17
Sep
Study of Biological Diversity Effects of Pesticide Mixtures Highlights Underestimated Risks to Ecosystems
(Beyond Pesticides, September 17, 2025) A study in Environmental Science & Technology finds additive effects of a synthetic pyrethroid insecticide (cypermethrin) and two fungicides (azoxystrobin and prochloraz) on biological control, biomass of major invertebrate trophic groups (position in food web), and soil ecosystem processes in arable systems (land suitable for growing crops). The study authors further highlight the failure of pesticide regulations to consider elaborate trophic interactions and pesticide mixtures, as well as additive and synergistic effects within their assessments, calling attention to the complexity of real-world exposures and the lack of research to fully understand the implications of chemical use for agricultural and land management purposes.
“Arable systems have a high dependence on diverse natural biota to support pest control, soil bioturbation, and nutrient recycling,†the researchers write. These communities rely on a balance of organisms within various trophic levels in order to function and provide vital ecosystem services. Disruptions caused by environmental contaminants, such as pesticides to nontarget organisms, impact entire ecosystems and overall biodiversity. As the authors state, current risk assessments underestimate the real-world risks of petrochemical pesticides and synthetic fertilizers that, despite a wide body of science connecting exposure to deleterious health and environmental effects, are still not fully understood.
There are complex interactions of chemical mixtures that occur as a result of the â€chemical soup†present in the environment, with underlying mechanisms that are incompletely comprehended. Rather than relying on a precautionary approach to protect health and the environment, the U.S. Environmental Protection Agency (EPA) utilizes unenforceable mitigation strategies to “reduce risks†and allows registrations based on insufficient scientific research. The risk assessment incorporates the assumption of 100% compliance with mitigation measures. (See more on the precautionary principle here, here, and here, as well as EPA failures here and here.)
Background and Relevance
The use of pesticides has grown exponentially since World War II, with many wartime chemicals transitioning to the consumer market, including the organochlorines (e.g., DDT) and organophosphates (e.g., malathion, chlorpyrifos). “The potential nontarget effects on biodiversity from these pesticides have required the development of regulatory procedures to mitigate risk,†the authors explain. They continue: “However, there is increasing concern that these fail to adequately capture the real-world consequences. Part of this failing relates to a focus during the regulatory process on single active ingredients, an emphasis on a restricted number of established model species (e.g., nontarget arthropods like honeybees, earthworms, springtails, and enchytraeids) from which risk is inferred and the lack of continuous instruction on the timing and dose of pesticide application.â€
The use of a limited group of model species, in particular, is concerning as they “fail to capture how differential species sensitivity propagates through trophic interactions to affect wider system-level processes,†the researchers emphasize. (See more information in Daily News coverage on varying sensitivity between species and risk assessment deficiencies here and here.)
The pesticides included in the experiment are cypermethrin, a synthetic pyrethroid insecticide, and two fungicides, azoxystrobin and prochloraz, which documentation shows cause health and environmental effects ranging from toxicity to aquatic organisms and bees to neurotoxicity, endocrine disruption, and oxidative stress.
Study Methodology and Results
The study, performed in an open growing area in Southern England, uses arable communities of pests (aphids and weevils), predators (e.g., beetles, spiders, and mites), and soil biota (including mites and earthworms) to assess interactions between cypermethrin, azoxystrobin, and prochloraz as well as within trophic interactions as a result of exposure to these active ingredients. “This study is intended to provide new insights into the risks posed by pesticide synergisms within the context of trophically complex communities critical to the support of agricultural ecosystem services,†the authors summarize.
They continue: “In this study, we use a model arable mesocosm [controlled, semi-natural experimental ecosystem] approach to test the interacting effects of a widely used pyrethroid cypermethrin in combination with an azole fungicide prochloraz that has an established biochemical mechanism resulting in synergisms. We also tested the effects of the strobilurin fungicide azoxystrobin, for which there is little evidence for synergisms with pyrethroids.â€
The results reveal that despite previous research supporting cypermethrin and prochloraz having increased interactions, cypermethrin and azoxystrobin have a greater than additive effect. “The synergism between cypermethrin and prochloraz was relatively small compared to that between cypermethrin and azoxystrobin,†the researchers state. “However, the synergism between cypermethrin and prochloraz was expected due to the inhibition of the cytochrome P450 monooxygenases,†which are proteins essential for metabolism.
