressor (individual pesticide or fertilizer) studies. (Beentjes et al., 2022) Urban streams that feed into the Amazon demonstrate high invertebrate risk from a multitude of compounds banned in the European Union, including chlorpyrifos, diuron, atrazine, and terbuthylazine. (Rico et al., 2022)
Previous Coverage
The U.S. regulatory system fails to assess the cumulative impacts of toxic pesticides, fertilizers, PFAS, and other exposure mechanisms based on potential additive or synergistic impacts.
A University of California, Los Angeles (UCLA) report, Building Capacity for Robust Pesticide Regulation: Part I – Cumulative Impacts, underscores the critical gaps in federal and state pesticide law and the opportunity for comprehensive reform to strengthen cumulative impact assessments for pesticide products. The main goal for this specific report is to develop a toolbox of scientific methodologies/approaches for California’s Department of Pesticide Regulation (DPR) and the local permitting process by county agricultural commissioners (CACs) to engage in more comprehensive and cumulative impact assessments under their purview. Before going into legal and regulatory history, the authors first distinguish between some key concepts, including cumulative exposure, cumulative risk, cumulative impact, and pesticide mixtures.Â
Cumulative exposure refers to the various pathways (e.g., soil, air, water) and routes (e.g., ingestion, dermal, inhalation) through which pesticide exposure occurs. Cumulative risk is the combined risk from multiple exposures, with cumulative impact stacking on additional dimensions (or “stressors,†as the report refers to them), including socio-economic status or heat stress, among others.
The report also describes three types of pesticide mixtures, some of which individuals or communities simultaneously face. These include the following:
- Product mixtures, where one registered pesticide product is a pre-mixed formulation of multiple active ingredients, “inert†or “other†nondisclosed ingredients, adjuvants, and other substances. For example, the new (as of 2024) Roundup Weed and Grass Killer “Exclusive Formula†consists of triclopyr, fluazifop, and diquat—three different active ingredients registered with EPA within one product.
- Field mixtures, where multiple pesticide products are added to a tank and simultaneously sprayed on crops—either because the EPA-registered label is “silent with respect to mixing, leaving the decision to the grower or applicator,†or “the application instructions on the product label require or encourage mixing with other pesticides or with materials such as emulsifiers or wetting agents.â€
- Coincidental mixtures, when separate applications from individual pesticide products and field mixtures from adjacent fields form into new combinations that could lead to additive (synergistic) or subtractive effects.
In terms of the various pesticide mixtures, the report offers different recommendations based on product mixtures, field mixtures, and coincidental mixtures. Beyond Pesticides notes that risk assessment methodology, unless it is considered in the context of a rigorous alternatives assessment, begins with the mostly false assumption that petrochemical pesticides are needed (or are essential) to achieve cost-effective pest management, agricultural productivity and profitability, and quality of life, when, in fact, this may not be the case. Therefore, improved risk calculations, while important to characterizing the harm and the gaps in fully assessing adverse effects associated with pesticide use, still impose some level of harm deemed by the government to be acceptable. For more information, see Daily News, Report Describes Complex Cumulative Risk Assessment Proposal to Implement California Law.
The decades-long chemical dependency crisis on top of the climate crisis is wreaking havoc on biodiversity in increasingly shocking outcomes. For example, researchers from the Helmholtz Centre for Environmental Research in Leipzig, Germany, analyze exposure to the pyrethroid insecticide esfenvalerate with two nonchemical environmental factors: elevated temperature and food limitation. In their recent publication in Environmental Pollution, the authors find the greatest synergistic effects when dapnids/water fleas, referred to as Daphnia magna (D. magna), are subjected to esfenvalerate under these aforementioned, climate-exacerbated conditions. (See Daily News here.) A separate study published in the Journal of Hazardous Materials by scientists at six Chinese universities and research centers examines the convergence in springtails (Folsomia candida)—tiny insect-like animals that live in soils worldwide and are commonly used as laboratory subjects. The researchers exposed springtails to the neonicotinoid insecticide imidacloprid at three concentrations and three temperatures. In addition to measuring the springtails’ direct mortality, the researchers also investigated the microbes in the animals’ guts, checking for expression of genes involved in antibiotic resistance. The evidence is unequivocal: imidacloprid exposure at a soil temperature consistent with current and expected warming (30°C, or 86° F) killed significant numbers of springtails. This common soil arthropod has clearly illustrated how this convergence creates synergistic effects: warming increases pesticide toxicity; pesticide toxicity triggers antibiotic resistance; antibiotic resistance spreads through horizontal gene transfer (movement through the environment to people) and predation. The consequences, not yet fully understood, are nevertheless emerging from accumulating research. (See Daily News here.)
Synergistic effects also translate into the multi-faceted stressors that face beneficial organisms that undergird agricultural systems, including honey bees and earthworms. For example, a study published in Pesticide Biochemistry and Physiology finds that the combination of Varroa mites with the neonicotinoid insecticide imidacloprid increases the risk of bee mortality and disrupts the larval gut microbiome, stressing a synergistic effect (a greater combined effect) between Varroa destructor, a parasitic mite that attacks and feeds on honey bees, and imidacloprid. (See Daily News here.) A study of earthworms published in Environmental Science & Technology highlights that chemical mixtures can have both synergistic and species-specific effects, threatening the soil microbiome and overall soil health. In exposing two species, Eisenia fetida and Metaphire guillelmi, to the weed killer glyphosate alone and in combination with urea, a synthetic fertilizer, the researchers find enhanced toxicity with co-exposure as well as varying health effects between the two species. (See Daily News here.)
Call to Action
There are multiple actions that you can take to urge elected officials and regulators to shift gears and embrace the precautionary approach to pesticide use in the United States.
- Tell EPA, FDA, and Congress that regulations must consider the effects of pesticides in the context in which they are used and with reference to the organic alternative. The target for this Action is the U.S. Congress, the U.S. Environmental Protection Agency, and the U.S. Food and Drug Administration under the Department of Health and Human Services.
- Tell Congress, FDA, and EPA that it is past time to stop the manufacture and use of all organophosphate pesticides, which damage the nervous system and brain at low levels. The target for this Action is the U.S. Congress, the U.S. Environmental Protection Agency, and the U.S. Food and Drug Administration under the Department of Health and Human Services.
- Accurately define and ban PFAS pesticides, starting in California. Don’t let EPA dismiss the serious health and environmental threat with a definition that defies international standards. This action targets Congress and EPA, and, for California residents, it additionally targets California state Senators.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
