78% of Walloon agricultural land contaminated with insecticides

Scientists* from the Earth and Life Institute (UCLouvain) have shown that 78% of agricultural soil in Wallonia is contaminated with neonicotinoids, insecticides that are particularly harmful to pollinators. 

Even more surprising is that 62% of sites that have never been treated with this insecticide are nevertheless contaminated. Neonicotinoid residues have also been detected in cover crops growing on soil that was treated several years ago. 

While the European Union has just approved a relaxation of certain environmental standards, scientists* from the Earth and Life Institute have published a groundbreaking study in the Journal of Hazardous Materials, highlighting a worrying reality: the massive and long-lasting contamination of Walloon agricultural soils by neonicotinoids, a family of insecticides known for their harmful effects on pollinators.

Between 2023 and 2025, researchers* sampled 86 agricultural plots in Wallonia, analyzing the top 30 centimeters of soil. The results are clear: neonicotinoid residues are present in 78% of the soils analyzed. 

Even more surprising is that 62% of the sites that have never been treated with this insecticide are nevertheless contaminated. This is the second study of its kind in Europe: the results show that Belgium is also affected by long-term and widespread soil contamination.

The two hypotheses that could explain this contamination (airborne transport of contaminated soil particles and transport by runoff water) could not be validated. The mechanisms of pesticide contamination at the landscape level remain poorly understood and difficult to anticipate. 

Cover crops: an unexpected ecological trap

Another part of the study focused on cover crops, an ecological practice that involves sowing plants between two crops to protect the soil and provide flowers for pollinators when food is scarce.

However, the results of the UCLouvain study show that these plants can absorb neonicotinoid residues present in the soil, sometimes up to three years after a single treatment.

Pesticides were found in the stems and leaves, in the flowers, but also in the pollen, an essential resource for pollinators. For solitary bees nesting in the ground, the risk of exposure reaches levels up to 26 times higher than the safety threshold!

A clear message for public policy 

These results highlight a key reality: agricultural soils retain a lasting “toxic memory”. In landscapes marked by a history of intensive neonicotinoid use, certain measures — even those that promote biodiversity, such as flower strips or grasslands — can expose pollinators, ultimately risking the stability of agricultural systems.

Even though current trends are moving towards relaxing standards, scientists* from UCLouvain are calling for greater caution in agro-environmental policies.

The way in which treatments are organized (not sowing in windy conditions or at the top of slopes, stopping repeated treatments, etc) and grasslands/grass strips are designed (as far away as possible from treated fields, waiting at least three years before converting a field into a flowered area, etc) appears to be crucial in limiting the persistent effects of these insecticides.

*Scientific team

For the first study: Maxime Buron (Earth and Life Institute - ELI), Émile Foguenne (ELI), Alodie Blondel (Walloon Agricultural Research Centre - CRA-W), Thomas Marchetti (CRA-W), Léna Jeannerod (ELI), Anne-Laure Jacquemart (ELI), Pierre Defourny (ELI), Julien Radoux (ELI), and Yannick Agnan (ELI).

For the second study: Inès Gancedo Tarano (ELI), Thomas Boumal (ELI), Marc De Toffoli (ELI), Maxime Buron (ELI), Tomasz Kiljanek (National Veterinary Research Institute, Poland), Anne-Claire Martel (ANSES, France), Anne-Laure Jacquemart (ELI), and Yannick Agnan (ELI).

References 

Buron, M., Foguenne, É., Blondel, A., Marchetti, T., Jeannerod, L., Jacquemart, A.-L., Defourny, P., Radoux, J., & Agnan, Y. (2026). Predicting soil neonicotinoid content in agricultural landscapes using indirect indicators. Journal of Hazardous Materials, 501, Article 140794. https://doi.org/10.1016/j.jhazmat.2025.140794 

Tarano, I. G., Boumal, T., De Toffoli, M., Buron, M., Kiljanek, T., Martel, A.-C., Jacquemart, A.-L., & Agnan, Y. (2025). Are cover crops a potential threat for pollinators due to clothianidin residues in floral resources? Environmental Monitoring and Assessment, 197(11), 1260. https://doi.org/10.1007/s10661-025-14741-9 

This article was originally written in French by the AREC team of UCLouvain. It is available to read here.