The study analyzes the effects of the three pesticides individually and in pairwise combinations to establish how “key aspects of arable farming systems, including aphid pest control by ladybirds, biomass of key trophic levels, overall community food web structure, and soil organic matter recycling†is impacted following exposure. The methodology includes a description of the 35 mesocosms containing blended loam topsoil with organic matter and a seeded soil surface with a mixture of organic powdered oats and dried yeast flakes to provide resources for soil microfauna.
Two months before the application of the pesticides, field beans were transplanted into the soil, and then a sequential addition of arthropod communities began to build up the trophic levels of detritivores (animals that feed on dead organic matter), herbivore pests, and predators. In total, the experiment includes seven treatments ranging from the control (no pesticides) to each active ingredient individually and in pairs.
Assessments following pesticide applications incorporate arthropod sampling and soil sampling, with an assessment of decomposition rates of the breakdown of organic matter by soil macrofauna. Trophic links are also estimated using the WebBuilder function of its statistical package “to predict trophic interactions between the terrestrial and soil arthropod communities for each mesocosm.†These links are based on observed and established species feeding relationships within the mesocosm.
Within the experiment, the arable crop mesocosm system records show 30 taxonomic units (including species, genus, and order classifications), representing a total of 317,064 individuals from the trophic levels of detritivores, herbivores, and predators over the 4-week sampling period. Impacts on the organisms throughout the experiment show:
- Ladybird abundance responds significantly to the interaction between pesticide treatments and sample week, with the lowest numbers recorded with exposure to cypermethrin.
- Predator biomass experiences a significant interaction between pesticide treatment and sample week, suggesting implications for single or acute exposure versus continued exposure.
- “[T]here was in general greater evidence for impacts of all three types of pesticides, with the use of cypermethrin either alone or with the fungicides having negative effects that decreased predator biomass over time. The use of either fungicide alone or in combination also appeared to have net negative effects on predator biomass.†Detritivore biomass declines over time are also noted.
- Within the predator food web architecture, there is a trend for reduced node richness with cypermethrin (alone and in combination with azoxystrobin).
- Additional “changes in the architecture of predatory trophic interactions did show evidence for synergistic effects between cypermethrin and the fungicides.â€
- There is a collapse in connectance (measure of network complexity), which represents the proportion of trophic links and is associated with system stability and robustness, which can impact biodiversity and its associated ecosystem processes. (See studies here and here.)
- Nutrient pathways and overall diversity in tropic interactions are affected by all three pesticides, implying “that these critical food webs have reduced functionality, with an equivalent reduction in this metric seen where communities suffer from overexploitation or invasive species.†The results suggest the diversity of nutrient pathways is additive with pesticide exposure.
The authors note that while the data highlights trophic level impacts with all three pesticides, and unexpected additive effects that are highest between cypermethrin and azoxystrobin, “more subtle effects may be occurring, resulting from sublethal behavioral changes (e.g., activity) that may affect their ability to impact soil ecosystem processes.†(See research here and here.) While mesocosms are able to act as an experimental model to study environmental changes under controlled conditions and offer insight, such as for complex interactions between trophic levels, they are inherently artificial and not representative of all factors within natural ecosystems that need to be considered.
Regulatory Deficiencies
As the researchers emphasize throughout the study, agricultural systems apply a wide range of pesticides, which are approved for use through regulatory processes that do not realistically consider all possible combinations and potential interactions between active ingredients. As a result of this, risks to health and the environment are often underestimated, as there is a potential for additive, or even greater than additive (synergistic), effects with pesticide mixtures. (See studies here, here, here, and here.)
