Reduction of pesticide use can increase earthworm populations in wheat crops in a European temperate region

• The negative effect of pesticides was greatest in earthworms living in close proximity to the soil surface.
• Insecticides have more negative influence on three earthworm species than herbicides and fungicides.
• A 50% reduction of pesticide use is likely to lead to large increases in earthworm densities.

Agricultural intensification has led to reduced soil biodiversity in arable lands. The potential benefits from organic farming and from low-input cropping systems have not yet been precisely assessed. Earthworm, having important agro-ecological functions, may be affected by pesticide applications, especially those species living mainly in the surface soil layer. We used a five-year experimental database including conventional and organic cropping systems to establish simple relationships between the Treatment Frequency Index – a phytosanitary indicator of pesticide pressure – and the abundance of three important earthworm species. We found that insecticides have more negative influence on earthworm species than herbicides and fungicides, and that species living in the soil's surface layer were the most affected by pesticides. Density of these earthworm species could be multiplied by a factor 1.5–4 if the Treatment Frequency Index was halved, as is currently required by some European regulations. Our results thus demonstrate that a reduction in pesticide application would increase earthworm population density in agricultural fields.