29
Apr
Pesticides Linked to Altered Gut Microbiota in Farmland Bird of Prey, Raising Biodiversity Concerns
(Beyond Pesticides, April 29, 2025) A study in Environment International finds pesticide-induced alterations in the gut microbiota of a farmland raptor species. In collecting blood and cloacal samples from Montagu’s harrier (Circus pygargus) nestlings, the authors “shed light on an overlooked collateral effect of pesticides, i.e., a general modification of gut bacterial assemblages,” which can lead to an imbalance of microorganisms (dysbiosis) and the promotion of potential pathogens, as well as negatively impact the health of birds of prey. “Additionally, our findings support the ‘One Health‘ framework, stressing the interconnectedness of wildlife, ecosystem, and human health, particularly in pesticide-affected agricultural areas,” the researchers share.
“The gut microbiota is crucial for host health and can be impacted by various environmental disruptions, yet the effects of multiple pesticide exposures on farmland organisms’ microbiomes remain largely unexplored,” the authors state. In the study, they “assessed microbiota changes in a wild apex predator exposed to multiple pesticides in agricultural landscapes,” which “provides evidence of pesticide impacts on wildlife gut microbiota, highlighting links between pesticide exposure and changes in microbiota composition,” the researchers note.
The Montagu’s harrier, as an apex predator, serves as a ‘bio-sentinel’ or bioindicator species for assessing ecosystem health. “As top-level predators in the food web, they are especially vulnerable to pesticide exposure due to biomagnification,” the authors say. (See study here.) They continue, “Studying the effects of pesticides on the microbiota of wild birds not only provides insights into the health of these organisms but also highlights broader ecological consequences, making them relevant biological models for understanding environmental and animal health impacts.”
As Montagu’s harriers choose primarily to build nests in farmland, the majority of which have been treated with pesticides prior to their breeding period, they are exposed to chemicals through both the environment and their food. Their prey is comprised mostly of voles, small birds, shrews, rabbits, lizards, and insects that are found near their nesting sites, which they make in immediate contact with contaminated soil.
“This threatened, protected species nests on the ground, mainly in cereal crops, making chicks and adults directly subject to agricultural activity,” the researchers point out. (See studies here and here.) In identifying pesticide residues within blood samples and examining their effects on the composition and structure of the gut microbiota, the study highlights the threats to the overall health of not only this avian species but also other organisms subjected to these chemicals as well.
“The gut microbiota is an essential modulator of host physiology and plays a key role in host health,” the authors state. They continue, “Its importance is such that, in organisms, including humans, its disruption is associated with several pathologies, including diabetes, cancer, cardiovascular events, and neurodegenerative diseases.”
The study, conducted in an area of intensive farming in France on a site called “Zone Atelier Plaine & Val de Sèvre,” involves blood and microbiota sampling from nestlings 26 ± 2 days old. The researchers consider “both the diversity of pesticides (i.e., the so-called cocktail effect) and the diversity of the gut microbiota” to test “the concordance between the gut microbiota community structure and the concentration of pesticides measured on chicks’ blood.”
As a result, the blood samples of 22 chicks reveal a total of 36 pesticides and 139 MOTUs (Molecular Taxonomic Operational Units), which are groupings of related DNA sequences for species of bacteria to help classify them within the analyses. From the collected data, statistical analyses were performed only for the pesticides and MOTUs that were observed in at least five different chicks in order to ensure robustness. This encompasses the 35 most prevalent MOTUs and 25 pesticides.
“The strongest correlations involved eight different MOTUs (4 Actinobacteriota and 4 Proteobacteria) and 12 pesticides (two fungicides, seven herbicides, three insecticides), among which three were banned in Europe (acetochlor: 2013, chlorpyrifos: 2020, quinoxyfen: 2019),” the authors report. They continue, “Pesticides, including acetochlor and quinoxyfen, which are supposed to be banned, were significantly positively correlated with certain key bacteria from Actinobacteria, Alphaproteobacteria and Gammaproteobacteria classes.”
This study shows “show a significant correlation between the composition of the gut microbiota of an organism exposed to a cocktail of different pesticides found in its blood, suggesting that the processes linking microbiota and pesticide effects cannot be drawn from a single molecule nor a bacterial species.” The findings also “suggest that pesticides may modulate the development of key bacterial taxa at early stages of the gut microbiota ontogeny, which may have later implications (due to priority effects) on the development of the adult microbiota and its health.”
As the presence of certain bacteria can alter the health of the organisms, such as through metabolism or by inducing pathogenicity, these results highlight the “potential collateral effect of pesticides on gut bacterial assemblages through unknown mechanisms” that could result in dysbiosis (see here and here) and promote pathogens. “[T]hese microbial shifts underline the broader ecological consequences of pesticide exposure, emphasizing the need for integrated biodiversity conservation and ecosystem management to protect environmental and public health,” the researchers conclude.
As previously reported by Beyond Pesticides, a 2024 study in Science of The Total Environment, featuring many of the same authors as the current study, finds lower pesticide load in Montagu’s harrier chicks in areas with organic farming. A correlation between lower numbers of pesticides in the blood of birds with the presence of organic farms surrounding the habitats was determined after analyzing 55 nestlings from 22 different nests in southwestern France. As the percentage of organic agriculture around the nests increased, there was a significant decrease in the quantity and types of pesticides detected within the chicks’ blood.
As a result of their analysis, “all chicks sampled (n = 55) were found to be contaminated with at least one pesticide, and the maximum number of pesticides detected per chick was 16,” the researchers report. Some of the detected pesticides include bifenthrin, boscalid, clothianidin, cypermethrin, cyprodinil, difenoconazole, dimethomorph, epoxiconazole, indoxacarb, mecoprop, myclobutanil, oxadiazon, piperonyl butoxide, propyzamide, quinoxyfen, and thiacloprid. Each of these pesticides is linked to health effects in humans that range from skin irritation to cancer, endocrine disruption, neurotoxicity, kidney and liver damage, and birth, development, and reproductive impacts. Many of these pesticides are toxic to aquatic organisms, bees, and birds have been banned in France. Their persistence in the environment is highlighted by their presence in the blood of the Montagu’s harrier chicks.
“The present study reveals that organic farming reduces the number of pesticides in Montagu’s harrier chicks, which may have a beneficial effect on its population, as chemical inputs have been shown to drive farmland bird population decline across Europe,” the study authors postulate. Elimination of this species’ exposure to pesticide cocktails means elimination of exposure to all organisms within the food chain. As the scientists mention, “because the Montagu’s harrier is at the top of the trophic chain and a specialist predator species of agricultural lands, studying its contamination with pesticides is particularly relevant as an indicator of larger contamination of the environment.” Creating a more sustainable environment by addressing the issue of pesticide exposure allows for a cascade of positive effects on soil, water, air, and biodiversity.
Help support Beyond Pesticides’ mission of eliminating petrochemical pesticides and synthetic fertilizers by becoming a member today. Learn more about the health and environmental benefits of organic land management here and here, and stay up to date on the hazards of pesticides, pesticide regulation and policy, pesticide alternatives, and cutting-edge science through the Daily News Blog.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Bariod, L. et al. (2025) Exposure to pesticides is correlated with gut microbiota alterations in a farmland raptor, Environment International. Available at: https://www.sciencedirect.com/science/article/pii/S0160412025001874.