The authors describe that, “Such synergistic risks can occur where pesticide active ingredients affect either: 1) the bioavailability of one or more compounds, e.g., by changing its half-life or altering interactions with organic matter to affect sorption; 2) toxicokinetics that change uptake and transport to target sites, e.g., via inhibition of transport proteins, competition for binding sites, metabolism, and/or excretion; or 3) toxicodynamics that change interactions with target sites or affect a species ability to combat the effects of exposure.†(See here, here, and here.)
There is significant potential for synergistic impacts on nontarget organisms, including those providing ecosystem services like pollination, natural pest control, and soil health benefits. However, regulatory assessments do not typically evaluate possible synergistic interactions, as well as the true impacts on community structure, which are highly complex, when exposed to pesticide mixtures.
“We have shown that while insecticide use has arguably predictable consequences for most invertebrates, it is only when consideration is given to complex measures of trophic interactions between species that synergistic risks between pyrethroids and strobilurin fungicides were identified,†the researchers conclude. They continue: “Yet, the current emphasis of pesticide regulatory approaches remains on a limited number of model species while rarely considering how changes in wider community composition can impact complex species interactions. Such interactions have implications for system resilience and ecosystem processes.â€
Previous Research on Arthropods
Daily News coverage entitled “Cultivating with Natural Predators Gets Farmers Off the Pesticide Treadmill, According to Study†highlights how many of the pesticides used against insects, including neonicotinoid seed treatments, also kill other beneficial arthropods, as well as other soil invertebrates important to cycling nutrients naturally. The study finds arthropod predators suffer sharp declines in fields applied with pesticides and where seeds have been treated with neonicotinoids, as well as how slug damage can be reduced when natural slug predators are encouraged, allowing farmers to avoid the endless cycle of pesticide dependency, pest resistance, genetically engineered crops, and synthetic fertilizers.
Prior research in Germany, summarized in the Daily News, Study Shows Drastic Loss of Arthropods in German Grasslands and Forests, shows a steep decline of arthropod (insect and spider) populations in grasslands and forests, which adds to the growing body of evidence documenting the ongoing “insect apocalypse.†As noted, it is clear, based on the scientific literature, that the use of pesticides in industrial agriculture is greatly contributing to the rapid decline of insect populations, which then causes cascading impacts throughout the food web, such as with birds and other wildlife.
Support for the Organic Solution
A myriad of studies highlight the importance of insect biodiversity for ecosystem functioning, which is protected through organic land management practices. One study finds uncultivated field margins contain almost twice as many beneficial insects as cropped areas around farm fields, according to research published in the Journal of Insect Science. The study notes that these predators and parasitoids overwinter in diverse vegetation and can provide farmers an important jump start on spring pest problems instead of relying on toxic pesticides.
Another study also promotes the value of natural predators, highlighting how bats are crucial in suppressing pest arthropods in agroecosystems and contribute vitally to sustainable agriculture. (Read more in the Daily News here.) Additional coverage this year of a study in Conservation Genetics, entitled “Organic farming fosters arthropod diversity of specific insect guilds – evidence from metabarcoding,†showcases the negative effect of chemical-intensive, conventional farm management on insect populations when compared to organically managed meadows.
Visit Beyond Pesticides’ What the Science Shows on Biodiversity page for more research on how population declines are linked to pesticide exposure. As an alternative to these harmful chemicals, Beyond Pesticides advocates for the elimination of all petrochemical pesticides and synthetic fertilizers that are insufficiently studied and regulated, putting all organisms, including humans, at risk.
The widespread adoption of organic practices can mitigate these risks by protecting and enhancing biodiversity, safeguarding public health, and mitigating climate change. Learn more about the health and environmental benefits of the organic solution here and here and take action to advance organic, sustainable, and regenerative practices and policies.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Woodcock, B. et al. (2025) Synergistic Pesticide Effects on Complex Arthropod Trophic Interactions in Arable Systems, Environmental Science & Technology. Available at: https://pubs.acs.org/doi/abs/10.1021/acs.est.5c04611.